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Advances in engineered surfaces for functional performance

A.A.G. Bruzzone (2)a,*, H.L. Costa b, P.M. Lonardo (1)a, D.A. Lucca (1)c

a Dipartimento di Ingegneria della Produzione, Termoenergetica e Modelli Matematici, University of Genoa, Genoa, Italyb Laborato rio de Tribologia e Materiais, Universidade Federal de Uberla ndia, Campus Santa Mo nica, Uberla ndia, Brazilc School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, USA

1. Introduction

The most important physical phenomena involving exchange of

energy and/or signal transmission take place on surfaces. The

advances in many fields of science over thelast century have takenplace as a result of a deeper knowledge of how surfaces influence

the mechanisms and kinetics of this energy and signal transfer.

In thelast fewdecades, theunderstanding of surface phenomena,

particularly at a micro- and nanometer scale, has played a funda-

mental role in the development of many advanced fields, such as:

electronics, information technology, energy, optics, tribology,

biology, biomimetics, etc. Furthermore, a rush towards miniatur-

ization can be observed, with a vast technological advance in the

area of microfabrication. The functional exploitation of several

physical phenomena becomes more and more important with the

reduction of dimensions. Since the availability of technologies that

permit the manufacture and control of micro/nano-surface features

is another key issue for miniaturization, many techniques are

currently available to create surface features with sizes in the

micrometer range. The manufacturing of features on the nanometer

scale is more challenging, and has also been the subject of intensive

research and development.

It has been nearly one decade since the last CIRP keynote paper

[78] established the state of the art for structured, textured or

engineered surfaces. The relationships between the technologies

for structuring surfaces, their functional properties and the

applications exploiting the surface functionality are a common

denominator of the research carried out in the last years. Since

then the aspects relating surfaces to production [44,91], metrology

[55,62,108,155], emerging applications [61] and chemical inter-

actions [29] have been analysed.

The relationships between the domains of applications, func-

tional properties and engineering technologies are rather complex

(Fig. 1).

As the dimension reduces, different surface phenomena and

functional properties can be exploited in order to carry out thesame application. On the other hand, an application can use

different surface properties and, furthermore, a specific functional

property can be at the basis of different applications. For example,

adhesion is controlled by surface wettability (i.e., free energy),

roughness [20] and its interlocking properties.

Considering the relationships between functional properties

and engineering technologies, the same technology adopted to

engineer or structure surfaces can enhance or use different func-

tional properties. Conversely, many functional properties can be

the core of a specific technology. Moreover, different engineering

technologies can exploit the same functional property.

A clear distinction between the set of applications, functional

properties, and engineering technologies must be made in order to

establish a sound framework for the scientific development of new

applications and/or processes.

The design-manufacturing process to implement an application

that exploits surface functional properties follows the schema

represented in Fig. 2 [154]. The goal, i.e., the specific application, is

achieved through the recognition of the functional property(ies)

required by the surface. The task of the design phase consists of

conceiving a way to implement the required function(s). The

production step relies definitely on the development, or use, of

techniques able to structure the surface(s), and these techniques

depend essentially on specific functional properties of surfaces.

The knowledge of physics, materials and processes is essential

during the design and production steps of a ‘‘surface-based’’

application.

This paper will report the advancements concerning the

functional properties, applications and engineering technologiesavailable to structure surfaces.

CIRP Annals - Manufacturing Technology 57 (2008) 750–769

A R T I C L E I N F O

Keywords:

Surface

Engineered surface

Engineering technology

A B S T R A C T

Surface phenomena play a decisive role in the behaviour of engineering parts; their understanding and

control are fundamental to the development of many advanced fields, such as: electronics, informationtechnology, energy, optics, tribology, biology and biomimetics. Engineered surfaces rely on the control of

surface characteristics to obtain a desired functional performance. This paper reportsthe advances in the

state of the art considering the relationships between the properties of functional surfaces, their

applications and the technologies to engineer surfaces.

ß 2008 CIRP.

* Corresponding author.

Contents lists available at ScienceDirect

CIRP Annals - Manufacturing Technology

journal homepage: http://ees.elsevier.com/cirp/default.asp

0007-8506/$ – see front matterß 2008 CIRP.

doi:10.1016/j.cirp.2008.09.003

http://www.sciencedirect.com/science/journal/00078506

http://dx.doi.org/10.1016/j.cirp.2008.09.003

http://dx.doi.org/10.1016/j.cirp.2008.09.003

http://www.sciencedirect.com/science/journal/00078506



2. Functional properties of surfaces

Surfaces represent the interface through which a large number

of phenomena occur. The engineering fields involved by surfacephenomena include chemistry, optics, mechanical engineering,

electrical engineering, etc. Many relevant industrial applications

depend on surface properties.

The availability of advanced instruments, such as scanning

probe microscopes, in addition to improved visualization

techniques, permits a better characterization of surfaces and

consequently facilitates the investigation of the relation-

ships between surface and functions. Surface characterization,

together with the measurement of all the phenomena related to

the required function, e.g., forces, temperature, deformation, etc.,

allows for improvement of the performance of the surface.

A first taxonomy of the physics of surfaces could distinguish

between contact and non-contact phenomena. Contact phenom-

ena concern all the cases in which the surface comes in contactwith another medium whose state is either solid or fluid (liquid

or gas). Non-contact phenomena are primarily related to the

radiation properties of surfaces.

Another classification of the interaction mechanism could

be made considering the fields of classical physics. However,

such a classification would not consider some relevant aspects,

such as the biological properties of surfaces and their related

phenomena.

It is also important to distinguish between genuine physical

properties and technological properties, the latter being coupled to

some exploitation of one or more physical phenomena. According

to this approach, the classification could be affected by the

advancement and industrial importance of one particular class of

properties.In this paper, the properties of surfaces will be discussed

according to the classification given in Table 1, following

the importance they have for the advancement of surface

engineering.

2.1. Physical properties

Among the various physical properties, the most important for

surface engineering are those concerning surface energy, optical

phenomena, thermal characteristics, and electrical and magnetic

properties.

2.1.1. Surface energy

Surface energy controls many interesting properties that play a

great role in the applications of engineered surfaces. Considering a

three phase system, wetting is the most important phenomenon

governed by surface energy.

Wetting refers to the contact between a solid surface and a

liquid and depends on intermolecular interactions. The minimiza-

tion of the energies (surface tension) of the interfaces controls the

phenomenon. Thedegree of wetting is given in terms of the contact

angle, i.e., the angle between the liquid–vapor interface and the

solid–liquid interface. A non-wettable surface has a contact angle

greater than or equal to 908, whereas wettable surfaces have lower

contact angles. When the liquid present is water, the term

hydrophilic is used and denotes wettable surfaces, while hydro-

phobic relates to non-wettable surfaces. Superhydrophobic

surfaces are characterized by a contact angle greater than 1508.

The lotus plant is an example of superhydrophobic surface that

exhibits a cleaning effect (lotus effect) discovered by Barthlott and

Neinhuis [12].In an equilibrium condition, the analysis of forces acting on the

interfaces of a three-phase system provides the following

equations:

g au þ g ub cosu þ g ab cosa ¼ 0 (1)

g au cosu þ g ub þ g ab cosb ¼ 0 (2)

g au cosaþ g ub cosu þ g ab ¼ 0 (3)

where a, b and u are the angles indicated in Fig. 3 and g ij is the

surface energy between the i and j phases. As a consequence of

equilibrium, the relationship g ij < g jk + g ik occurs, i.e., none of the

surface tensions can exceed the sum of the other two. Whenwetting concerns only a flat rigidsurface the equilibrium equations

reduce to the Young equation:

g DS þ g DB cosu À g SB ¼ 0 (4)

where g DS, g DB and g SB are the surface tensions for the solid liquid

drop, liquid-background fluid, and solid-background fluid inter-

faces, respectively, and u is the contact angle of the drop on the

solid surface. Consequently, neither g SB nor g DS can be larger than

the sum of the other two surface energies.

Fig. 1. Surface, applications, functions and technologies.

Fig. 2. Development process of ‘surfaces-based’ applications [154].

Table 1

Main functional properties of engineered surfaces.

Physical Surface energy Capillarity, wetting, adhesion

Optical Reflectivity, light absorption,

diffraction

Thermal

Biological Adsorption

Technological Mechanical Fatigue, hardness

Hydrodynamical

T ribo logica l Friction, wear, stic ti on

Fig. 3. Three phases contact.

A.A.G. Bruzzone et al. / CIRP Annals - Manufacturing Technology 57 (2008) 750–769 751



To measure wetting the spreading parameter S can be defined:

S ¼ g SB À ðg DS þ g DBÞ (5)

The liquid wets the surface completely when S > 0, and

incompletely when S < 0.

When the system is not in equilibrium, e.g., when a droplet

expands or retracts, different mechanics apply. Hysteresis in the

contact angle can be observed; the static contact angle after

expansion is higher than that observed after a contraction. In

dynamic conditions the contact line does not move regularly but

incrementally, apparently with a stick-slip mechanism. These

aspects of dynamic wetting are currently the subjects of research

by many scientists.

Wetting and de-wetting of structured and imprinted surfaces

has been theoretically studied by Lipowsky et al. [150]. A

theoretical study regarding the influence of surface roughness

on superhydrophobicity is reported in Ref. [246].

Recently, Seemann et al. [198] provided an experimental and

theoretical analysis of wetting morphologies considering silicon

surfaces etched with grooves having a rectangular cross section

with depth of 100–900 nm, and width of 400 nm–3 mm (groove

aspect ratio ranging from 0.04 to 0.60). Silicon substrates were

chemically modified to vary the contact angle u for low molecular

weight polystyrene withinthe range 5–808. Vapor condensation of

the polystyrene provided the liquid droplets that were solidified bylowering the temperature below the glass transition temperature.

Fig. 4 shows the AFM images of the droplet morphologies

(images refer to different contact angles). The study introduced a

global morphology diagram of clustering droplet geometries

according to the aspect ratio of the groove and the contact angle

of the substrate.

Another important effect is due to the dependence of surface

tension on the temperature and liquid composition. The variability

of one of these factors determines the variation of surface tension.

This in turn causes the Marangoni effect, i.e., the liquid flows away

from regions of low surface tension. Marangoni convection plays

an important role in hydrodynamics and heat transfer in fluid

films.

Considering heat transfer, the structured surfaces used in heatexchangers to enhance performance determine a non-uniform

thickness in the thin liquid layer. Since the temperature of the

surface–liquid interface is higher where the liquid is thinner and

lower where the film is thicker, the liquid surface tension changes

locally, influencing thermocapillary stresses and liquid flow.

Recently, Skornyakov et al. [204] reported the experimental

results on the visualization of the Marangoni effect on glass

microstructured surfaces performed through the background-

oriented Schlieren (BOS) method. Triangular grooves with a depth

ranging between 250 and 1000 mm, and angles between 608 and

908 were examined. Both non-volatile liquids (silicone oil) and

volatile liquids (water and alcohols) were used. The experiment

indicated that the velocity field inside a single groove has a vortex

shape.Surface energy can be changed by applying an electric field,

which modifies the capillary forces at the interface. This

phenomenon, known as electrowetting, was reported in 1875 by

Lippmann [151], who constructed an electrometer based on the

capillary rise of a polarizedmercury–electrolyte interface to obtain

first-grade microscopic information about liquid interfaces.

In the last decades, the introduction of surface engineering led

to the enhancement of the electrowetting effect, e.g.,by implanting

electroactive-terminated alkane-thiol molecules into a metallic

surface [207] or by coating a metallic substrate with an insulating

film of macroscopic thickness, it is possible to obtain electrowet-

ting on insulator-coated electrodes (EICE) [19].

Recently, a survey of the developments of electrowetting wasreported by Quilliet and Berge [189] and numerical models for the

application of electrowetting to droplet actuation are described by

Arzpeyma et al. [9].

Similar to wetting, adhesion is an effect due to molecular

attraction exerted between bodies in contact. Five possible

mechanisms could determine adhesion:

Mechanical adhesion: is due to interlocking as seen in joints on

Velcro1 [63]. Chemical adhesion: depends on the chemical characteristics of

the joining materials and their susceptibility to ionic, covalent or

hydrogen bonding. Dispersive adhesion (or adsorption): takes place when two

materials are held together by van der Waals forces. Electrostatic adhesion: relies on a difference in electrical charge at

the joint that determines a structure similar to a capacitor and

produces an attractive electrostatic force between the materials. Diffusive adhesion: the joint is due to diffusion occurring when

the molecules of both materials are mobile and soluble in each

other. This mechanism typically occurs during sintering of

polymer chains, metals and ceramics.

Studies of the effects of surface micro-geometry on joints made

through structural adhesives are reported in Refs. [32,42,153].

2.1.2. Optical properties

One of the most important optical properties of surfaces is the

reflectance. This parameter is typical of each material and is

affected by the roughness of the reflecting surface.

When a light beam is incident onto a perfectly planar surfaceseparating two media of differentrefractive indices, the reflectance

r is defined as the ratio between the reflected and incident fluxes.

It can be expressed as a function of the relative index n:

r ¼ n À 1

n þ 1

2

(6)

In case of transparent materials, r is usually very small,

approaching zero as n approaches 1.

When the reflecting material is a metal, the index n is complex,

n = nÀ jx, where n is the refractive index of the metal and x is the

extinction index of the metal.

Consequently the reflectance becomes:

r ¼ ðnÀ 1Þ2 þ x2

ðnþ 1Þ2 þ x2(7)

The r value is relevant for the gloss of the metal, varying

between 0.33 and 0.94 for iron and silver, respectively, for a mid

visual light frequency [237].

The extinction index establishes the decay law of the light

intensity through the metal:

I z ¼ I 0 eÀð4px=lÞ z (8)

where I 0 and I z are the intensity of the beam at depths 0 and z ,

respectively. The transparency of the material decreases with the

value of x.

When a monochromatic, linearly polarized wave is incident tothe surface of a metal, it produces a reflected wave, which is

elliptically polarized. In fact, the reflected components that are

parallel and transverse to the incidence plane, E 00p and E 00t ,

respectively, show differences in their modulus and phase.

Fig. 4. AFM images of droplet morphologies (source [198], Copyright 2005 National

Academy of Sciences, U.S.A.).

A.A.G. Bruzzone et al. / CIRP Annals - Manufacturing Technology 57 (2008) 750–769752



The relationships between the reflected components and the

incident components, E p and E t, are given by the Fresnel formulae:

E 00p ¼ E pn cosu À cosu

0

n cosu þ cosu 0 ; E 00t ¼ E t

cosu À n cosu 0

n cosu þ n cosu 0 (9)

The influence of roughness on the surface reflectance has been

extensively studied.

A classic study by Bennet and Porteus [17] established that for a

normal incidence and a Gaussian distribution of the topographyheights, the relationship between the reflectance r of a rough

surface and the reflectance r0 of a smooth surface of the same

material is

r

r0

¼ eÀð4pRqlÞ

2

þ 32p4 Rq

l

4e

2Rdq

2

(10)

where l is the light wavelength and e is the aperture angle of the

receiving optical system. The first term of theequation refers to the

specular reflection and the second term to the contribution of the

diffuse reflection in the normal direction. For small values of the

ratio Rq/l, the reflectance canbe assumed dependentonly upon Rq.

Based on this principle, optical instruments measuring the

roughness through the measurement of reflectance have been

proposed.Also, the effect of roughness on the polarization of the reflected

light has been exploited. Experimental studies [152] have found

relationships between the phase and azimuth angles of polariza-

tion and roughness values.

Another optical property of surfaces associated with their

micro-geometrical texture is diffraction. This phenomenon is

generated when an interference of two or more waves occurs.

When an opaque surface is illuminated by a coherent light, the

radiation passing through an aperture produces diffraction. By

assuming the Fraunhofer approximation, the amplitude U ( x, y) o n a

screen located at a distance r from the surface is described by

U

ð x; y

Þ ¼C Z Z t ð x; yÞexp

j2p

lr ð xX

þ yY

Þ d X dY (11)

where X , Y are the coordinates of the illuminated aperture and x, y

are the coordinates of the screen.

The amplitude is the Fourier transform of the aperture

transmittance t ( x, y) when the aperture coordinates are measured

inunits of lr /2p. A particularlyuseful diffractionpattern is the one

generated by a rectangular aperture. For a long aperture having a

width s parallel to the y axis, the light intensity I ( x) onthescreen is

I ð xÞ ¼ I 0sinðpsx=lr Þpsx=lr

2

(12)

where I 0 is the illumination at x = y = 0.

If the slit is repeated periodically n times, the resulting

diffraction pattern is

I 0ð xÞ ¼ sinðpLx=lr Þn sinðp px=lr Þ

2

I ð xÞ (13)

where p is the period and L = np is the total length of the array.

Instead of a series of slits, a reflecting grating can be used with

the groves shaped to reflect the majority of light in a direction

corresponding to a high order of diffraction (Fig. 5) [145]. These

gratings are used in many technical applications.

2.1.3. Thermal properties

Thermal properties refer to the heat transfer mechanisms that

can take place by conduction or radiation. Convective heat transfer

relies on the combined effects of conduction and fluid flow and is

considered a third mechanism of heat transfer.Fourier’s law for heat conduction is

@Q

@t ¼ Àk

I S rT dS (14)

It establishes a relationship between the time rate of heat Q

transferred through a material and the negative gradient in the

temperature (5T ) and the area S through whichthe heat is flowing.

The thermal conductivity k indicates the capability of a material to

conduct heat. For metals, thermal conductivity essentially follows

electrical conductivity according to the Wiedemann–Franz law.

Heat transfer through thermal radiation depends on the

electromagnetic radiation emitted from the surface, which is

due to the temperature of the object. The power output W from a

surface of area A at a temperature T is given by the Stefan–

Boltzmann law:

W ¼ eðT Þs AT 4 (15)

where s is the Stefan–Boltzmann constant and e(T ) is a correction

factor taking into account the emissivity of real surfaces which are

not black bodies.

Convective heat transfer implies thetransfer of mass that canbe

natural or forced. The propensity to natural convection is

represented by the Grashof number (Gr ), which is a ratio between

buoyancy force and viscous force. Forced convection is character-

izedby the Peclet number (Pe). Convective heat transfer represents

a very active research field where microstructured surfaces have a

fundamental importance.

2.2. Biological properties

Biological properties of surfaces have been recently extensively

reviewed in the CIRP 2007 keynote paper by Ramsden et al. [190].

2.3. Technological properties

Mechanical, hydrodynamic and tribological properties are

technological properties that have a relevant impact on engineer-

ing.

2.3.1. Mechanical properties

Mechanical properties of surfaces involve the effect of surface

characteristics on the resistance of the part delimited by the

surface. The concept of resistance includes the stress–strain

resistance, the aero and hydrodynamic resistance, and the effects

of interactions between surfaces belonging to different parts or

media.Contact mechanics considers the load supported by the

asperities and the real contact area. These aspects are of

fundamental importance for sliding surfaces (in tribology) as well

as for surfaces that statically transmit load.

In many engineering applications, the contact between two

surfaces is non-conforming, i.e., thecontactarea is very small when

comparedwith the geometry of thebodies in contact.Also,even for

conforming contacts, the contact between the asperities that

compose the surface topography is known to be non-conforming.

When two non-conforming bodies are brought into contact,

they initially touch at a single point or along a line. Under the

action of the highest load, they deform in the vicinity of the initial

contact points, so that the contact area grows to a finite value,

though small when compared to the dimensions of the bodies.Historically, the first model for contact mechanics was

proposed by Hertz in 1882. Hertz theory predicts the shape of

this area of contact andhow it grows in size with increasing load, as

well as the magnitude and the distribution of surface stresses,

Fig. 5. Reflecting grating.




normal and possibly tangential, transmitted across the interface.

Hertz theory was based on some key assumptions: (i) the surfaces

are continuous and non-conforming; (ii) the strains are very small,

which guarantees that the contact area (a) is much smaller than

the radii of curvature of the bodies (R) (where a ( R); (iii) the

contact is fully elastic; (iv) the surfaces are frictionless.

Table 2 summarises the contact widths and contact pressures

for circular (sphere-on-sphere, sphere-on-plane, crossed cylin-

ders) and line (cylinder-on-cylinder and cylinder-on-plane)

contacts as predicted by Hertz theory.

To overcome the elastic limit of the Hertz model, plasticity

indices (c) have been introduced to quantify the severity of the

contact:

c ¼ 2E Ã

H

s sR

1=2

(16)

where H is the hardness of the softer material and s s is the height

distribution.

The Hertz model neglects the effects of the surface roughness.

These effects were addressed by the work of Greenwood and

Williamson [96] that analysed a nominally flat surface covered

with a large number of asperities with spherical ends in contact

with a rigid plane.

Several models have been successively proposed. Chang et al.

[37] described an elasto-plastic model based on the conservation

of an asperity control volume during plastic deformation. Zhao

etal. [250] developeda model thatincorporates the transition fromelastic deformation to fully plastic flow and Aramaki et al. [7]

proposed a contact model expressly dedicated to surfaces with

purely longitudinal roughness. In this model, the surface profile is

approximated by parabolas located on a reference plane whose

positions change by varying the load.

Pugliese et al. [187] reported an experimental assessment of

different contact mechanics models assuming roughness profiles

described by simple parabolic functions. Starting from the profiles

of five engineering surfaces with different roughness character-

istics the study determines the amplitudes of the contact zone and

the load as a functionof theinterference of each profile with a rigid

smooth flat surface.

A contact model considering the properties of engineering sur-

faces was developed and implemented using finite element analysisby Sellgren et al. [200]. The study shows that the height distribution

of the topography has a significant influence on the contact stiffness

but that the curvature of the roughness is of minor importance.

Fatigue is another important phenomenon for engineering

applications. The mechanisms that influence fatigue crack growth,

such as the stress ratio, environment and other parameters have

been widely studied since the first article of Albert [4] in 1838. The

model for the stage II crack growth, according to Paris, widely used

since the 1960s, establishes the relationships between the crack

length ( 2a), the number of cycles N , and the cyclical componentDK

of the stress intensity factor K :

@a

@N ¼ C ðDK Þm

(17)

where m has values between 3 and 5.

In the 1970s, the introduction of the crack closure concept by

Elber [74] depicted that the mechanisms and importance of crack

‘non-closure’ slowing the growth of a fatigue crack is due to the

wedging effect of plastic deformation left behind the tip of the

crack. The reduction of DK thresholds with load determines crack

closure influenced essentially by plasticity, oxide or crack surface

roughness.

The quality of the surface, surface micro-geometry, defects,

etc., determine stress concentrations or supply crack nucleation

sites that lower the fatigue life. Notably surface stress can be

intentionally controlled to increase fatigue life. Surface engineer-

ing treatments like shot peening and ballizing are industriallyused to cause compressive stress in the surface and therefore

improve fatigue life by preventing surface crack formation. The

same effect can also be obtained by utilizing modern day

techniquessuch as laser peening and ultrasonic impact treatment

[103,166].

However, it must be remarked that although surface micro-

geometry, hardness and residual stress are recognized to have a

primary role in fatigue behaviour [244,245], no general model

exists to describe the effects of surface preparation [84,139].

Hardness is the mechanical characteristic of a solid that

quantifies the material’s resistance to plastic deformation. Hard-

ness depends on atomic, intermolecular and lattice bonds: it is

measured through indentation tests performed on a macroscopic

or microscopic scale.

Hardness cannot be considered a fundamental material

property, as it is dependent upon the specific procedure (standard)

used for the test. Consequently, hardness is a measurement that

can be used to compare materials properties, provided that the

same standard procedure is adopted.

Considering small scales, typical of engineered surfaces and of

emerging materials, like thin films, conventional uniaxial tensile

indentation tests give no reliable results. With scale reduction, the

postulations of continuous, homogeneous and isotropic materials

no longer hold. The consequences are of paramount importance on

many micro-engineering technologies [5].

For brittle materials like monocrystalline Si and LiNbO3, brittle

to ductile transition in micromachining and microindentation can

be observed. Shimada et al. [202] reported that microindentation

testing can evaluate the intrinsic ductility and critical scale of machining for ductile mode machining.

Microindentation tests are quasi-non-destructive techniques

that can be used for evaluating mechanical properties such as

hardness and Young’s modulus in a nanometer range [26], for

modelling abrasive wear [53] or for investigating phenomena such

as sub-surface damage during grinding [220].

Vickers and the Knoop tests are the best known microindenta-

tion tests. The ISO14577 standard [121] establishes the procedures

for instrumented indentation tests. Part 1 of the standard

(ISO14577-1) indicates three ranges: macro (test forces from

2 N to 30 kN), micro (test force less than 2 N; indentation depth

greater than 0.2mm) and nano (indentation depth equal to or less

than 0.2mm). Part 4 of the ISO standard (ISO14577-4) specifies a

method for testing coatings that is particularly suitable for testingin the nano/micro range applicable to thin coatings.

A recent assessment on nanoindentation was reported by

Hermann [104]. The influence of indenter tip geometries and

calibration procedures on the measurement of a coating was

analysed by Bouzakis and Michailidis [24].

2.3.2. Hydrodynamic properties

Hydrodynamic properties have a great influence on the

applications of structured surfaces.

At small scales (channel diameters of around 100 nm to several

hundred micrometers), surface forces prevail in relation to mass

forces, and factors such as surface tension, energy dissipation, and

fluidic resistance govern the system.

Hence thefluidflow at the micro-scale acts differentlyfrom thatin the macroscopic scale. Complete reviews on fluid mechanics in

micro/nano-scale are reported in [88,210,217].

At small scales, the Reynolds number (Re) assumes values at

least one order of magnitude smaller than unity. Re is defined as

Table 2

Stress and deformation formulae for normal contact of non-conforming elastic

bodies according to Hertz; E* is the effective elastic modulus.

Parameter Circular contact

(diam. = 2a, load = W )

Line contact (width = 2a,

load = W 0/unit length)

Semi-contact radius

or width

3WR

4E Ã

1=3 W 0R4E Ã

1=2

Contact pressure6WE Ã2

p3

R

2 !1=2

W 0E Ã

pR

1=2




the ratio between inertial forces and viscous forces:

Re ¼ rvsL

m(18)

where r is the density of the fluid in kg/m3, vs is the mean fluid

velocity in m/s, m is the absolute dynamic fluid viscosity in N s/m2

and L is the characteristic length in m, of the system. Typical values

of Reynolds number range from $1 Â 10À2 (spermatozoa) to

$5 Â 10

9

(large ship).With the reduction of scale and of the Reynolds number, the

flow patterns that come from the Navier–Stokes equations also

change [112]. Essentially these equations, originating from the

conservation of mass, momentum, and energy, simply equal the

changes of momentum in infinitesimal fluid volumes to the sum of

dissipative viscous forces, changes in pressure, gravity, and other

forces acting inside the fluid.

For incompressible flow of Newtonian fluids, equations in

vector form are

r@n

@t þ nrn

¼ Àr pþmr2nþ f (19)

where the term on the left, representing inertia that depends on

unsteady acceleration (@n

/@

t ) and convective acceleration (nÁ5n

),

equals the term on the right given by the sum of pressure gradient

(À5 p), viscosity (m52n) and the external forces f .

These scaling effects are very important in microfluidics, a

discipline embracing engineering, physics, chemistry, microtech-

nology and biotechnology. Microfluidics concerns the study of the

behaviour and techniques to control and use microliter and

nanoliter volumes. It dates back to the 1980s, specifically to the

development of the first inkjet printheads. Nowadays, thanks to its

multidisciplinary technology, microfluidics allows for the produc-

tion of DNA chips, micro-propulsion, micro-thermal technologies,

and lab-on-a-chip devices.

Surface structure and characteristics are fundamental for the

progress of microfluidics applications. In microfluidics, inertial

forces play an irrelevant role, particularly when dimensions

reduce.

Considering small scales, typical of microfluidics, the con-

vective acceleration vanishes. Without this non-linear convection,

the flow patterns basically become laminar and deterministic. The

velocity profiles are parabolicin pressure-driven flows,plug-like in

electro-osmotic flows, or a superposition of both.

Microfluidics applications exploit the laminar flow, e.g.,

genomic material can be transported easily without shearing in

a lab-on-a-chip device.

With laminar flow, when two fluids join, they do not mix

rapidly by turbulence, but slowly by diffusion, obeying Fick’s laws.

Mixing in the micro-scale devices depends on the molecular

diffusion and could be unacceptably slow. The rate of advection of

a flow to its rate of diffusion is captured by the dimensionless

Peclet number (Pe):

Pe ¼ Lv

D(20)

where L is the characteristic length, v is the velocity and D is the

mass diffusivity.

Peclet number gives an indication of the relative importance of

diffusion and convection. In order to enhance mixing in the

microfluidic devices, passive and active techniques have been

proposed [173].

Passive mixing needs no external forces and is performed

through transverse flow induced by special geometries (grooves,

rivets, or posts) set in microchannels, or local circulations induced

by heterogeneous surface pattern in electrokinetic flows.

Active mixing requires external fields such as acoustics,pressure, temperature, or electrohydrodynamics for the flow

disturbance.

Furthermore, reduction of dimensions enhances capillary

action and changes the way in which fluids flow through micro-

scale channels. In addition to capillarity, governed by surface

energy, other phenomena, such as viscoelasticity and electro-

kinetic effects, complicate flow patterns, since their non-linearity

increases at small scales. Besides the Reynolds number Re and the

Peclet number Pe, additional variables should be considered when

facing microfluidics, such as the capillary number Ca, expressing

the importance of interfacial tension, the Deborah, Weissenberg,

and elasticity numbers De, Wi, and El, describing elastic effects due

to deformable microstructural elements, the Grashof and Rayleighnumbers Gr and Ra, describing density-driven flows and the

Knudsen number Kn, describing the importance of noncontinuum

molecular effects [210].

The phenomena become more complex when multiphase flow

is considered.Cubaud et al. [51] reported an experimental study on

two-phase flow in microchannels with hydrophobic and hydro-

philic surfaces. Kuo and Peles [137] investigated flow boiling in

200 mm Â 253 mm parallel microchannels with structured reen-

trant cavities, demonstrating that structured surfaces promote

nucleation of bubbles with a better uniformity and enhance the

critical heat flux when compared with plain wall microchannels.

A first classification of microfluidics technologies concerns

continuous or digital (droplet-based) flow.

Continuous-flow technologies rely on the manipulation of

continuous liquid flow through microfabricated channels. Fluid

flow is obtained through external pressure sources or internal

devices such as integrated mechanical micropumps, or through

electrokinetic systems.

Advantages of continuous-flow microfluidics technologies are

the simple implementation and the low sensitivity to protein

fouling problems. The main drawbacks are the difficulties of

integrating and scaling closed channel processes and the low

flexibility and reconfigurability.

Digital microfluidics is based on the manipulation of discrete,

independently controllable droplets on an open substrate. By these

technologies, microfluidics functions can be implemented by

arranging basicoperations, moving the single droplet over one unit

of distance.

Analogous to integrated circuits in microelectronics, digitalmicrofluidic technologies have a great potential in the large-scale

automation of chemistry and biology; they provide a greater

flexibility, since the digitization permits the adoption of a

hierarchical and cell-based approach for microfluidic chip design.

Furthermore, the single droplet control endorses dynamic

reconfigurability, i.e., the implemented function can be changed

during operations.

2.3.3. Tribological properties

Tribologyis defined as the science and technology of interacting

surfaces in relative motionand embraces the study of friction, wear

and lubrication [115]. Today, around 30% of the world energy

consumption is used to overcome friction [238]. In most

mechanical components, wear is responsible for a large numberof failures. Although in some cases the worn component can be

replaced before failure, the economic costs associated with wear

surpass 5% of the GDP of developed countries [127,128].

Both friction and wear are surface phenomena that need to be

properly investigated to reduce such costs. However, due to its

surface/interface nature, tribological phenomena are very com-

plex, generally irreversible, evolve with time and therefore are

highly systemic.

A recent trend is the search for a multi-scale approach to study

tribology [31,52,156], so that the understanding of the energy

dissipative mechanisms at the interface between solids at atomic

and molecular levels, using both experimental techniques, such as

AFM [21,105,159], nanoindentation [104], FIB-HRTEM [68,76] and

theory [109], mainly using molecular dynamics [157,182], couldhelp to explain macro-tribological aspects.

The texturing of surfaces as a means of improving lubrication

and reducing friction and wear between moving parts has been

largely proposed in the literature and started to be systematically




investigated in the last two decades. For example, the potential of

the texturing of surfaces with patterns containing plateaux and

valleys to improve the tribological performance and shorten the

running-in period of components has been the focus in the

production of engine components. This section will discuss the

main theoretical aspects related to tribology and the latest

achievements in this area towards a more fundamental under-

standing of friction and wear. It will include surface characteriza-

tion, friction, wear and lubrication. Eachsub-section reports brieflythe main models.

2.3.4. Surface characterization

All solid surfaces present some surface topography when

regarded at a sufficiently small scale. The real contact between

sliding surfaces occurs at the summits of the higher asperities,

where friction and wear occur. Techniques to analyse this surface

topography include light or electron microscopy [186], stylus

profilometry [133,135], optical interferometry [40], tunnelling

microscopy [93], AFM [135,147] or indirect measurements such as

electrical [60] or thermal [65] measurements. The quantificationof

surface topography makes use of roughness, waviness and form

error parameters, which have been intensively discussed and used

in engineering [14,89,99].

2.3.5. Contact mechanics for sliding surfaces

In order to understand the tribology of textured surfaces, it is

important to first define some concepts related to the contact

mechanics of two sliding surfaces. In this section, the basic

theoretical concepts related to contact mechanics in non-

conforming contacts for tribology are summarised.

In tribology, the contact pressure between the asperities is

generally very high and therefore plastic in many cases. For

example, if the indenter (or asperity) is spherical, indentation

hardness testing theory predicts that plastic flow first occurs at a

depth of about 0.47a at a mean contact pressure of 1.1Y (uniaxial

yield stress). With the increase of the normal load, plastic

deformation extends from beneath the contact until it eventually

reaches the surface. At this point the mean contact pressureachieves approximately 3Y and does not increase further. Such

findings lead to the assumption that for any shape of asperity

pressed against an opposing surface, the mean contact pressure

will always be around three times the unixial yield stress (Y ) of the

softer materials and the area of contact will be directly

proportional to the load [115].

In real surfaces, statistical theories of multiple asperity contact

take into consideration the fact that surface irregularities are

statistically distributed and analyse the probability that one

asperity will touch an opposing surface [96].

2.3.6. Lubrication of sliding surfaces

The presence of a lubricant film between two sliding surfaces

reduces friction and wear. If the surfaces are sliding against eachother with a certain velocity and a lubricant fluid is present in

the contact, a hydrodynamic film may form spontaneously

between them. This condition is known as hydrodynamic or full-

film lubrication. However, depending on the operating conditions,

the formation of a thick fluid film between the surfaces may not

occur and the lubrication will need to rely on other mechanisms.

When the surfaces possess a micro-topography, it can

considerably change the pressure distribution, the hydrodynamic

action of the fluid and the deformation of the surfaces, leading to

differences in the lubrication conditions of the surfaces. These

differences can be positive or negative in terms of the tribological

performance, depending on the characteristics of the topography

[197].

For situations involving full-film lubrication, the effect textur-ing has on increasing the hydrodynamic pressure between the two

sliding surfaces has been analysed both via numerical simulation

and experimental investigation. The first evidences of the effect of

irregularities on the hydrodynamic lubrication was found by

Hamilton et al. [102]. This was based on visual observation of non-

continuous lubrication films in optically flat, transparent seals. The

film was interrupted by numerous long, narrow cavitation

streamers, which appeared to originate and terminate on locations

corresponding to irregularities on the stator surface. Based on

these observations, the authors produced a simple analytical

model to predict the effect of the geometry of micro-irregularities

on the load support of face seals by hydrodynamic lubrication.

They used the Reynolds equation reduced to the two-dimensionalform of the Laplace equation to describe the flow.

In order to understand the possible effect of a micro-texture in

increasing the hydrodynamic pressure between two sliding

surfaces, it is necessary to examine the hydrodynamic effect that

occurs dueto thepresence of a geometrical wedge. The variationof

lubricant pressure in a bearing is described by the Reynolds

equation, which can be applied to model the hydrodynamic action

on the basis of a convergent clearance space through the length of

the bearing. It allows the pressuredistribution andload capacity to

be determined [212,238].

For textured surfaces, the micro-topographies that have offered

the best tribological results consist of small cavities uniformly

distributed over the sliding surface. Considering the micro-

converging wedges generated by each one of these pockets,

textured surfaces composed of a plurality of pockets could be

regarded as a set of micro-bearings [47].

In non-conforming contacts, such as gear teeth,piston rings and

cam followers, the pressure is very high. If hydrodynamic

equations are applied to suchcontacts, the predictedfilm thickness

is very small and would not prevent metallic contact of the surface

asperities. However, in practice these systems operate quite

satisfactorily. This indicates that thicker films must be present. The

mechanisms responsible for this are the variation of the viscosity

with high pressures and, to a large extent, the elastic deformation

of the bearing surfaces [101]. This condition is called elastohy-

drodynamic lubrication (EHL).

It has been established recently that some engineering

components can have roughness features that are of the same

order as or even significantly larger than the predicted thickness of the EHL film. Consequently, they operate under conditions

described as partial, mixed or micro-EHL [75].

Studies of EHL and micro-EHL of surfaces containing dents and

grooves have also been intensively conducted [130,178,209].

Although the main objective of these studies is to determine

stresses around surface defects that compose the surface

topography of engineering components, in order to understand

surface-originated fatigue failures, they show that the surface

topography itself is intimately involved in the lubrication process.

These studies have been mostly carried out by optical inter-

ferometry of artificially produced surface features. Obviously, they

can also help to understand the effect of a well-defined surface

texture on EHL and micro-EHL.

Undervery high contact pressures, or at very lowsliding speeds,hydrodynamic forces are insufficient to maintain even a thin EHL

film and direct contact will occur between the asperities. In these

cases, the surface must be protected by a suitable boundary

lubricant and this regime is known as boundary lubrication [115].

2.3.7. Friction of textured surfaces

Frictionis definedas theresistanceencounteredby onebody in

moving over another. It occurs as a result of the interactionsin the

real contact area of solids, i.e., around the contacting asperities.

Friction depends on both the materials in contact and the

tribological system; therefore it is not an intrinsic property of

the contacting pair.

The texturing of surfaces, especially in meso-scale or macro-

scale, is traditionally used to increase the friction of parts[120,183]. Coefficients of friction as high as 1.6 were measured

for the sliding of specially designed textures [183].

Friction between sliding surfaces can also be reduced by

texturing in a micrometric scale. The two main reasons ascribed to




this reduction of friction are entrapping of wear debris inside the

pattern features [16,72,131,184,197,225,230], and improvement

of lubrication [22,48,77,92,110,194,196,231–233].

2.3.8. Wear of textured surfaces

Wear is themaincausefor materialswasteand efficiency loss in

mechanical components.

In many situations, the wear between two sliding surfaces can

be reduced by surface texturing. Various authors have studied thewear of textured surfaces [86,123,184,213]. These studies include

mainly experimental evaluation of textured surfaces.

Three main effects seem to contribute to reduce the wear of

textured surfaces. The first is the entrapment of wear debris inside

the pockets that compose the texture, avoiding their presence

between the two surfaces, which could cause abrasive wear. The

second is the fact that very smooth lubricated surfaces for both

moving and stationary contacting parts usually show poor

behaviour because they may not have sufficient oil-holding

capacity to ensure lubrication at the contact. The existence of

pockets of lubricant could be particularly useful as a secondary

source of lubricant, especially in conditions involving high plastic

deformation. The third is the enhancement of a hydrodynamic

pressure between the surfaces due to the converging wedges

constituted by the pockets.

3. Applications

A great number of applications are based on engineered

surfaces. Many industrial applications for engineered surfaces are

described or suggested in literature and patent databases. The

most interesting emerging applications for industry and for the

research community are reported. The list, which is not exhaustive,

includes applications and advances in the fields of energy,

microfluidics, optics, bioengineering, cleaning, and manufacturing.

3.1. Energy

Interesting applications related to energy transfer have beendeveloped in different industrial sectors, such as aerospace, power

plants and electronics.

The role played by structured surfaces in heat transfer

phenomena, based particularly on spray and boiling mechanisms,

has been investigated by many authors. Microstructured surfaces

increase the coolant-solid contact area. However, this aspect alone

does not explain the observed enhancement in heat transfer rate.

A fundamental design parameter in advanced applications of

heat transfer is thecritical heat flux (CHF) that canbe attained;it is

defined as the heat supplied to the heater that just balances the

liquid heat removal capability. For spray cooling, CHF is usually

higher than for boiling, since the vapor produced at the surface by

phase change can be much more easily removed from the surface.

3.1.1. Spray cooling

Spray cooling is very efficient for dissipating high heat fluxes

with low coolant mass fluxes at low and high wall superheats

[132]. Applications of this technology include the Cray X-1

computer cooling system and the Space Shuttle’s open loop flash

evaporator system (FES), where the cyclic water spray of surfaces

enhanced with triangular grooves cools Freon-based heat exchan-

gers [171]. Besides cooling applications used in supercomputing

and space explorations for on-board flight system components,

spray cooling has been used in laser-diode arrays, multi-chip

modules, metal quenching and medical treatments.

Spray cooling is a multiphase convective process that yields

high heat fluxes, greater than 100 W/cm2 using Fluorinerts, and

over 1000 W/cm2

with water. It permits precise temperaturecontrol (Æ2 8C) at low coolant fluid flow rates.

Spray cooling heat transfer can operate with three different

regimes. At low temperatures, the liquid evaporates at the free

liquid–vapor interface. At high temperatures, vapor bubbles are

produced in the liquid films at the superheated hot wall. The

transition from the low temperature to the high temperature

region characterizes the third regime.

At low wall temperatures, heat transfer is controlled by single-

phase convection. At higher wall temperatures, bubble nucleation

appears, providing considerable agitation; phase change phenom-

ena prevail andenhance heat flux. Heat transfer mechanisms in the

two-phase regime are possibly caused by: (i) thin liquid film

evaporation on the surface; (ii) secondary nucleation due to gaslayer of striking droplets; (iii) extension of the three-phase contact

line where solid, liquid and vapor phases coexist.

Several factors influence spray heat transfer. Volumetric spray

flux is preeminent on other hydrodynamicparameters of the spray.

Other factors are the ejected fluid temperature, chamber environ-

mental conditions and surface roughness.

Preliminary studies on the surface influence indicated that the

heat transfer coefficient increases with the use of smooth surfaces

(Ra < 0.1mm) for gas atomized sprays, while the opposite was

observed for liquid atomized sprays [199].

Recently, Stodke and Stephan [215] compared heat transfer

with water spray cooling of a polished plain surface with

Ra < 0.3mm to three different surfaces structured with micro-

pyramids with dimensions much larger than the roughness

studied by Sehmbey et al. Pyramids with 75, 150 and 225mm

heights and corresponding width of 150, 300 and 450 mm were

manufactured onto a 20 mm diameter copper cylinder (Fig. 6).

These structures caused an increase in surface area compared to

the smooth surface by a factor of ffiffiffi

2p

.

Infrared camera analysis of the plain surface suggested a

correlationbetween the heat flux andthe lengthof thethree phase

(solid, liquid and vapor) line. This observation explains the

improved cooling performance of pyramidal surfaces compared

with smooth surfaces at the same wall superheat. An increase in

heatflux witha factorfrom 2 to5 for surface superheat inthe range

2–3 K was experienced with the larger micro-pyramidal struc-

tures. The smaller pyramidal structures showed the steepest

temperature gradient, suggesting this geometry for the applica-

tions that require a constantwall temperature over a wide range of heat fluxes.

Other studies on the effect of structured surfaces on spray

cooling were recently carried out.

Amon et al. [6] analysed three different surface textures

obtained on silicon surface by using deep reactive ion etching

(DRIE). The examined geometries are square studs with height of

500mm, different sizes (120 and 480mm) and different channel

widths (120 and 360 mm). These geometries are distinguished by

different heat transfer surface areas and capillary forces of the

liquid between the studs.

Water spray cooling experiments revealed four regimes

characterized by different heat transfer mechanisms correspond-

ing to increasing wall temperatures: (i) a flooded regime, governed

by forced convection of the impinging droplet; (ii) a thin filmregime, controlled by the evaporation of the thin liquidfilm within

the channels; (iii) a partial dryout regime, analogous to the

transition-boiling regime in pool boiling; (iv) a dryout regime,

controlled by the volume of the liquid droplets impinging the

Fig. 6. Geometry of the structured surface.




surface. The structured surface with finerstuds provides the higher

heat flux and the most uniform surface temperature.

Analogous results were found by Hsieh and Yao [111] for silicon

and aluminum surfaces with the same square stud geometries.

They report that the Bond number (Bo) can characterize the

capillary forces between the micro-studs and, therefore, wett-

ability. Microstructured surfaces improve the spreading of the

impinging droplets, the formation of a thinner liquid film and

permit the full liquid film wetting in order to achieve higher heatfluxes. Bond number compares the gravitational force and the

surface tension force:

Bo ¼ G ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffig =ðrl À rvÞ g

p (21)

where G is the groove width (m), g is the surface tension (N/m), rland rv are the liquid and vapor densities (kg/m3) and g is the

gravitational constant (N/s2).

Smaller Bo values characterize microstructures with narrower

grooves and higher thermal flux. The effect of capillary forces

disappears when grooves are too small for allowing good liquid

penetration and flow. Structured surfaces give important improve-

ments in the thin film and in the partial dryout regime. In the

flooded and dryout regime their contribution is insignificant.Also, Silk et al. [203] studied the impact of structured surfaces

on spraycooling and demonstrated that enhanced surfaces provide

larger heat transfer relative to smooth surfaces. They examined

three geometries structured with cubic pin fins, pyramids and

straight fins (Fig. 7) made of oxygen-free high conductivity copper

and used Fluorinert PF-5060 as the working fluid.

Both in the single-phase and multiphase regime, the experi-

mental results indicated that the straight fin geometry had the

highest heat transfer performance (reaching a CHF value of 126 W/

cm2), followed by the cubic pin finned and the pyramid surfaces.

Noticeably the heat transfer is not directly proportional to the

wetted surface area in any of the regimes suggesting that the

differences could be ascribed to the liquid management on the

heater surface, a more efficient use of the wetted area, or their

combination.

Further investigation on spray cooling of straight finned copper

surfaces was carried out by Coursey et al. [50] using FC-72 as the

working fluid. Fins were 500mm wide with a pitch of 860 mm and

heights of 0.25–5 mm. The observed data indicated a significantly

higher heat flux compared to the flat surface. All the authors

remark the complexity of spray cooling and recognize that the

mechanisms are not yet fully understood and deserve further

research.

3.1.2. Boiling

Boiling heat transfer plays an important role in many industrial

and domestic applications where vaporizers are used to transmit

thermal power. Boiling consists of the creation of vapor bubbles at

the heating surface, specifically at nucleation sites. Nucleation canbe homogeneous, when occurring within the liquid, or hetero-

geneous, when taking place at the liquid–solid interface. Due to its

industrial relevance, heterogeneous nucleation has been studied

by many researchers in the past 50 years.

In spite of these efforts, nucleate boiling is one of the least

known subjects of thermal engineering due to the large number of

interlinked complex processes. Many factors influence nucleation:

topography of the surface, properties of the solid and liquid and

operating parameters such as pressure and temperature.

Nucleation site density is of paramount importance for reliable

boiling models wanting to predict the relationships between

boiling, wall superheat, wettability and surface structures. In order

to enhance the rate of heat transfer many techniques for artificialnucleation sites of appropriate design have been proposed and

investigated [56,57].

An outstanding survey of the evolution of the enhanced boiling

surfaces is reported by Webb [235]. Research regarding the effect

of roughness and structured surfaces on nucleate boiling dates

back to 1931, when Jakob and Fritz studied the performance of

sandblasted and structured surfaces, as later reported by Jakob

[122]. When compared with a smooth surface, the sandblasted

surface provided an initial improvement up to 15%. The surface

structured with a square grid geometry (0.016 mm square with

0.48 mm spacing) increased its boiling coefficients by a factor of

three. However, these results vanished withina fewdays due to the

degassing process of the cavities that eventually didnot stably trap

vapor. Scientific interest renewed in the 1950s [18,45,138]

establishing that boiling enhancement depends on the site density

of stable nucleation sites and suggested investigations to multiply

artificial nucleation sites.

Griffith and Wallis [97] first studied the effects of artificial

nucleation sites by boiling water over a paraffin-coated horizontal

surface with 37 evenly spaced artificial cavities. They proposed the

first fundamental model of artificial nucleation outlining the

importance of reentrant cavity geometry. The cavity mouth

diameter settles the superheat needed to start boiling and its

form controls the process stability once boiling initiates.

Reentrant cavities also permit the vapor nucleus to exist in the

presence of subcooled liquid, leading to a very stable vapor trap.

During nucleate boiling, the contact angle controls the stability of

the bubble within the cavity. For contact angles between the half-

conical angle of the cavity and 908, the superheat required toactivate nucleation is governed by the cavity mouth radius.

In the mid-1960s, commercial applications of tubes with

engineered surfaces for enhancing boiling appeared. The employed

technologies were porous coating and cold metal re-working of the

tube to obtain nucleation cavities. Milton [165] patented a tube

characterized by sintered copper coating with a thickness of

0.25 mm (HIGH-FLUXTM tube).

A cold worked metal pipe was patented by Kun and Czikk [136]

after manufacturing a flat aluminum plate with closely pitched

parallel grooves of 0.13 mm pitch and 0.25 mm deep. These

grooves are deformed perpendicularly to their direction in order to

obtain sub-surface cavities with restricted openings at the top.

Since the 1970s, tubular geometries with structured outer

boiling surface are widely used and presently all water chillersemploy such components.

Nakayama et al. [169] proposed the first dynamic model

describing the influence of pore and tunnel geometries on boiling

heat transfer. The model supposes a suction–evaporation mode

where the total heatflux isthe sum ofa latentheat generated inthe

tunnel (cavity) and a single-phase external convection heat flux.

Hwang and Moran [116] studied cavities of 3–15 mm mouth

diameters drilled by laser over a vertical silicon test surface. By

boiling FC-86 over the surface for a large degree of superheat, they

established that a higher heat transfer performance in the nucleate

boiling region and a higher CHF can be obtained using artificial

nucleation sites.

This result was also experienced by Miller et al. [164] in their

study on a vertical silicon chip with hexagonal dimples (9.4mm indiameter and 3.3mm deep) immersed in FC-72. Later on, Phadke

et al. [185] compared boiling heat transfer from vertical silicon

chips with a smooth surface and four types of treated surfaces

with an array of reentrant cavities (mouth diameter equal to 0.23–

Fig. 7. Surface structures examined by Silk et al. [203] reprinted from International

Journal of Heat and Mass Transfer, Vol 49/25 –26, Silk, E.A., Kim, J., Kiger, K., Spray

cooling of enhaced surfaces: impact of structured surface geometry and spray axis

inclination, Pages No. 4910–4920, Copyright 2006, with permission from Elsevier.




0.49 mm) immersed in R-113. Data concerning saturated boiling

indicated that all the treated surfaces decrease the required

superheat considerably when compared with a smooth surface.

In 1998, Chien and Webb [39] demonstrated, using flow

visualization, the correctness of the Nakayama et al. suction and

evaporation model for saturated boiling at typical operational heat

fluxes.

The models of Nakayamaet al. and Chein and Webb did not face

liquid refill stage. Recently Das et al. [56] developed a mechanistic

model of liquid intake whose predictions shows a good agreement

with the experimental data reported in the literature (Fig. 8).

Heat transfer enhancement rate depends both on nucleation

site density and the activation of cavity. Liang and Yang [148]

determined that the geometry of the cavity and wetting property

of a liquid controls the activation of the site.

By using micro-electronic fabrication technologies, Kubo et al.

[134] manufactured inline arrangements of micro-reentrant

cavities with mouth diameter of 1.6–3.1mm and observed a

higher heat transfer performance for these surfaces than plain

surfaces also in the high heat flux region.

Experimental investigations of the role of surface structure in

heterogeneous boiling were carried out by Qi et al. [188].

Specifically the measurements of nucleation site densities on

stainless steel and brass surfaces polished with the same siliconsandpaper (400 mesh size) indicated a larger number of sites on

the brass surfaces. The authors suggested that the difference could

depend on the smaller cavity sizes of the stainless steel or on the

larger population of vapor trapping cavities of brass. According to

this study only the deep cavities can trap vapor in contrast to the

conclusion of Messina and Park [162] that shallow pits are more

efficient for nucleation site formation.

Nimkar et al. [175] studied the effect of nucleation site spacing

on pool boiling characteristics. They considered a vertical silicon

surface structured with a grid of pyramidal reentrant cavities

having a 240 mm square base and a 40 mm square mouth (Fig. 9);

the test was performed on grids with pitches of 0.5, 0.75 and

1.0 mm. Boiling tests were carried out with single-phase immer-

sion cooling in saturated FC-72 at atmospheric pressure. Thecomparison of the boiling curves for the structured surfaces with

the boiling curve for the plain surface showed that a surface with

0.5 mm pitch and the highest nucleation density had a worse

performance than the plain surface. A surface with 0.75 mm

spacing had the highest heat transfer and the highest active site

density. The experiments indicated an optimum inter-cavity

distance for which the largest part of the surface is active over a

wide range of heat fluxes. No effect of cavity spacing was observed

on the departure diameter of bubbles and frequency.

Recently Das et al. [57] examined horizontal copper surfaces

with regular arrays of discrete nucleation sites manufactured bymicro-drilling. The holes with diameters of 600 mm and depth to

2 mm,were placedon a squaregrid with pitches of 10 mm,7.5 mm

and 5 mm.Boilingin distilledwater indicateda 100% improvement

in the heat flux for the 10 mm pitch with respect to the smooth

surface. Reducing the grid pitch increased the number of the

nucleation sites but the gain on the heat flux reduced fast due to

the interactionsbetween the nucleation sitesthat behaved as small

pumps.

3.2. Microfluidics

Key application areas of microfluidics range from biology to

energy. The first studies date back to the 1960s when Furmidge

[87] analysed the effects of the surface properties of the spray

liquid/solid combination on the retention of pesticide sprays by

sliding drops of water and solutions of surfactants on wax and

cellulose acetate surfaces.

Recently many technologies have been studied for implement-

ing digital microfluidics such as electrowetting-on-dielectric

(EWOD) and acoustically induced droplet transport with Surface

Acoustic Waves. Surface fouling due to proteins is a limiting factor

for applying EWOD to biological samples.

Microfluidics applications involving engineered surfaces have

been proposed by many authors. Aggarwal et al. [3] suggested a

method for moving liquid along surfaces by using purely

electrostatic effects where liquid drops are confined to specific

regions of the substrate by a printed pattern that has appropriate

hydrophilic and hydrophobic wetting characteristics (Fig. 10).

By using localized electrostatic fields and changing the appliedfield pattern, a blob of oil kept on a hydrophobic region of the

surface can be split into droplets and moved up to 15 cm/s causing

liquid flow.

Buguin et al. [34] investigated the ratchet-like topological

structures for the control of microdrops. Specifically liquid

microdrops confined in asymmetrically structured geometries

Fig. 8. Evaporation process in the sub-surface tunnel for one bubble cycle: (a)

preparatory, (b) growth, (c) refill, and (d) depressurization phases.

Fig. 11. Scheme of drop actuated by electrowetting on structured surfaces [34]

(with kind permission from Springer Science+Business Media: Applied Physics A,

Ratchet-like topological structures for the control of microdrops, Vol 75/2, 2002,

pages 209, Buguin, A., Talini, L., Silberzan, P, figure 4).

Fig.9. Structuredsiliconsurface with0.75 mm inter-cavity spacing [175] (reprinted

from International Journal of Heat and Mass Transfer, Vol 49/17–18, Nimkar, N.D.,

Bhavnani, S.H., Jaeger, R.C., Effect of nucleation site spacing on the pool boiling

characteristics of a structured surface, Pages No. 2829–2839, Copyright 2006, with

permission from Elsevier).

Fig. 10. Closed loop geometry of hydrophobic regions [3] (reprinted with

permission from Journal of Applied Physics, Vol 99, Aggarwal, J., Kotlicki, A.,

Mossman, M., Whitehead,L., Liquidtransport based on electrostatic deformation of

fluid interfaces, page 104904, Copyright 2006, American Institute of Physics).




were considered and fluctuations in the drop shape and wetting

properties were produced by an on/off electric field acting

transversally to the motion, a low-frequency electric field of zero

mean value along the motion, or by vibrating the substrate. Fig. 11

shows a schematic view of water drop actuation by electrowetting

in a micro-channel with structured surfaces. The width of the

paraffin channel was 1 mm, the periodicity of the surface ranged

between 100 and 200 mm and the amplitude and frequency of the

applied voltages wereup toa few kV and 50 Hz. Fig. 12 shows three

framesof an actuated drop.The drop bridging thesurfacesdeforms

to minimize the electrostatic energy of the capacitor resulting

eventually in the drop displacement with a speed up to 100 mm/s.

Other experimental researches on droplets actuations capable

of leaving no contamination on the channel surface are reported in

the literature. The phenomena utilized are the Marangoni effect

due to a thermal gradient [227], the difference of pressures applied

across a drop on a textured surface [43], the dielectrophoretic

actuation [126], an electrostatic traveling wave generated by

addressing microelectrodes underneath the drop [234], dc and ac

fields [106,107], as well as macroscopic gradients due to diverse

effects: chemical composition of the surface [38], temperature [33]

or light on photosensitive liquids or surfactants [117].

Droplets actuation on super-hydrophobic surfaces (with water

contact angle > 1508) by electrostatic forces was demonstrated by

Torkkeli [226]. Motion and deformation of a water-based magnetic

fluidon a hydrophobic surface were investigated under gravity and

a magnetic field by Asakura et al. [10]. Linke et al. [149] report anexperimental analysis on liquids performing self-propelled motion

when placed in contact with hot surfaces with asymmetric

(ratchet-like) topology.

When the liquid is in the Leidenfrost regime (the film-boiling

regime)droplets are separated from thesurface by a vapor layer and

hover on their ownvapor cushion along a direction perpendicular to

the thermal gradient. Motion is controlled by the asymmetric saw-

tooth-shapedsurface geometry anddropletscan reachvelocity up to

5 cm/s. Liquids with boiling temperatures ranging from À196 to

+151 8C such as refrigerant R134a, nitrogen, acetone, methanol,

ethanol, water and hexadecane were examined. The observed

results show the effect takes place independently from surface

material andwithsaw-toothprofileshavinglength rangingfrom 1 to

3 mmand height ranging from 0.1 to 0.3 mm. The authors concludethat liquid motion is driven by a viscous force exertedvia vaporflow

between the solid and the liquid.

An interesting application of microfluidics, that could be

exploited to manufacture structured and engineered surfaces, is

reported by Jeon et al. [124] where a microfluidic network with

three inlets and one output channel is used to generate a gradient

in topography in SiO2. Dilute hydrofluoric acid solution (5% in

water) fed the middle inlet of the microfluidic network and water

fed the two outer inlets. The microfluidic network provided a

gradient in concentration of HF along the section of the output

channel. As a result the surface was hatched with different depths.

A review of microfluidics applications to the study of the

nervous system was presented by Gross et al. [98].

3.3. Optics

Structured surfaces have a great role in optics. A class of appli-

cations regards the use of optics for surface metrology. Hocken

et al. [108] reported a comprehensive study on the technologies

based on optical phenomena for surface measurement.

Another important area of applications is related to the

advancement of micro-optics which has shifted from refractive,

based on cylinder lenses, to diffractive optical elements in order to

satisfy the miniaturization requirements arising from the com-

munications, computing, consumer electronics, automotive and

aerospace industries.

Consequently, processes for volume and cost effective manu-

facturing of micro-optics component are required.

Diffractive micro-optics are usually planar or flat devices and

are manufactured with lithographic and micro-mechanical meth-

ods like diamond milling, turning or planning [30,59,118,222].

Recentreviews of thestate of theart on precisioncuttingprocesses

for manufacturing micro-optical elements are reported in [28] and

in [193]. Mechanical technologies however cannot manufacture

certain optical microstructures such as V-shaped concave dots.

An innovative production process that overcomes this limit andpermits the manufacture of molds for the replication of prismatic

microstructures has been recently proposed by the researchers’s

group led by Brinksmeier [83]. This technology, diamond micro-

chiseling (DMC), exploits a monocrystalline diamond tool,

properly designed and manufactured, that cuts microcavities with

V-shaped profiles (Fig. 13). The same principle was used to

improve surface quality in a diamond turning process to

manufacture diffractive optical patterns reported by Brinksmeier

et al. [27].

Another technology based on cylindrical coordinate machining

(CCM) to manufacture optical freeform surfaces such as compound

eye lenses has been recently proposed by Fang et al. [81].

Lithographic techniques were reviewed by Reimer et al. [191].

Applications of gray-tone lithography range from anti-counter-feiting [144] to the fabrication of micro-mirror [143] or hexagonal

micro-lens arrays [247].

Besides lithographic and precision cutting processes, other

technologies have been reported for engineering optical surfaces.

Yum et al. [249] described a laser technique for manufacturing

low-cost and high-performance anti-reflection (AR) surfaces with

nano-structured sinusoidal or moth-eye surfaces on polymer films.

Applications of electrowetting phenomenon used to change the

reflectance of water drops was reported in [141]. Specifically by

electrowetting, the shape of the water drop was changed from its

natural ‘‘beaded up’’ state, reflecting approximately 3% of incident

light, to a hemispherical state, reflecting about 43% of the light.

3.4. Bioengineering

The research in nanotechnologies, micro-engineering, and bio-

engineering now allows for the characterization, design, fabrication

and customization of the functional properties of nanoparticles and

Fig. 12. Snapshots of a drop actuated by a 700 V, 50 Hz sine electric field [34] (with

kindpermission fromSpringerScience+Business Media:Applied Physics A, Ratchet-

like topological structures for the control of microdrops, Vol 75/2, 2002, pages 209,

Buguin, A., Talini, L., Silberzan, P, figure 5).

Fig. 13. V-grooveswith decreasing width from 200to 12.5mm [83] (reprintedfrom

Industrial Diamond Review, Vol 67, Flucke, C., Glabe, R., Brinksmeier, E.,

Manufacturing of moulds for the replication of prismatic microstructures by a

novel diamond cutting process, Page 29 Copyright 2007, with permission from

Industrial Diamond Review).




surfaces for a large number of biomedical applications such as

tissues and organ regeneration, bone reconstruction, ceramic

scaffolding and biochips.

The industrial relevance of technologies related to structured

surfaces in the medical field can be checked through a patent

database query. Over 100 patents and applications are reported

using the keywords ‘‘structured surface’’ and ‘‘medical’’ from the

USPTO database.

A detailed description of the applications concerning implantswas reported in the CIRP 2007 keynote by Ramsden et al. [190].

Considering the medical diagnostic field, surface engineering

permits the development of microfluidic applications suchasmTAS

(micro-total analysis system) and LOC (laboratory on chip).

Particularly,biochips seem the most challenging products. They

are the result of an emerging interdisciplinary industry where

surface properties and technologies have a fundamental role:

molecular techniques for enzymatic analysis (e.g., glucose and

lactate assays), DNA analysis (e.g., polymerase chain reaction and

high-throughput sequencing), proteomics, clinical pathology,

especially the immediate point-of-care diagnosis of diseases and

real-time testing of air/water samples are just a few examples of

possible applications.

Early biochips, like the GeneChip1DNA array from Affymetrix

[2], DNA microarray from Infineon AG [119], or NanoChip1

microarray from Nanogen [170], were based on the concept of a

DNA microarray. Analogous to a DNA microarray, the protein

arrays areminiature arrays where a multitude of different capture

agents, e.g.,monoclonal antibodies, are placed on a chip surface to

detect the presence and/or amount of proteins in biological

samples.A disadvantageof DNAand protein arrays is that they are

neither reconfigurable nor scalable after manufacture. Micro-

fluidics chips overcome this drawback and permit the develop-

ment of mTAS that are more versatile and complex than

microarrays [219].

Self-assembled monolayers for biosensors were described by

Wink et al. [239]. The development and application of cell-

based biosensors were discussed by Pancrazio and his group

[95,180,214], Aravanis et al. [8], Baudoin et al. [15]. Recent studieson the strategy for patterned cell adhesion based on chemical

surface modification were presented by Franks et al. [85].

Chemical micropatterning techniques were also discussed by

Falconnet et al. [80]. Yoshino et al. [248] addressed the properties

of engineered surfaces and the manufacturing technologies and

the development of DNA microarray chips.

An architecturefor isolationof axons incorporating tiny grooves

of adequate size and length was developed by Taylor et al. [223].

An analysis of the effects of surface chemistry and topography

on the cell adherence and the interaction between the tissues and

devices was recently reported by Palyvoda et al. [179].

3.5. Cleaning

Methods based on the lotus effect have been proposed to

produce paints, roof tiles, fabrics and other surfaces that can be

made dry and clean. This effect can be attained through a physical/

chemical treatment or by texturing the surfaces.

Chemical treatments can be based on fluorochemicals, silicone,

or a combination of polyethylene glycol with glucose and sucrose.

A technology for manufacturing lotus-like structured surfaces

with the possibility of changing wettability from superhydrophilic

to superhydrophobicwas presented by Wu andShi [240]. A plasma

technique to produce durable hydrophilic surfaces on hydrophobic

polymer materials was investigated by Kuzuya et al. [140].

Water repellent clothing made by Nano-Care1 has been

developed and marketed by Gap and Dockers. Analogously, water

repelling shower glass is already available on the market by Saint-Gobain-Solaglass (SGG Aquacontrol1 Showers).

Sto Lotusan1 is a water repellent paint that is incorrectly

supposed to exploit the lotus effect. It actually contains titanium

dioxide that produces superhydrophilicity so that water, attracted

to the paint, washes debris away in sheets of water rather than

droplets.

Fingerprint-resistant stainless steel (AL Clean1 by ATI Proper-

ties, ProtectaClearTM by Everbrite) exploits an organic coating on

steel that provides resistance to fingerprints and smudges.

3.6. Manufacturing

Structured surfaces can be usedto enhancethe productivity and

efficiency of manufacturing processes.

In polishing and finishing processes the efficiency depends on

the abrasive surface. Standard and aggregate coated abrasive

products have abrasive grains randomly placed on a backing and

this casual geometry produces unreliable grinding results in

terms of abrasive life. By shaping the abrasives into patterned

geometries, each consisting of an erodable structure containing

multiple abrasive particles, heat generation is reduced and belt life

is extended [216,236]. Engineering technologies have also been

used to produce scented abrasives [221] as well as easy

identification of the abrasive belt by indicia [172].

Another interesting study concerns the application of func-

tional surfaces for gripping micro-objects [90]. Fig. 14 shows threetypologies of the investigated surfaces and the gripping config-

uration. For plastic parts a smaller contact area resulted in a better

control of gripping and releasing; for micro-screws the results

showed that reducing the contact area did not provide the same

positive effects.

4. Surface engineering technologies

Methods to modify surface properties have been developed by

many industries: technologies range from sand blasting to

innovative grinding systems that exploit mechanical and electro-

chemical phenomena [176].

This section discusses the modification of surface topography in

relation to the manufacturing methods that improve or exploitsurface functions; it mainly considers microtexturing, i.e., the

methods of modifying surface topography,creating regular patterns

of depressions, or microasperities, on an otherwise flat surface.

The state of the art will be assessed through a critical literature

survey. The main attributes, advantages, disadvantages and

applications of each texturing method are reported. The review

presents the same difficulties since surface texturing is currently

the subject of extensive studies, focusing both on new texturing

methods and on applications of textured surfaces.

Texturing techniques are widely exploited by the semiconduc-

tor industry, which has invented many micro- and nano-scale

manufacturing methods [163,242]. These methods can be

regarded, in general, as possible techniques for texturing surfaces.

However, they are mostly specific to a particular class of materials,highly sophisticated and expensive.

Alternative manufacturing technologies for surface texturing

are currently needed to overcome the challenges of volume

production. Thetechnologies should be cheapand flexible in terms

Fig. 14. Three functional surfaces for gripping micro-objects [90] (reprinted from

theProceedingsof 7thEUSPENInt.Conf., Gegeckaite, A.,Hansen, H.N., De Chiffre,L.,

Pocius, P., Handling of microobjects: investigation of mechanicalgripper functional

surfaces, page 187, Copyright 2007, with permission from euspen ltd.).




of both the shapes of the features and the surfaces to be textured.

For example, some innovative methods might be inspired by

biology and/or chemistry, such as the concept of self-assembly,

namely ‘things making themselves’ [25,69,120,243].

The methods presented here are reviewed either because they

have already been widely used in industrial applications or

because they appear to present improved characteristics. The

methods will have their most important attributes identified and

then will be categorized according to their physical principles intofour main groups:

Adding material: the pattern features are created by addition of

material to the desired surface, creating small areas of relief. Removing material: the features are created by removal of

material of the surface, creating small depressions. Moving material: the change in the surface structure is

attributable to plastic deformation and redistribution of material

from some parts of the surface to others. Self-forming : wear-resistant regions are formed on a surface, so

that a texture develops through wear of the surface, with the

wear-resistant regions being left positioned above the surround-

ing material.

4.1. Adding material technologies

Adding material technologies can be classified according to

Table 3. Texturing methods that involve adding material are

mostly related to the deposition of patterned coatings. Localized

deposition is normally controlled by locally changing chemical or

physical properties of the surfaces before deposition of the coating,

which either promotes or inhibits the deposition in those areas.

Chemical processes involve printing on the surface with inks

that can inhibit or promote some chemical or electrochemical

reaction, in order to generate a patterned coating. However, it is

important to emphasize that when metallic substrates and/or

coatings are involved, the texturing method can expose the

substrate material, so that a galvanic pair can be formed on the

surface. Therefore, in most applications, it is vital to have a processthat produces a very thin coating, followed by the masking step,

and then the final coating that will be textured.

In another approach, features with special characteristics are

deposited onto the surface. For example, wear-resistant reliefs

composed of ceramic particles can be deposited by inkjet printing.

The range of materials that can be inkjet printed on a surface

includes sol–gel materials, conducting polymers, structural poly-

mers, organic transistors, ceramics and metals.

The current resolution of the technique is believed to be

approximately 15mm.

Focused ion beam technique (FIB) can also be used for local

deposition of material, with excellent resolution (<100 nm),

however, the process is very slow.

Thermoset polymers cannot be made more plastic throughheating, as is currently done for the embossing of thermoplastic

polymers. However, when a textured coating of a thermoset

polymer is desired, a liquid resin can be deposited on the surface,

and a tool with thedesired texturing then used to embossthe resin.

The tool must be used when the resin is still plastic, which occurs

during curing [206].

In physical vapor deposition, masking shields can be used to

mask the PVD stream [113].

Nano-surface textures can be produced by spin coating a

colloidal nanoparticle solution onto a flat substrate. Its mainadvantage is the flexibility of the process in terms of the material,

size and density of the nano-texture produced, because a large

variety of nanoparticles are commercially available [252].

Self-organizing effects during the growth of thin polycrystalline

films can lead to coatings with columnar microstructures and

surface corrugations in the 100 nm range [36]. The cleaning of the

surface prior to the coating is crucial and it involves fairly

sophisticated deposition equipment. The selection of the deposi-

tion parameters must be very carefully chosen to ensure columnar

growth. TiAlN and TiO2 have been successfully deposited with

nano-sized diamond-like relief on silicon surfaces, but the growth

mode has not been detected to be dependent upon the substrate

material [241].

4.1.1. Removing material technologies

Table 4 classifies the texturing processes based on material

removal. The most representative texturing methods of this class

will be discussed.

4.1.2. Chemical texturing

In this group of methods, chemical reagents are selectively

applied to a surface to remove material in specific regions, in order

to produce texturing. Material is removed from selected areas by

immersing it in a chemical etchant. Etching only in selected areas

can be achieved either by masking, or by self-assembling of solid

particles and/oror liquids to form a mask. Material removal results

from a microscopic electrochemical cell action, equivalent to that

responsible for corrosion or chemical dissolution of a metal.

The resolution of the features thatcompose the texture dependsstrongly on the method used to mask the surface.

Depending on the printing method, texturing can also be

applied to surfaces with irregular shapes. Also, there are few

restrictions related to the shape and distribution of the features

that compose the surface texture.

The overall speed of these methods is restricted by the speed of

both the printing method and etching process. Normally, these

methods change the surface topography, but they do not change

the mechanical properties of the textured region. The main

equipment costs are related to the printing method.

Fig. 15 shows an example of a texture obtained on copper by

using photochemical etching (PCE) and the corresponding pseudo-

color map of the surface.

The copper surface can be etched in different conditions:changing the rate of reagent diffusion, the temperature of the

Table 3

Classification of adding material technologies.

Chemical conversion coatings Patterned chromating [46]

Patterned phosphating [46]

Chemical deposition coatings Patterned chemical deposition (CVD) [228]

Chemical processes Patterned autocatalytic plating [114,242]

Electrolytic coating

Anodising [46]

Electro-deposition [67,129]

Patterned precipitation coating [13]

Physical deposition Inkjet for wear-resistant patterns [167,168,208]

Patterned curing [206]

Focused Ion Beam for wear-resistant patterns [54]Patterned physical vapor deposition [113]

Patterned organic coating (painting)

Deposition of micro- or nanoparticles [252]

Self-assembling in polycrystalline films [241]




eroding bath andthe geometry of the masks protecting parts of the

substrate.

Change from a reaction-limited etching to a diffusion-limited

etching can be observed as the eroding bath temperature increases.

A Monte Carlo model of the chemical etching process can take into

account the different erosion conditions [192].

Other factors such as local gradients of the etching media

determine particular patterns. Fig. 16 shows the surface of an

etched hole and the corresponding pseudo-color map with a

characteristic pattern due to convective streams in the liquid

etcher.

Costa [46] also investigated other possible masking methods to

be used before etching for chemical texturing. The use of inkjet

printing for masking steel surfaces, with subsequent etching and

ink stripping, proved to be a versatile, fast and highly feasible

technique for texturing steel surfaces [49]. Other surfaces couldpotentially be textured by this technique using adequate alter-

native etchants. Figs. 17 and 18 show the inkjet printing of a steel

surface with parallel gaps and in multi-steps with chevron-like

gaps, respectively.

Fig. 15. PCE patterned surface.

Fig. 16. Hole etched on copper by PCE.

Table 4

Classification of removing material technologies.

High temperature Laser methods Lasertext (LT) [66,181,201]

Masked excimer laser [41,91,251]

Laser honing [86]

CNC focused lasers [23,46,73]

Femto-second lasers [73,158,160]

Electrical discharg e machi ning (ED M) E lectric al discharge texturing (E DT) [11,181]

Micro EDM [46]

Electrical beam texturing (EBT) [64]

Ion beam texturing (IBT) [54,142]Chemical etching Masking methods Chemical texturing [46,79,154,192]

Electrochemical texturing [58,229]

Non-masking methods Self-assembling [25,46,120,211]

Maskless laser assisted etching [161]

Maskless electrochemical texturing [46,195]

Anisotropic etching [94,183]

Mechanical CNC ultrasonic machining [46]

Mechanical honing [16,146]

Grinding [174]

Free abrasive machining [70,224]

Microcutting [28,46,70,83,225]

Patterned erosion [1,205]

Fig. 17. Inkjet printing of a steel surface with parallel gaps.




Some special etching methods that do not require prior

masking can be envisaged. For example, methods involving

simultaneous self-assembling of particles [25], droplets [120], or

bubbles [211], followed by etching or methods involving the

formation of locally etched regions created by acid droplets [46],

could considerably reduce texturing time and process costs. These

methods generally rely on the hydrophobicity and hydrophilicity

of surfaces, which allow small particles or droplets to arrange

themselves, forming patterns. The assembly process depends on

the differences in surface energy between the various components.

4.1.3. Maskless electrochemical texturing

Costa [46] proposed and implemented the maskless electro-

chemical texturing (MECT), a simple and cheap method to texture

metallic surface by electrochemical machining. Fig. 19 shows an

example of a texture obtained using the optimized conditions and

Fig. 20 shows the profile of an individual pocket as imaged by

optical interferometry.

4.1.4. Mechanical methods

Material can also be removed from a surface by mechanical

action. A cutter could be used to remove material from selected

areas of a sample. If the surface is maskedwith an erosion-resistant

mask, material can be removed by the action of a high-velocity

stream of abrasive particles. If no masking is applied, the

topography can be controlled by conventional surface finishing

methods, such as mechanical honing, grinding, polishing, etc. In

abrasive ultrasonic machining (USM), if a CNC table moves the

sample, different patterns can be created on the surface.

4.2. Moving material technologies

A classification of texturing technologies based on materialmoving is reported in Table 5.

In this group, plastic deformation of surfaces is used to form the

surface topography. Some increase in surface hardness can be

obtained by strain hardening, but it is generally less pronounced

than that obtained by high-temperature material removal

methods. These methods are generally restricted to ductile

surfaces, such as thermoplastic polymeric materials or metals of

high ductility, but very hard embossing tools can also be used to

emboss ceramic surfaces.

In shot blasting (SB), hard shot particles impact the surface at

high velocity to cause impact damage and roughening of the

surface. The change in the surface structure is attributable to

plastic deformation. It is difficult to precisely control the texture

because of the large number of variables inherent in the process,such as the size, shape,velocity andhardness of theshot. Also, shot

blasting can only form random textures.

To allow for the ease of embossing of thermoplastic materials,

the surface to be textured must be heated until it becomes plastic.

The embossing tools do not need to be very hard, which lowers the

tooling costs. This is a simple and versatile way to create well-

resolved structures on thermoplastic substrates. The ductility of

the substrate permits it to be embossed into regular, micrometric-

sized structures [218]. For harder materials, microstructured

diamond tools are normally used [183].

Vibrorolling uses fine-scale plastic deformation of metals. A

hard tool, normally a highly hardened ball or a rounded diamond

tip, is forced into the surface. Complicated kinematics of the

movement of the tool and the sample produces a uniform surfacemicrorelief, with regularly shaped asperities [35].

Chemical reactions can also be used to move material and

therefore produce a surface texture. Dos Santos et al. [71] have

reported the fabrication of nano-structured gratings formed with

mass transport arising from a light-driven molecular migration

mechanism due to the use of a laser beam. Some UV-curable

photopolymers can undergo contraction during polymerization. It

is possible to use this contraction to form a textured polymer

Fig. 18. Inkjet printing of steel in multi-steps with chevron-like gaps.

Fig. 19. SEM of a surface texture obtained with MECT.

Fig. 20. Profile of an individual pocket obtained with MECT.

Table 5

Classification of moving material technologies.

Mechanical Shot blasting [181]

Embossing Patterned embossing

tools [105,183,218]

Vibrorolling [35]

Chemical Molecular migration [71]

UV contraction [177]




surface if a metal mask is placed between the UV source and the

liquid monomer [177].

4.3. Self-forming technologies

Table 6 reports the classification of texturing techniques based

on self-forming by wear.

Once produced, a surface texture may not have sufficient

mechanical resistance to survive in highly loaded systems. Severe

stresses, especially the shear stress, may wear away surface

textures withinthe first few cycles. This happens because the wear

properties of the textured surface remain virtually uniform at all

points. Raised areas of the surface, which support a larger

proportion of the applied loads than do lower areas of the surface,wear more rapidly than the surrounding material. Some authors

[82,213] claim that to create a surface structure that is maintained

over a long functional period requires a method where the process

of wear itself is used to produce the required surface structure.

These methods are called ‘self-forming by wear’. They use localized

wear-resistant regions on a workpiece surface (Fig. 21); these

regions can be achieved by having a local gradient in mechanical

properties, which ensures that wear occurs at different rates in

different regions, leading to the development of geometrical

structures.

A gradient in mechanical properties can be created by using

known metallurgical treatments. Shot blasting and subsequent

polishing prior to the use of the surface wears the substrate at a

rate higher than that of the hard regions, and therefore produces asurface texture [46].

5. Conclusions

Since the last CIRP keynote paper [77] the use of engineered

surfaces has increased its industrial importance and expanded to

new areas that take advantage of a greater knowledge of the

phenomena occurring on the engineered surfaces. Energy transfer,

microfluidics and biomedical diagnostics are particularly emerging

fields for innovative applications, The availability of affordable

technologies for mass production of engineered surfaces discloses

nowadays wider markets.

However, the potential of engineered surfaces must face the

complexity of the relationships between surface properties,surface-based applications and surface engineering technologies.

In this keynote paper, the principal physical and technological

properties of surfaces have been reported. An analysis of the

most interesting and advanced applications depending on surface

characteristics outlines the spectrum of possibilities for engi-

neered surfaces. The emerging technologies used for texturing

surface have been reviewed. Some technologies are currently used

in industry as conventional surface structuring methods, while

others are used for microfabrication, but could be adapted to

engineer surfaces.

Issues related to the metrology of engineered surfaces should

also be considered in order to support the industrial diffusion of

new applications [125].

Acknowledgments

The authors would like to thank the colleagues from the STC S

for their valuable contributions: E. Brinksmeier, L. De Chiffre, F.Z.

Fang, A.S. Korhonen, O. Riemer.

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