contact angle 69326_17

Aqueous and Non-aqueous Liquids on SuperhydrophobicSurfaces: Recent Developments

Michele Ferrari

CNR-National Research Council — IENI, Institute for Energetics and Interphases, via De Marini 6,16149 Genova, Italy

AbstractIn this paper recent developments regarding design and preparation of superhydrophobic substrates and thewetting behavior of water based solutions, engineered and non-aqueous liquids in contact with such surfacesare summarized, considering application in a range of basic research and industrial fields.

The combination of highly water repellent surfaces with engineered liquids is of great interest in openingnew trends in liquid handling and manipulation, especially as regards to small volumes.

Reference data and related studies are still not sufficient to cover the several aspects of ultralyophobicity,in particular in those fields where the research deals with specific problems related to compatibility orsolubility, to name a few. The exploitation of these studies in switching between wetting states has beena topic of some investigations, while only very few reports are available on immiscible liquids.

KeywordsSuperhydrophobicity, wetting, non-aqueous liquids

1. Introduction

In this paper an overview about the behavior of water-based solutions and non-aqueous liquids in contact with surfaces or coatings with extreme water repellencyis provided. The terms superhydrophobic or ultrahydrophobic refer to low energysurfaces with a water contact angle (CA) greater than 150◦ and since the introduc-tion of the idea of the Lotus effect [1, 2] to enhance and exploit the self-cleaningproperties of solid surfaces, many researchers have introduced plenty of techniquesto obtain superhydrophobic surfaces, most of them being based on controlling theroughness or topography of low energy surfaces (Fig. 1).

The birth of new disciplines like biomimetics indicates the strong attention givento the exploitation of structural properties of plant or animal surfaces: starting frommicro or submicroscopical observations, the characterization has led to a more andmore increased “market” of structures with specific tailored features. The combina-

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Figure 1. AFM image of a FAS-TEOS superhydrophobic surface showing nanoscale roughness.

tion of such highly water repellent surfaces with engineered liquids is and will be ofgreat interest in opening new trends in liquid handling and manipulation, especiallyas regards to small volumes.

In particular, recent developments regarding design and preparation of superhy-drophobic coatings or substrates will be considered here in terms of their applica-tions in a wide range of basic research and industrial fields.

The importance of the studies regarding non-aqueous liquid–solid interactionsso far has not been investigated in detail; therefore, the literature available has beenreviewed focusing on application aspects of ultrahydrophobic and ultralyophobicstates.

2. Superhydrophobic States: Learning from Nature

The first lesson coming from nature deals with the well-known self-cleaning proper-ties of lotus leaves, which have been under investigation for their particularly strongwater repellency due to a unique structure with a highly hydrophobic character.

This structure was recently studied in detail by Marmur [3, 4] by electron mi-croscopy, showing a 2D roughness in leaf appendixes of several micrometer in sizeand covered with small wax crystals.

Since the time of seminal works of Wenzel [5, 6] and later of Cassie and Bax-ter [7], investigation of birds and insects wings or beetle shields [8, 9], together withdifferent kinds of plants leaves, has led to the key conclusion that such a strong hy-drophobic effect arises from combining a micro- and nano-patterned surface witha particular geometry with a coating of natural organic compounds of different na-ture [1].

Aqueous and Non-aqueous Liquids on Superhydrophobic Surfaces 271

The models of Wenzel and Cassie–Baxter have been utilized to interpret theroughness effect on the wettability properties of a solid surface.

The Wenzel’s approach assumes the liquid to fill the space between the pro-trusions on the surface, linking the apparent contact angle θ ′ and thermodynamiccontact angle θ as

cos θ ′ = r cos θ, (1)

where r (roughness factor) is the ratio between the true surface area and its hori-zontal projection.

According to Cassie and Baxter the surface traps air in the hollow spaces of therough surface, and the superhydrophobicity can be interpreted as follows:

cos θ ′ = fLS cos θ − fLV, (2)

where fLS is the fraction of liquid area in contact with the solid and fLV is thefraction of liquid area in contact with the trapped air (fLS + fLV = 1).

The two states can be distinguished by the contact angle hysteresis. At largerhysteresis they can be regarded as belonging to the Wenzel’s one, while at smallerhysteresis values, the Cassie–Baxter approach is applicable: the surface can be re-garded as composed by a pillar-like structure supporting the liquid and reducing theavailable area.

This combination of micrometer and nanometer-scale roughness with low sur-face energy in a non-homogeneous wetting regime (Cassie–Baxter regime) ledOnda and co-workers [10] to create “artificial” superhydrophobic surfaces.

In order to apply the water repellent properties of duck feathers for engineer-ing new materials featuring such non-wetting behavior, the microstructure of thefeathers was investigated by Liu et al. [11] with a scanning electron microscope(SEM) via a method based on a different magnification stages procedure. The re-sults showed that superhydrophobic behaviour of duck feathers was the result of thecombination of this particular structure and the presence of the preening oil.

A novel method, based on surface solution precipitation (SSP) was introduced tosimulate the feather microstructure on textile substrates using chitosan as buildingblocks, and then the textile substrates were further modified with a silicone com-pound to lower the surface energy. Highly water repellent properties were observedin textiles showing bionic superhydrophobic surfaces prepared on soft substrates bya simple procedure involving flexible biopolymeric materials.

3. Superhydrophobic Surfaces and Drop Manipulation

Handling small volumes of liquids has provided a great stimulus for designingappropriate surface topography. Surface roughness is known to be crucial for su-perhydrophobicity, however, new approaches are required to expand the concept ofsurface roughness, such as hierarchical roughness in more complex mechanisms ofwetting, CA hysteresis and wetting regime transition.

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A simple casting method was used by Hou et al. [12] for preparing a polypropy-lene/methylsilicone based superhydrophobic surface. The surface microstructurecould be tuned by varying the ratio of polypropylene and methylsilicone result-ing in different surface features. The wetting behaviour of the as-prepared surfacewas investigated. A polypropylene monolithic material was also prepared and itssuperhydrophobicity was still retained when the material was cut or abraded. Theas-prepared material can also be used to separate some organic solvents from wa-ter.

A different application of bio-inspired superhydrophobic surfaces was found byNosonovsky and Bhushan [13] for adhesion reduction between micro/nanoelectro-mechanical systems (MEMS/NEMS) components.

The authors suggest that the dissipation mechanisms is related to hierarchicalroughness and may lead to self-organized criticality.

To obtain the desired surface roughness, various techniques like chemical andplasma etching, laser treatment, chemical vapour deposition and electrodeposition,dipping and spraying, have been used together with surface modification by me-chanical methods or photopatterning [14].

As suggested by Extrand [15] and Sedev et al. [16], roughened surfaces obtainedby micropatterning, machining or etching show higher values of CA due to inhibi-tion of the liquid by the rough surface in such a way that drop spreading is hinderedby the edges of the grooves.

In addition, a systematic study on different rough surfaces with a well-definedsurface chemistry was the topic of Spori et al. [17] where water CA measurementswere performed for a better understanding of liquids in contact with rough surfacesranging from sandblasted glass slides to sandblasted titanium. In particular, theyfound that photolithographically fabricated golf-tee shaped micropillars (GTMs)showed Cassie-type hydrophobicity, even in the presence of hydrophilic surfacechemistry.

Despite the hydrophilic nature of the rough surfaces, CAs are shifted to morehydrophobic values because of pinning effects, unless roughness or surface energyare such that capillary forces become significant, leading to complete wetting.

The observed hydrophobicity is thus not consistent with the well-known Wenzelequation. The authors show that surface chemistry does not influence the pinningstrength of the surface if capillary forces or air pockets are not involved. By plottingCAs on rough versus flat surfaces, the authors described the pinning strength by theintercept of the plots at fixed surface chemistry.

Electrowetting has also been recently employed [18] for the micromanipulationof a liquid droplet which can be picked up and handled by controlling the wettingproperty between the liquid and the substrate itself (Fig. 2). The authors providea precise analysis through a numerical method of the process behind the rupture ofthe liquid bridge between the conical gripper and the substrate. The authors foundit possible to control the efficiency of micromanipulation in different conditionsof CAs between the liquid and the gripper taking into account the distribution ra-


Figure 2. Micromanipulation by electrowetting of a liquid droplet which can be picked up and handledby controlling the wetting between the liquid and the substrate.

tio between the droplet volume retained by the substrate and the whole volumeof the liquid droplet during the rupture. An optimal efficiency was attained whichwas supported by a theoretical analysis that helped to find the best conditions andparameters for the micromanipulation process experimentally demonstrated on thestandard probe of an AFM.

Another interesting application of such a technique for nanomaterials can befound in Brunet et al. [19] where experiments on drop impact impalement (Wenzelstate) and electrowetting were performed to compare the wetting properties of su-perhydrophobic silicon nanowires. In this paper the authors provide a comparisonbetween the resistance to impalement by electrowetting and drop impact.

From this study it becomes evident that there is a proportional, direct relationshipbetween the increase of the length and density of nanowires and the thresholds fordrop impact, observing an increase of the electrowetting irreversibility while theCA hysteresis after impalement decreases. The threshold to impalement of sucha surface results to be up to three times higher than most of the surfaces or coatingstested providing a good preservation of the reversibility.

The design of hydrophobic materials and coatings with tailored superhydropho-bic features of the surface finds theoretical support in Boinovich and Emelya-nenko [20] where the authors discuss the possibilities of the formation of orderedtextures with high CAs on the surfaces of hydrophobic materials. They providean analysis of the necessary conditions for thermodynamic stability of the het-erogeneous wetting regime of such surfaces facing the problems of ageing anddegradation of superhydrophobic coatings. The authors give examples of the useof superhydrophobic materials for industrial applications.

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4. Superhydrophobic Surfaces and Aqueous Mixtures of Organic Liquids

Basic research investigations on exploiting biosurfaces in contact with liquids canbe found in the work by Fang et al. [21] where the wetting properties of distilledwater and methanol solution on the wings of butterflies were studied by CA mea-surements. The scale structures of the wings were observed using scanning electronmicroscopy and the influence of the structure scale on the wettability was investi-gated.

Results show strong hydrophobic behaviour in numerous species with CAsgreater than 150 degrees. In some cases the CA of distilled water on the wingsurfaces varies from 134 to 159 degrees. The wing surfaces of some species arenot only hydrophobic but also resist wetting by methanol solution with 55% con-centration. Because of the structure features (spindle-like and pinnule-like shapes)only two species with these shapes cannot resist wetting, showing large differencecompared to the other species. Spreading/wetting on the wing surfaces of differ-ent species was observed for a large concentration range (from 70% to 95%) ofmethanol solutions. After contact with methanol solution for 10 min, the wing sur-face showed an enhanced capacity against wetting by distilled water with a CAincrease up to two degrees.

Because of the lack of adequate literature in this field, authors like Shirt-cliffe [22], have focused on the topic of interaction between non-aqueous liquidsand highly water repellent surfaces. In this work they studied the switching behav-ior from superhydrophobic to hydrophilic properties by varying physico-chemicalparameters. In particular, they investigated aqueous solution of ethanol in contactwith porous superhydrophobic substrates finding that in low surface tension condi-tions such liquids could enter surface grooves.

For superhydrophobic surfaces Rao et al. [23] also studied porous substrates forindustrial purposes. They showed how the surface tension could be the reason forthe transition from the Cassie state to the Wenzel state. Even without performingCA measurements of pure liquids, it was reported that with lower surface tensionthe non-aqueous pure liquids, but not the pure water, penetrated the surface grooves.

Investigations on water–alcohol mixtures were reported also by Fujita et al. [24].They measured high advancing angles (above 150◦) for highly hydrophobic pat-terned surfaces with water and water–glycerol mixtures, but below 10◦ for ethanol(γLV = 22.3 mN/mm), confirming the Cassie–Wenzel transition with the decreasein surface tension.

Organic and inorganic coatings were studied by Shibuchi and co-workers [25,26]. They first explored the potential offered by fractal surface produced sponta-neously by solidification from the melt of alkylketene dimer, AKD (a kind of wax)and obtained highly water repellent substrates showing a CA on the order of 174◦with drops rolling off the surface at a small tilt angle.

The CAs of some water/1,4-dioxane mixtures on the fractal and the flat AKDsurfaces were determined and a decrease in CA with increase of dioxane fractionwas observed. The surface tension of the liquid changes from 36 mN/m for pure 1,4-


dioxane to 72 mN/m for pure water, depending on the concentration of 1,4-dioxane.As a function of the fractal parameters they also studied the wetting properties ofthe mixtures at different water contents, finding low CAs (15–20◦) for the lowestwater ratio (20%). It has been demonstrated by this work that the fractal concept isa powerful tool to develop some novel functional materials, for example which canact as a switch from the Cassie to the Wenzel mode.

In [26] the authors investigated an aluminum substrate roughened first by elec-trochemical etching and then hydrophobized by a fluorination treatment. The alu-minum was processed by anodic oxidation obtaining a highly water wettable sur-face, which was then modified to ultra water repellent (CA of 160◦ for waterdroplets) by treatment with perfluorooctyltrichlorosilane.

In this study they reported super water- and oil-repellent surfaces made byexploiting the fractal structure of the surface. Fluorinated monoalkyl phosphateswere used as hydrophobizing agent for the aluminum substrate to obtain super oil-repellent surfaces. They found a CA of about 150◦ for rapeseed oil (surface tensionof about 35 mN/m) on the super oil-repellent surface where droplets rolled off thesurface without significant attachment. In other cases for oils with a surface tensiongreater than 23 mN/m, CAs greater than 120◦ on the super-oil-repellent surfaceswere found. Shibuichi et al. explained their results in terms of fractal dimensionand influences of chemical structure on smooth and rough surfaces.

In [27] several organic liquids of different chemical nature characterised by lowsurface tension were used to study the superlyophobic behavior of nanostructuredsurfaces. Among the wide variety of tested liquids were alcohols (aliphatic andalicyclic, short chained), water–alcohol mixtures, aromatic hydrocarbons, ethers,esters, and silicone oils. These liquids formed on these surfaces droplets with highmobility, low hysteresis, with related CAs larger than 131◦. When no voltage wasapplied, these surfaces showed, in the initial state, CAs as high as 150◦ for liquidswith surface tensions ranging from ethanol to water. Once applied, the electricalvoltage induced a transition from the superlyophobic state to wetting, whose naturewas investigated both experimentally and theoretically by the authors. These resultscan be considered a promising method for manipulating liquids on the microscale,in fact showing, for the first time, dynamically tunable surfaces, such as nanonails,capable of driving a transition from a remarkable superlyophobic behavior to almostcomplete wetting.

Mohammadi et al. [28] studied pure liquids and surfactant solutions on super-hydrophobic substrates featuring a rough microstructure produced as in [26] byspontaneous formation of AKD crystals. As in the paper of Shibuichi et al. [26],the advancing CAs showed a discontinuous drop with pure liquids at surface ten-sion values of about 45 mN/m.

In the paper by Chen et al. [29] the preparation of both ultrahydrophobic and ul-tralyophobic surfaces using several techniques is described. Plasma polymerizationof a fluoroacrylate on poly(ethylene terephthalate) (PET) produced surfaces withhigh CAs >170◦ and hysteresis of about 1◦. Argon plasma etching of polypropy-

276 M. Ferrari

lene in the presence of poly(tetrafluoroethylene) also produces surfaces with similarCAs but larger hysteresis of 3◦.

By compressing spherical particles of poly(etrafluoroethylene) (PTFE) of submi-crometer diameter range, superhydrophobic surfaces were prepared which showedhigh CAs and low CAH for water, methylene iodide and hexadecane (CAs of about140◦), in particular the latter result being of great value if compared with previousdata [30] on smooth surfaces. Even considering a standard interpretation concern-ing a Wenzel state behaviour in this case the authors observe an ultralyophobicfeature. A helpful explanation can be found in [31] in which the geometry of thesurface is regarded as responsible for high CAs despite the low surface tension ofthe liquids.

The role of CA hysteresis in characterizing lyophobicity instead of looking atthe maximum CA must be emphasized, as these surfaces are usually rough atthe micrometer and submicrometer scales, and water drops roll easily from all ofthem.

The authors in [29] also report about smooth ultralyophobic surfaces preparedby silanization of silicon wafers. These surfaces exhibit much lower CAs but littleor no hysteresis, and droplets of water, hexadecane and methylene iodide slide offeasily on them. This behaviour has been interpreted with the liquid nature and theflexibility of the monolayers assuming that droplets in contact with them experiencevery low energy barriers. Their conclusion is that topography of the roughness isimportant in controlling the continuity of the three-phase contact line and thus thehysteresis.

5. Non-aqueous Liquids at Superhydrophobic Surfaces

Studies regarding the wetting of smooth hydrophobic and ultrahydrophobic sur-faces by non-aqueous pure liquids or mixtures are only limited.

Egatz-Gómez et al. [32] follow a microfluidics approach for a faster and moreflexible control over drop movement. They describe a method to control drop mo-tion on superhydrophobic surfaces by means of magnetic fields operating under thesurface (Fig. 3). In this way they can move liquid nano-drops prepared with lowpercentage of paramagnetic particles (0.1% weight) relatively fast coalescing witha static drop.

Recently new engineered liquids have been under investigation by Bormashenkoet al. [33] for their unique properties in microfluidics. A microfluidic device basedon ferrofluidic marbles has been described.

In particular, a study on the motion on flat polymer substrates containing fer-rofluidic marbles, prepared by dispersing nanopowder of poly(vinylidene fluoride)and γ -Fe2O3, evidences the behaviour of such a fluid in presence of a magneticfield. The sliding of ferrofluidic drops on superhydrophobic surfaces was studiedafter activation of the marbles by means of an external magnetic field. It is shownthat drop radius influences the drop displacement with a linear dependence on the


Figure 3. Motion of a nano-drop containing low percentage of paramagnetic particles controlled bymeans of a magnetic field operating under the superhydrophobic surface.

threshold magnetic force evidencing the role played by the processes at the contactline.

In [34] the present author shows how the increased topography, from a simplepolymeric fluorine-based coating to a mixed nanoparticles–polymer, increases theCA in a water–hexane system with a jump of almost 40◦, where water drops rolloff from the surfaces without sliding (Fig. 4a–c). In combination with surfactantsolutions with different oil solubilities, these systems offer the opportunity of wet-ting control as a switching effect from a Cassie–Baxter to a Wenzel state that canbe effectively reversed to superhydrophobic behaviour by exploiting the surfactantdistribution between the liquid phases.

6. Conclusions

After more than two decades, the developments regarding design and preparation ofsuperhydrophobic coatings or substrates are ongoing and seem to give new insightsin terms of a wide range of basic research and industrial application fields. Thecombination of such highly water repellent surfaces with engineered liquid is andwill be of great interest in opening new trends in liquid handling and manipulation,especially regarding small volumes.

Moreover, the behaviour with water, water-based solutions, or organic solvents,despite the enormous potential applications, has not been studied adequately andthe related studies are still not sufficient to cover the several aspects of ultralyopho-bicity, in particular in fields, like microfluidics, where one has to deal with specificproblems related to compatibility or solubility, to name a few.

278 M. Ferrari

(a)

(b)

(c)

Figure 4. Water droplet in hexane on glass (a), on fluorinated polymer coated glass (b), on mixednanoparticle-fluorinated polymer coated glass (c).


The exploitation of these studies in switching between wetting states has beenthe topic of some investigatons, while very few studies are available on immiscibleliquids, and it is clear that topography and a given surface chemistry play importantroles in controlling this transition.

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