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Nanomaterials and their Optical Applications Winter Semester 2012
Lecture 09
[email protected] Lecture 09
Schedule until the end of the semester 2
9 10.12.2012 Paper 9, Optofluidic 1
x 17.12.2012 no lecture
x 24.12.2012 Holidays, no lecturex 31.12.2012 Holidays, no lecture
10 07.01.2013 Turn in homework 4, Optofluidic II11 14.01.2013 Nanomarkers12 21.01.2013 Seminars presentations by students (2)13 28.01.2013 Seminars presentations by students (2)x 04.02.2013 no lecture
Lecture, Mondays 16-17.30
Turn in HW 3 on Tuesday 17.12.2012, scan or give it to Can !
Seminar, Tuesdays 5 11.12.2012
Paper 8 & 9 / Tipps for Homework 3 & 4/ Solutions HW2
x 25.12.2012 Holidays, no seminar
6 08.01.2013 Paper 10 / Solutions HW 4 / Q's for talk7 22.01.2013
Examination : February 14th Beutenberg campus, IAP, 14.30-16.30
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Choose your slot 3
Topics for a 25 minutes oral presentation with slides and questions (5 minutes)
Lecture : Nanomaterials and their optical applications
Date Room Time Speaker Title of the talk
21.01 IAP 16.00
16.30
17.00
28.01 IAP 16.00
16.30
17.00
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Location Institute of Applied Physics 4
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Outline: Optofluidics 5
• What is optofluidic ?
• History of micro-nano-opto-fluidic
• Basic properties of fluids
• Nanoscale forces and scale law
• Optofluidic: fabrication and applications
13.12.2011, Le Monde, AP, Michael Sohn
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What is optofluidics ? 6
A combination of optics and micro/nanofluidics
Optics: the study of light and its interactions with matter. Fluidics: the study of materials that deform under a shear stress.
Optical (em) field interacts with the fluid in a micro/nano scale system Light manipulates fluid and/or fluid manipulates light
V.R. Horowitz, D. D. Awschalom, S. Pennathur, Optofluidics: field or technique? Lab Chip, 8 1856–1863 (2008)
Aim: investigating and developing miniature devices which can sense, pump, mix, monitor and control small volumes of fluids
2 types of interactions:
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Advantages 7
• Work with small volume
• Increase speed of reaction
• Better performance with lower power
• Precise mixing, dosage and heating
• Integrate with other devices on a chip
• Ease of disposing device or fluid
• Reduce costs of reagents and power consumption
• High surface to volume ratio, low Reynolds number
• Minimize dead space, void volume and sample carryover
• Multiplex capability: increased number of parameters monitored per assay
Adam T Woolley, Brigham Young University (BYU)
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Challenges 8
• Sensitiviy limited by sample volume
• Integrating microvalves and micropumps
• Packaging
• Control algorithms, data processsing and communication
UC Davis biomedical engineer ProfESSOR Alexander Revzin has developed a "lab on a chip" device for HIV testing.
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Few milestones 1970-2010 9
1970 - 1990 : Essentially no microfluidic (except the Stanford gas chromatographer)
1990 : First liquid chromatograph - (Manz, Graber, Widmer, Sens.Actuator, 1991)
1995 : Microfluidic community reaches a critical mass
1990 -1998 : First elementary microfluidic systems (micromixers, microreactors, …)
1997 : First MicroTAS meeting, held in North Holland ~150 participants
1998 : Invention of soft lithography technology (Whitesides et al). All sorts of
microfluidic systems with various levels of complexity are made.
2003 First use of the term optofluidics (Psaltis Nature 2006)
2004 Nanofluidics (van den Berg review paper in 2005)
2011 Optofluidic for Energy (Psaltis Nature Photonics 2011)
S.C.Terry,J.H.Jerman and J.B.Angell:A Gas Chromatographic Air Analyzer Fabricated on a Silicon Wafer,IEEE Trans.Electron Devices,ED-26,12(1979)1880-1886.
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Dimensions 10
Protein, lipid molecule of the membrane: 1 nm Virus: 10 nm Cell: 1-10 μm
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Dimensions 11
Nanofluidics deals with fluids flowing in systems whose characteristic sizes range between 10 and 300 nm
Nanofluidics deals with fluids flowing in conditions where interactions between micro and macroscopic scales play a crucial role.
THE BEHAVIOUR OF SIMPLE LIQUIDS AT THE NANOSCALE (Tabor, Israelachvilii ~1980)
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Questions 12
• Does fluid mechanics holds at such small dimensions ?
• Which type of flow is playing a role ?
• Liquid, gaz: how do they behave ?
• How to create fluid movement at this scale ?
• Which hydrodynamics concepts can be minaturized ?
• Which models ?
• Small volume thus surfaces play a huge role ?
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Basic properties of fluids 13
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Nanoscale forces 14
Electrostatic forces As far as the electrical double layer 1-100nm Repulsive or attractive depending on the electrolyte concentration
Van der Waals forces At distances smaller than 2 nm Always attractive
Steric forces Repulsive forces due to freely fluctuating polymer brushes
Capillary forces Attractive
1. Eijkel, J.C.T. & Berg, A.V.D. Nanofluidics: what is it and what can we expect from it? Microfluidics and Nanofluidics 1, 249-267 (2005).
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Nanoscale forces and other related phenomena 15
Surface roughness
No zero wall velocity but slippery walls !
Optical and mechanical tweezers
Dielectrophoretic forces • a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field • In rapidly changing high electrical field gradient Less important forces Gravitational and inertial forces
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New balances of forces at the microscale 16
Scaling laws Insects can walk on water
Compare the exponents of the scaling law: the smaller wins at the microscale
Insects can be easily caught by a water droplet
Advantages : jump high disadvantages capillary forces are even stronger
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New balances of forces at the microscale 17
A contact line occurs on the legs of these insects. The leg surface is hydrophobic, and therefore the surface is curved downwards, which creates an upward tension force.
Insects can walk on water ! Ants can lift 1000 ants !
Muscular force:
P= stress from a fiber
Number of people you can lift:
Human: l = 1 m Ant : l = 1 mm
http://en.wikibooks.org/wiki/Microfluidics
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Theoretical descriptions 18
Best description with accounting for the forces between individual atoms • Problem of computational power • Deviation from classical continuum theory in space smaller than 10
molecular diameters
2-10 nm : Molecular dynamics for such systems computer simulation of physical movements of atoms and molecules
Kalra et al. PNAS, 2003
green: carbon atomes Red/white : water molecules
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Theoretical descriptions 19
10-100 nm: classical Navier-Stokes theory accounting for electrostatic effects by the Poisson-Boltzmann equation
A fluid particle is a volume, large enough to contain smooth molecular variations, but small compared to the system size. It has a mesoscopic character, intermediate between a microscopic (molecular) and macroscopic description.
A continuum approach is possible if…
A 10 nm cubic volume of liquid contains already 30000 molecules
A 10 nm cubic volume of gaz contains only 20 molecules (not always standard approach)
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Theoretical descriptions 20
A 10 nm cubic volume of liquid contains already 30000 molecules
A 10 nm cubic volume of gaz contains only 20 molecules (not always standard approach)
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Theoretical descriptions 21
Validity of the classical hydrodynamic approach depends on the Knudsen number for gazes:
l = microchannel heights for instance λ = mean free path of the gaz
4 regimes: 1. Kn < 0.01 : continuum regime, classical hydrodynamic 2. 0.01< Kn< 0.3 : slip regime, classical hydrodynamic and slippery walls 3. 0.3< Kn< 10 : transition shift to Burnett equation 4. 10< Kn : free molecular flow regime, only molecular dynamics models
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Theoretical descriptions 22
These equations arise from applying Newton's second law to fluid motion, together with the assumption that the fluid stress is the sum of a diffusing viscous term (proportional to the gradient of velocity), plus a pressure term.
Mass conservation :
For incompressible fluids (which is almost the case with liquids, and gases at velocities small in front of the speed of sound), ρ = cst, and mass conservation implies a specific flow field that is divergence free
Forces within a fluid: pressure and shear forces due to friction
n normal to the volume dS small surface element τ shear stress tensor µ viscosity
Classical electrodynamics : Navier-Stokes equations
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Theoretical descriptions 23
Navier-Stokes equations For a Newtonian fluid the Navier-stokes equation writes
comes from the inertia of the fluid. It is a non-linear term that produces history dependent effect very present at the human scale or larger scale
From viscous effect that are preponderant at small scales
The Reynolds number provides an estimation of the ratio inertial forces/viscous forces: if the typical velocity of the fluid is u, and the typical size l
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Theoretical descriptions 24
Reynolds number: small in microsystems
~ l2
Measurement of the turbulence Re <2100 : laminar (Stokes) flow regime, no inertial effect, strong viscous interaction between fluid and wall Re > 4000: unstable laminar flow, turbulent flow regime
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Theoretical descriptions 25
The velocity field solution of these equations is linear in applied stresses meaning: • Uniqueness: the solution is unique (whereas the full Navier-Stokes equation
gives rise to turbulence and instabilities) • Reversibility : the solution is reversed when the forces are reversed: it is
impossible to create a fluid "diode" at small scales • It can also be shown that the solution minimizes the total dissipated power.
Flow properties at small Reynolds number
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Theoretical descriptions 26
• Reversibility : the solution is reversed when the forces are reversed: it is impossible to create a fluid "diode" at small scales
Flow properties at small Reynolds number
Physical Not physical
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Theoretical descriptions: capillarity 27
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Theoretical descriptions: capillarity 28
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Microfluidic components and devices 29
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Optofluidics: why combining optics and fluid ? 30
http://www.biophot.caltech.edu/optofluidics/publications/industry_workshop/tang.pdf
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Optofluidics: why combining optics and fluid ? 31
http://www.biophot.caltech.edu/optofluidics/publications/industry_workshop/tang.pdf
1. Naturally smooth • Static: minimization of surface tension • Dynamic: laminar flow 2. Deformable • can change geometry without extra stress/strain
Why fluid ?
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Optofluidics: why combining optics and fluid ? 32
PDMS chip: soft lithography silicon-based organic polymer optically clear, inert, non-toxic,
non-flammable, viscoelastic
Integration of optics and microfluidics for cheap, compact, disposable devices with very low sample volume
Microfluidics channels
Microfluidics valves and pumps
Optical structure: photonic crystal structures, sensors, sources and waveguides
Psaltis, Quake, and Yang, Nature 442, 381, 2006
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Soft lithography 33
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PDMS as material for optical devices 35
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Soft lithography 36
1. The mask = glass substrate with a patterned layer of chromium (Cr) • Cr not transparent to UV • Draw your idea of pattern with a software (CleWin)
Area exposed to light are removed
Area exposed to light remains
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Soft lithography 37
2. Template: silicon wafer with epoxy structure (su8) • Substrate cleaning • Resist spin • Pre-exposure - soft bake • Exposure - postbake • development
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Soft lithography 38
3. Stamp silicone: polydimethylsiloxane (PDMS)
• Weigh the silicone
• Mix the base and the curing agent
• Degas in exsicator using vaccum
• Dispense on the molded substrate and spread
• Curing
• Peal off,
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Soft lithography 39
4. Assembling In a plasma chamber
Make the PDMS hydrophilic
To bond PDMS with a glass slide
5. Experiments 5. Tubing
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Optofluidic Devices 40
3 categories
• Solid-liquid hybrid devices • Complete fluid based devices • Colloid based systems
Fluids in solids: optofluidic microscope, DFB laser, photonic bandgap structures
Fluids in fluids: liquid core / liquid cladding for waveguiding, liquid lens
Solids in fluids: tweezing of colloidal particles
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Optofluidic Devices : Fluids in solids 41
1. Optofluidic microscope
• Portable imaging device without bulk optic • High resolution and speed • Integration friendly
100 µm
Slanted hole array Y: sub-wavelength resolution : 300 nm
Main assumtion: Micro-organism flows with constant speed, shape and orientation
Heng, X. et al. Optofluidic microscopy--a method for implementing a high resolution optical microscope on a chip. Lab on a Chip 6, 1274-1276 (2006).
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Optofluidic Devices : Fluids in solids 42
1. Optofluidic microscope
Hole size: 200 x 700 nm Channel width : 10 x 40 µm
Slanted hole array: fabrication by e-beam lithography
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Optofluidic Devices : Fluids in solids 43
1. Optofluidic microscope
Point-by-point mapping : apertures in metal film map to pixels on a camera
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Optofluidic Devices : Fluids in solids 44
2. Optofluidic dye lasers Liquid medium :optical gain, the lasing wavelength, spatial modes and tunability
1. Advantages compared to bulk dye lasers 2. Unique optical performances not possible with solid-state lasers, like the waelength 3. Lab-on-chip system
Single mode operation using a Bragg grating
The reflected wavelength (λB), called the Bragg wavelength, is defined by the relationship,
,
How ? by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength specific dielectric
mirror. used as an inline optical filter to block
certain wavelengths, or as a wavelength-specific reflector.
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Optofluidic Devices : Fluids in solids 45
2. Optofluidic dye lasers
the lasing wavelength
free spectral range (FSR)
How to get a single mode laser ? • Confinement of the modes to get
only E11 for instance • Free spectral range (FSR) of the
cavity must be larger than the gain spectral bandwidth
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Optofluidic Devices : Fluids in solids 46
2. Optofluidic dye lasers
Periodic structure = PDMS post (grating spaced by Λ) that reflects light with wavelength equal to 2nΛ/m = feed back for the laser operation
n beeing the refractive index of the structure Thus tuning the frequency by changing n
How to get a single mode laser ? • Confinement of the modes to get only
E11 for instance • Free spectral range (FSR) of the
cavity must be larger than the gain spectral bandwidth
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Optofluidic Devices : Fluids in solids 47
2. Optofluidic dye lasers Solution or organic dyes in a solvent Rodhamine 6G Liquid dye laser characteristics
Stokes-shift
4 level system Triplet states may disturb: use short pulses and fast dye circulation
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Optofluidic Devices : Fluids in solids 48
2. Optofluidic dye lasers Liquid medium :optical gain, the lasing wavelength, spatial modes and tunability
1. Advantages compared to bulk dye lasers 2. Unique optical performances not possible with solid-state lasers, like the waelength 3. Lab-on-chip system
Li, Z. & Psaltis, D. Optofluidic dye lasers. Microfluidics and Nanofluidics 4, 145-158 (2007).
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Optofluidic Devices : Fluids in solids 49
2. Optofluidic dye lasers Periodic structure (grating spaced by Λ) that reflects light with wavelength equal to 2nΛ/m
n beeing the refractive index of the structure Thus tuning the frequency by changing n
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Paper & Outlook 50
Heng, X. et al. Optofluidic microscopy--a method for implementing a high resolution optical microscope on a chip. Lab on a Chip 6, 1274-1276 (2006).
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