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[email protected] Lecture 09 http://www.iap.uni-jena.de/multiphoton

Nanomaterials and their Optical Applications Winter Semester 2012

Lecture 09

[email protected]

[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


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


Location Institute of Applied Physics 4


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


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:


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)


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.


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.


Dimensions 10

Protein, lipid molecule of the membrane: 1 nm Virus: 10 nm Cell: 1-10 μm


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)


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 ?


Basic properties of fluids 13


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).


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


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


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


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



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)



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)



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



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



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



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



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



• 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


Theoretical descriptions: capillarity 27


Theoretical descriptions: capillarity 28


Microfluidic components and devices 29


Optofluidics: why combining optics and fluid ? 30

http://www.biophot.caltech.edu/optofluidics/publications/industry_workshop/tang.pdf



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 ?



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


Soft lithography 33


34

Whitesides, Annu. Rev. Mater: Sci. 1998. 28: 153-84


PDMS as material for optical devices 35


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


Soft lithography 37

2. Template: silicon wafer with epoxy structure (su8) • Substrate cleaning • Resist spin • Pre-exposure - soft bake • Exposure - postbake • development


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,


Soft lithography 39

4. Assembling In a plasma chamber

Make the PDMS hydrophilic

To bond PDMS with a glass slide

5. Experiments 5. Tubing


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


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).




Hole size: 200 x 700 nm Channel width : 10 x 40 µm

Slanted hole array: fabrication by e-beam lithography




Point-by-point mapping : apertures in metal film map to pixels on a camera



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.



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



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



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



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).



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


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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