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ME 381R Fall Lecture 24:

Micro-Nano Scale Thermal-Fluid Measurement Techniques

Dr. Li Shi

Department of Mechanical Engineering The University of Texas at Austin

Austin, TX 78712www.me.utexas.edu/~lishi

lishi@mail.utexas.edu

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• Caged fluorescence• Micro Particle Image Velocimetry (PIV)

Visualization of Microflows

References: 1. A particle image velocimetry system for microfluidics, Santiago, J.G et al.

Experiments in Fluids, 25, pp. 316-319. (1998)

2. PIV measurements of a microchannel flow, Meinhart et al. Experiments in Fluids, 27, pp. 414-419 (1999)

3. J.I. Molho, A.E. Herr, T.W. Kenny, M.G. Mungal, P.M. St.John, M.G. Garguilo, P.H . Paul, M. Deshpande, and J.R. Gilbert, "Fluid Transport Mechanisms in Microflui dic Devices", Micro-Electro-Mechanical Systems (MEMS), 1998 ASME International Mechanical Engineering Congress and Exposition (DSC-Vol.66) 

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•Fluorescent dye chemically locked in a stable molecule until hit with Nd:YAG laser which “uncages” it.

•Uncaged dye is pumped with Microblue diode pumped laser.

•Fluorescence is imaged with CCD camera.(Molho. Et.at. 1998)

Caged Fluorescence

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Results

Experiment matches prediction for uniform “plug flow” for some cases studied.

No discernable boundary layers, but some diffusion.http://microfluidics.stanford.edu/caged.htm

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More Results

In other cases though, flow looks very much like a pressure-driven Poiseuille flow

Electro-Kinetic Flow can actually induce a pressure gradient in a capillary flow and thus alter the basic flow structurehttp://microfluidics.stanford.edu/caged.htm

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Comparison with CFD

Electro-Osmotic flow is relatively simple to model with standard CFD solvers.

For pressure driven micro-capillary flow, CFD predicts flow field remarkably well, as shown in comparison of experimental and computational results at left.(Molho et.al. 1998)

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Particle Image Velocimetry (PIV)

Cross-correlation

Velocity vector

Raw velocity field Mean velocity subtractedTurbulent velocity field

Particle fields1024 x 1024 pixels

21 x 21 mm

Interrogation windows 32x32 pixels, 0.6 x 0.6 mm

• Seed flow with particles– Don’t affect fluid characteristics– Accurately follow the flow

• Illuminate flow at two time instances separated by t (e.g. using Nd:YAG laser)• Record images of particle fields (e.g. CCD camera)• Determine particle displacement• Calculate velocity as V x/ t

Images from Tsurikov and Clemens (2002)

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The Need for -PIV

• The physics is not very clear in micro flows (e.g. surface tension)

• Typical length scales of 1-100 m, traditional flow diagnostics cannot be employed

• Most micro-flow measurements were limited to bulk properties of the flow like wall pressure and bulk velocity

• PIV enables measurements of velocity field in two dimensions

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Other efforts

• Particle streak imaging by Brody et al. (1996)

– Less accurate than pulsed velocimetry measurements

• Lanzilloto et al. (1997) used X-ray micro-imaging of emulsion droplets

– Emulsion is deformable, large and not a good tracker of the flowfield

• Optical Doppler Tomographic imaging by Chen et al. (1997) using Michelson interferometry

– Single point measurement

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-PIV

• Particles used must be small enough to

– Follow the flow

– Should not clog the device

• They must also be large enough to

– Emit sufficient light

– Sufficiently damp out Brownian motion

• Particles are tagged with a fluorescent dye; hence actually imaging the fluorescence

– Elastic scattering measurements are more difficult to employ in the micro-scale

– Inelastic scattering like fluorescence can be readily filtered out

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• Errors in measurement due to Brownian motion when measuring velocities of 10 m/sec

• Error induced by Brownian motion sets a lower limit on the time separation between the images

t

D

ux

sB

21

2/12

-PIV

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First -PIV system

• Essentially a microscope imaging fluorescence from the seed particles

From Santiago et al. (1998)

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State of the art -PIV system

http://microfluidics.stanford.edu/piv.htm

From Meinhart et al. (1999)

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Demonstration of -PIV

• Hele-Shaw flow (Re=3e-4) – used the first -PIV system discussed before

• Micro-channel flow– Uses the laser based system

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Velocity fields: Hele-Shaw

• Shows instantaneous and average images• Effect of Brownian motion goes away on averaging• Spatial resolution 6.9 m x 6.9 m x 1.5 m

From Santiago et al. (1998)

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From Meinhart et al. (1999)

Velocity Fields in a Micro-channel

• Shows mean velocity profiles in a micro-channel

• Measurements agree within 2% to analytical solutions

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Comparison to analytical solution

From Meinhart et al. (1999)

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Infrared Thermometry 1-10 m*

Laser Surface Reflectance 1 m*

Raman Spectroscopy 1 m*

Liquid Crystals 1 m*

Near-Field Optical Thermometry < 100 nm

Scanning Thermal Microscopy (SThM) < 100 nm

Techniques Spatial Resolution

*Diffraction limit for far-field optics

Thermometry of Nanoelectronics

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X-Y-Z Actuator

Scanning Thermal Microscopy

Sample

Temperature sensor

Laser

Atomic Force Microscope (AFM) + Thermal Probe

CantileverDeflectionSensing

Thermal

X

TTopographic

X

Z

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Microfabricated Thermal Probes

Pt-Cr Junction

Shi, Kwon, Miner, Majumdar, J. MicroElectroMechanical Sys., 10, p. 370 (2001)

10 m

Pt Line

Cr Line

TipLaser Reflector

SiNx Cantilever

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Thermal Imaging of Nanotubes

Multiwall Carbon Nanotube

1 m

Topography

1 m

Topography

3 V88 A

Distance (nm)

Th

erm

al s

ign

al ( V

) 30

20

10

0

4002000-200-400

50 nm

Distance (nm)

Hei

ght

(nm

)

30 nm

10

5

0

4002000-200-400

Distance (nm)

Hei

ght

(nm

)

30 nm

10

5

0

4002000-200-400

Thermal

Shi, Plyosunov, Bachtold, McEuen, Majumdar, Appl. Phys. Lett., 77, p. 4295 (2000)

Spatial Resolution

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Metallic Single Wall Nanotube

-20

0

20

200010000-1000-2000

Bias voltage (mV)

Cu

rren

t (

A)

AB C D

Bias voltage (mV)

Cu

rren

t (

A)

AB C D

Topographic Thermal

1 m

A B C D

Low bias:

Ballistic

High bias:

Dissipative (optical phonon emission)

Ttip

2 K

0

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Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

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Ideal MOSFET

VG>0

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Pinch-Off & IV

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Thermal Circuit

Particle transport theory

Fourier’s law of heat conduction

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Joule Heating inHigh-Field Devices

Localized heat generationnear the pinch-off point

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SiGe Devices

Future Challenge:Temperature Mapping of NanotransistorsSOI Devices

• Low thermal conductivities of SiO2 and SiGe

• Interface thermal resistance• Short (10-100 nm) channel effects (ballistic transport, quantum transport)• Phonon “bottleneck” (optical-acoustic phonon decay length > channel length)• Few thermal measurements are available to verify simulation results