1 me 381r fall lecture 24: micro-nano scale thermal-fluid measurement techniques dr. li shi...
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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
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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