1. 1 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 78712 www.me.utexas.edu/~lishi lishi@mail.utexas.edu

2. 2 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)

3. 3 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

4. 4 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

5. 5 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 structure http://microfluidics.stanford.edu/caged.htm

6. 6 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)

7. 7 Particle Image Velocimetry (PIV) Cross-correlation Velocity vector Raw velocity fieldMean velocity subtracted Turbulent velocity field Particle fields 1024 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)

8. 8 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

9. 9 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

10. 10  -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

11. 11 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  -PIV

12. 12 First  -PIV system Essentially a microscope imaging fluorescence from the seed particles From Santiago et al. (1998)

13. 13 State of the art  -PIV system http://microfluidics.stanford.edu/piv.htm From Meinhart et al. (1999)

14. 14 Demonstration of  -PIV Hele-Shaw flow (Re=3e-4) –used the first  -PIV system discussed before Micro-channel flow –Uses the laser based system

15. 15 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)

16. 16 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

17. 17 Comparison to analytical solution From Meinhart et al. (1999)

18. 18 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

19. 19 X-Y-Z Actuator Scanning Thermal Microscopy Sample Temperature sensor Laser Atomic Force Microscope (AFM) + Thermal Probe Cantilever Deflection Sensing Thermal X T Topographic X Z

20. 20 Microfabricated Thermal Probes Pt-Cr Junction Shi, Kwon, Miner, Majumdar, J. MicroElectroMechanical Sys., 10, p. 370 (2001) 10  m Pt Line Cr Line Tip Laser Reflector SiN x Cantilever

21. 21 Thermal Imaging of Nanotubes Multiwall Carbon Nanotube 1  m Topography 1  m 3 V 88  A Distance (nm) Thermal signal (  V) 30 20 10 0 4002000-200-400 50 nm Distance (nm) Height (nm) 30 nm 10 5 0 4002000-200-400 Distance (nm) Height (nm) 30 nm 10 5 0 4002000-200-400 Thermal Shi, Plyosunov, Bachtold, McEuen, Majumdar, Appl. Phys. Lett., 77, p. 4295 (2000) Spatial Resolution

22. 22 Metallic Single Wall Nanotube TopographicThermal 1  m ABCD Low bias: Ballistic High bias: Dissipative (optical phonon emission)  T tip 2 K 0

23. 23 Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

24. 24 Ideal MOSFET V G >0

25. 25 Pinch-Off & IV

26. 26 Thermal Circuit Particle transport theory Fourier ’ s law of heat conduction

27. 27 Joule Heating in High-Field Devices Localized heat generation near the pinch-off point

28. 28 SiGe Devices Future Challenge: Temperature Mapping of Nanotransistors SOI Devices Low thermal conductivities of SiO 2 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