Pamela Norris, PhD, Professor, Department of Mechanical and Aerospace Engineering, University of Virginia
Scattering Events Affecting Transport Around Solid Interfaces in Nanomaterial Systems

The ongoing trend of miniaturization at the nanoscale has created many new thermal challenges for device engineers and scientists. As transistors shrink to length scales on the order of carrier mean-free-paths, the solid/solid interfaces between the transistors and the packaging systems are responsible for the major thermal resistances that are impeding thermal dissipation. Development of an understanding of the fundamental processes that contribute to thermal boundary conductance (h_BD) is an important area for nanomaterial system design and engineering.

This presentation will overview research on the scattering processes that contribute to h_BD at solid interfaces. In particular, three different assumptions/phenomena are investigated using results from transient thermoreflectance (TTR) measurements.

• Phonon interfacial transport is typically assumed to occur at a perfectly abrupt interface while most real interfaces demonstrate some degree of interatomic diffusion and mixing. Results from a series of Cr/Si samples that were fabricated subject to differing deposition conditions are given to examine the effects of atomic mixing on h_BD.
• A common assumption in predicting phonon interfacial transport is that phonons can only transmit energy across the interface by scattering with a phonon of the same frequency - i.e., elastic scattering, which can lead to an order of magnitude under-prediction of h_BD. A series of metal/dielectric interfaces with a wide range of vibrational similarity were studied at temperatures above and around the Debye temperatures of the materials to examine the effects of inelastic scattering on h_BD. Models are presented that predict h_BD and its relative dependency on elastic and inelastic scattering events.
• High frequency microelectronic devices often experience an electron-phonon nonequilibrium, which can add another form of resistance to thermal dissipation. As material length scales decrease to the nanoscale, interfacial transport can affect the rate at which the electrons and phonons re-equilibrate. The electron-phonon coupling factor was measured in several Au films with varying thicknesses and substrates, and subjected to differing laser fluences to study the effect that h_BD has on electron-phonon equilibration.
• Interfacial effects in the electron-phonon coupling measurements are described and are quantitatively explained with a three temperature model.

Dongqing Li, PhD, H. Fort Flowers Professor, Department of Mechanical Engineering, Vanderbilt University
Applications of Electrokinetic Microfluidics in Lab-on-a-Chip Devices

Imagine holding a business-card-sized, fully functional biomedical diagnostic lab in your hand! A lab-on-a-chip (LOC) is a miniaturized biomedical laboratory built on a thin glass or plastic plate with a network of microchannels, electrodes, sensors and electronic circuits. The lab-on-a-chip devices can duplicate the specialized functions as their room-sized counterparts, such as clinical diagnoses of bacteria, viruses and cancers. The advantages of these labs on a chip include significantly reduced sample/reagent consumption, very short analysis time, high throughput, automation and portability.

The key microfluidic functions required in lab-on-a-chip devices include pumping and mixing liquids, controlling bio-reactions, dispensing samples or reagents, and separating molecules or particles. Essentially all on-chip microfluidic processes are electrokinetic processes. Basic understanding, modeling and controlling of these microfluidic processes are essential to systematic design and operation control of the lab-on-a-chip systems. This presentation will explain the principles of these electrokinetic microfluidic processes and how they are used in lab-on-a-chip devices. Some fundamental research in microfluidics and several types of lab-on-a-chip devices such as real-time PCR chip, DNA sensor chip, Immunoassay chip developed and cellular lab-on-a-chip will be introduced.

Yogesh Jaluria, PhD, Board of Governors Professor, Mechanical and Aerospace Engineering Department, Rutgers University
Microscale Transport Phenomena in Materials Processing

Microscale transport mechanisms play a critical role in the thermal processing of materials because changes in the structure and characteristics of the material largely occur at these or smaller length scales. The heat transfer and fluid flow considerations determine the properties of the final product, such as in a crystal drawn from silicon melt or a gel from the chemical conversion of a biopolymer. Also, a wide variety of material fabrication processes, such as the manufacture of optical glass fiber for telecommunications, fabrication of thin films by chemical vapor deposition and surface coating, involve microscale length scales due to the requirements on the devices and applications for which they are intended. For example, hollow fibers, which are used for sensors and power delivery, typically need fairly precise micro-scale wall thicknesses and hole diameters for satisfactory operation. The basic transport mechanisms underlying these processes are discussed. The importance of material characterization in accurate modeling and experimentation is brought out, along with the coupling between the process and the resulting properties such as uniformity, concentricity and diameter. Of particular interest are thermally induced defects and other imperfections that may arise due to the transport phenomena involved. Additional aspects such as surface tension, stability, and free surface characteristics are also discussed. Some of the important methods to treat these problems and challenges are presented. Characteristic numerical and experimental results are discussed for some new and emerging areas. The implications of such results in improving practical systems and processes, including enhanced process feasibility and product quality, are also discussed.

Ping Cheng, PhD, Chair Professor and Director, Engineering Thermophysics, School of Mechanical and Power Engineering, Shanghai Jiaotong University
Boiling and Condensation in Microchannels

The unique characteristics of flow boiling and condensation processes in a microchannel under heating or cooling conditions are discussed in this paper. It has found that stable and unstable flow boiling modes exit in a microchannel. Stable flow boiling mode with constant temperature variations exits when the size of the nucleated bubble at the exit is less than that of the microchannel diameter, while unstable boiling mode exists when the size of the bubble at the exit is greater than the microchannel diameter. The latter is owing to the fact that when a bubble grows to the size of the microchannel, it will expand in both upstream and downstream directions. Subsequently, the reversed flow of vapor bubble is swept away by the incoming subcooled liquid, leading to large cyclic fluctuations in temperature and pressure. It is found that the amplitude and frequency of these fluctuations depend greatly on the inlet/outlet configurations and the exit quality. By fabrication of inlet restriction on each of the microchannel, it is found that the reversed flow of vapor can be suppressed, resulting in a stable flow boiling mode. Boiling heat transfer coefficient and pressure drop in a microchannel have been obtained under stable flow boiling conditions. These data are found to be substantially different from the correlations obtained based on flow boiling in macrochannels. For condensation in a microchannel, it was found that mist flow, annular flow, injection flow, slug/bubbly flow exist in a microchannel depends on mass flux, condensation heat flux, and the location in the microchannel. The occurrence of the injection flow is owing to the instability of the liquid/vapor interface because of the surface tension effects is predominant. The location at which the injection occurs depends on the mass flux and the cooling rate of steam. It is also found that increase of steam mass flux, decrease of cooling rate, or decrease of the microchannel diameter tends to enhance instability of the condensate film on the wall, resulting in occurrence of the injection flow further toward the outlet with an increase in occurrence frequency. At low mass fluxes, the pressure drop and heat transfer data obtained for microchannels are substantially different from the correlation equations obtained from condensation in macrochannels because of different flow patterns.

Steve Choi, PhD, Professor and KIER Fellow, Korea Institute of Energy Research & University of Illinois-Chicago, High Efficiency Energy Research Department
Nanofluids: From Vision to Reality Through Research

Nanofluids are a new class of nanotechnology-based heat transfer fluids engineered by dispersing and stably suspending nanoparticles with typical length scales on the order of 1 to 50 nm in traditional heat transfer fluids. During the past decade, pioneering scientists and engineers have made phenomenal discoveries that a very small amount (< 1 volume %) of guest nanoparticles can provide dramatic improvements in the thermal properties of the host fluids. For example, some nanofluids exhibit superior thermal properties such as anomalously high thermal conductivity at low nanoparticle concentrations, strong temperature- and size-dependent thermal conductivity, a nonlinear relationship between thermal conductivity and concentration, and a three-fold increase in the critical heat flux at a small particle concentration of the order of 10 ppm. Nanofluids are of great scientific interest because these unprecedented thermal transport phenomena surpass the fundamental limits of conventional macroscopic theories of suspensions. Therefore, numerous mechanisms and models have been proposed to account for these unexpected, intriguing thermal properties of nanofluids. These discoveries also show that nanofluids technology can provide exciting new opportunities to develop nanotechnology-based coolants for a variety of innovative engineering and medical applications. As a result, the study of nanofluids has emerged as a new field of scientific research and innovative applications. Hence, the subject of nanofluids is of great interest worldwide for basic and applied research. In this talk I will highlight recent advances in this new field of research and show future directions in nanofluids research through which the vision of nanofluids can be turned into reality.