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IN THIS ISSUE

### Foreword

J. Heat Transfer. 2013;135(9):090501-090501-1. doi:10.1115/1.4024353.

The third ASME Micro/Nanoscale Heat and Mass Transfer International Conference (MNHMT2012) was held in Atlanta, GA on March 3–6, 2012 (http://www.asmeconferences.org/MNHMT2012). The conference was cosponsored by the ASME Heat Transfer Division and Georgia Institute of Technology and was organized by G. P. “Bud” Peterson as the General Chair and Zhuomin Zhang as the Program Chair. This conference series is dedicated to Dr. Chang-Lin Tien (1935–2002), a world renowned scholar, a leader in higher education, and a close friend and colleague. Professor Tien’s tremendous intellect and unique vision have continued to inspire researchers to expand the frontiers of micro/nanoscale heat and mass transfer. The first two conferences were held in Tainan (January 2008) and Shanghai (December 2009).

Commentary by Dr. Valentin Fuster

### Research Papers

J. Heat Transfer. 2013;135(9):091001-091001-18. doi:10.1115/1.4024354.

Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091101-091101-9. doi:10.1115/1.4024355.

We compare two methods for the calculation of mode dependent ballistic phonon transmission in nanoscale waveguides. The first method is based on continuum acoustic waveguide theory and uses an eigenmode expansion to solve for phonon transmission coefficients. The second method uses lattice dynamics (LD)-computed mode shapes to excite guided phonon wavepackets in a nonequilibrium molecular dynamics (MD) simulation and calculates phonon transmission from the final distribution of system energy. The two methods are compared for the case of shear-horizontal (SH) phonons propagating in a planar waveguide with a T-stub irregularity, a geometry which has been proposed for the tuning of phonon transmission and nanostructure thermal conductance. Our comparison highlights advantages and disadvantages of the two methods and illustrates regimes when atomistic effects are prominent and continuum approaches are not appropriate.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091102-091102-6. doi:10.1115/1.4024356.

Thermal properties and transport control are important for many applications, for example, low thermal conductivity is desirable for thermoelectrics. Knowledge of mode-wise phonon properties is crucial to identify dominant phonon modes for thermal transport and to design effective phonon barriers for thermal transport control. In this paper, we adopt time-domain (TD) and frequency-domain (FD) normal-mode analyses to investigate mode-wise phonon properties and to calculate phonon dispersion relations and phonon relaxation times in bismuth telluride. Our simulation results agree with the previously reported data obtained from ultrafast time-resolved measurements. By combining frequency-dependent anharmonic phonon group velocities and lifetimes, mode-wise thermal conductivities are predicted to reveal the contributions of heat carriers with different wavelengths and polarizations.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091103-091103-7. doi:10.1115/1.4024357.

Accurate thermal conductivity values are essential for the successful modeling, design, and thermal management of microelectromechanical systems (MEMS) and devices. However, the experimental technique best suited to measure the thermal conductivity of these systems, as well as the thermal conductivity itself, varies with the device materials, fabrication processes, geometry, and operating conditions. In this study, the thermal conductivities of boron doped single-crystal silicon microbridges fabricated using silicon-on-insulator (SOI) wafers are measured over the temperature range from 80 to 350 K. The microbridges are 4.6 mm long, 125 μm tall, and either 50 or 85 μm wide. Measurements on the 85 μm wide microbridges are made using both steady-state electrical resistance thermometry (SSERT) and optical time-domain thermoreflectance (TDTR). A thermal conductivity of 77 Wm−1 K−1 is measured for both microbridge widths at room temperature, where the results of both experimental techniques agree. However, increasing discrepancies between the thermal conductivities measured by each technique are found with decreasing temperatures below 300 K. The reduction in thermal conductivity measured by TDTR is primarily attributed to a ballistic thermal resistance contributed by phonons with mean free paths larger than the TDTR pump beam diameter. Boltzmann transport equation (BTE) modeling under the relaxation time approximation (RTA) is used to investigate the discrepancies and emphasizes the role of different interaction volumes in explaining the underprediction of TDTR measurements.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091201-091201-8. doi:10.1115/1.4024359.

Methods used to measure the temperature of AlxGa1−xN based ultraviolet light emitting diodes (UV LEDs) are based on optical or electrical phenomena that are sensitive to either local, surface, or average temperatures within the LED. A comparative study of the temperature rise of AlxGa1−xN UV LEDs measured by micro-Raman spectroscopy, infrared (IR) thermography, and the forward voltage method is presented. Experimental temperature measurements are provided for UV LEDs with micropixel and interdigitated contact geometries, as well as for a number of different packaging configurations. It was found that IR spectroscopy was sensitive to optical properties of the device layers, while forward voltage method provided higher temperatures, in general. Raman spectroscopy was used to measure specific layers within the LED, showing that growth substrate temperatures in the flip-chip LEDs agreed more closely to IR measurements while layers closer to the multiple quantum wells (MQWs) agreed more closely with Forward Voltage measurements.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091202-091202-7. doi:10.1115/1.4024389.

Laser micromachining of an aluminum film on a glass substrate is investigated using a time-resolved transmission imaging technique with nanosecond resolution. Micromachining is performed using a 7 ns pulse-width Nd:YAG laser operating at the 1064 nm wavelength for fluences ranging from 2.2 to 14.5 J/cm2. A nitrogen laser-pumped dye laser with a 3 ns pulse-width and 500 nm wavelength is used as a light source for visualizing the transient hole area. The dye laser is incident on the free surface and a CCD camera behind the sample captures the transmitted light. Images are taken from the back of the sample at various time delays with respect to the beginning of the ablation process, allowing the transient hole area to be measured. For low fluences, the hole opening process is delayed long after the laser pulse and there is significant scatter in the data due to weak driving forces for hole opening. However, for fluences at and above 3.5 J/cm2, the starting time of the process converges to a limiting minimum value of 12 ns, independent of laser fluence. At these fluences, the rate of hole opening is rapid, with the major portion of the holes opened within 25 ns. The second stage of the process is slower and lasts between 100 and 200 ns. The rapid hole opening process at high fluences can be attributed to recoil pressure from explosive phase change. Measurements of the transient shock wave position using the imaging apparatus in shadowgraph mode are used to estimate the pressure behind the shock wave. Recoil pressure estimates indicate pressure values over 90 atm at the highest fluence, which decays rapidly with time due to expansion of the ablation plume. The recoil pressure for all fluences above 3.1 J/cm2 is higher than that required for recoil pressure driven flow due to the transition to explosive phase change above this fluence.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091301-091301-8. doi:10.1115/1.4024390.

This paper describes results from an experimental study of the effect of an electric field on nucleate boiling and the critical heat flux (CHF) in pool boiling at atmospheric pressure. A dielectric liquid of HFE-7100 (3 M Co.) was used as working fluid. A heating surface was polished with the surface roughness (Ra) of 0.05 μm. A microsized electrode, in which the slits were provided, was designed in order to generate non uniform high electric fields and to produce electrohydrodynamic (EHD) effects with the application of high voltages. The obtained results confirmed the enhancement of CHF since the EHD effects increased the CHF to 47 W/cm2 at the voltage of −1500 V, which was three times as much as CHF for the free convection boiling. From the observations of the behavior of bubbles over the electrode and of the boiling surface condition, the instability between the liquid and the vapor increased the heat flux, the heat transfer coefficient (HTC), and the CHF. The usual traveling wave on the bubble interface induced by the Kelvin-Helmholtz instability was modified by adding the EHD effects. The ratio of critical heat flux increase with and without the electric field was sufficiently predicted by the frequency ratio of liquid–vapor surface at the gap between the boiling surface and the electrode.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091302-091302-8. doi:10.1115/1.4024422.

With regard to potential application in pressurized water reactors (PWRs), a nanoporous heated surface was tested in pool boiling of an aqueous solution of boric acid (H3BO3), or borated water (1% volume concentration). The effect of system pressure and surface orientation on pool boiling heat transfer (BHT) was studied. The nanoporous surface consisted of a coating of alumina nanoparticles applied on a 1 cm2 flat copper surface through nanofluid boiling. An uncoated surface in borated water was similarly tested, and due to boric acid deposition, the BHT degraded and the critical heat flux (CHF) enhanced relative to pure water. Also, the possibility of transient pool boiling behavior of borated water was investigated but none was detected. With pressure and orientation variation, the nanoporous surface imposed on borated water showed a trend of further CHF enhancement to the CHF limit produced by the nanoporous surface in pure water. Over the nanoporous surface, the CHF of borated water was increasingly better with decreasing pressure, than that over the plain surface. However, BHT degraded slightly further. Boric acid deposition over the nanoporous surface was believed to be the source of this BHT degradation, but played no apparent role in the further CHF enhancement.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091303-091303-6. doi:10.1115/1.4024423.

In this work, thermophysical properties, microstructure, and pool boiling characteristics of water-in-polyalphaolefin (PAO) nanoemulsion fluids have been measured in the water concentration range of 0–10.3 vol. %, in order to gain basic data for nanoemulsion boiling. Water-in-PAO nanoemulsion fluids are formed via self-assembly with surfactant: sodium sullfosuccinate (AOT). Thermal conductivity of these fluids is found to increase monotonically with water concentration, as expected from the Maxwell equation. Unlike thermal conductivity, their dynamic viscosity first increases with water concentration, reaches a maximum at 5.3 vol. %, and then decreases. The observed maximum viscosity could be attributed to the attractive forces among water droplets. The microstructures of the water-in-PAO nanoemulsion fluids are measured via the small-angle neutron scattering (SANS) technique, which shows a transition from sphere to elongated cylinder when the water concentration increases above 5.3 vol. %. The pool boiling heat transfer of these water-in-PAO nanoemulsion fluids is measured on a horizontal Pt wire at room temperature (25 °C, subcooled condition). One interesting phenomenon observed is that the pool boiling follows two different curves randomly when the water concentration is in the range of 5.3 vol. % to 7.8 vol. %.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091304-091304-12. doi:10.1115/1.4024424.

Condensation is an important process in both emerging and traditional power generation and water desalination technologies. Superhydrophobic nanostructures promise enhanced condensation heat transfer by reducing the characteristic size of departing droplets via coalescence-induced shedding. In this work, we investigated a scalable synthesis technique to produce functionalized oxide nanostructures on copper surfaces capable of sustaining superhydrophobic condensation and characterized the growth and departure behavior of the condensed droplets. Nanostructured copper oxide (CuO) films were formed via chemical oxidation in an alkaline solution resulting in dense arrays of sharp CuO nanostructures with characteristic heights and widths of ≈1 μm and ≈300 nm, respectively. To make the CuO surfaces superhydrophobic, they were functionalized by direct deposition of a fluorinated silane molecular film or by sputtering a thin gold film before depositing a fluorinated thiol molecular film. Condensation on these surfaces was characterized using optical microscopy and environmental scanning electron microscopy to quantify the distribution of nucleation sites and elucidate the growth behavior of individual droplets with characteristic radii of ≈1–10 μm at supersaturations ≤1.5. Comparison of the measured individual droplet growth behavior to our developed heat transfer model for condensation on superhydrophobic surfaces showed good agreement. Prediction of the overall heat transfer enhancement in comparison to a typical dropwise condensing surface having an identical nucleation density suggests a restricted regime of enhancement limited to droplet shedding radii $<~$2.5 μm due to the large apparent contact angles of condensed droplets on the fabricated CuO surfaces. The findings demonstrate that superhydrophobic condensation typified by coalescence-induced droplet shedding may not necessarily enhance heat transfer and highlights the need for further quantification of the effects of surface structure on nucleation density and careful surface design to minimize parasitic thermal resistances.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091401-091401-10. doi:10.1115/1.4024425.

An extension of a recently proposed single domain formulation of conjugated conduction–convection heat transfer problems is presented, taking into account the axial diffusion effects at both the walls and fluid regions, which are often of relevance in microchannels flows. The single domain formulation simultaneously models the heat transfer phenomena at both the fluid stream and the channel walls, by making use of coefficients represented as space variable functions, with abrupt transitions occurring at the fluid-wall interface. The generalized integral transform technique (GITT) is then employed in the hybrid numerical–analytical solution of the resulting convection–diffusion problem with variable coefficients. With axial diffusion included in the formulation, a nonclassical eigenvalue problem may be preferred in the solution procedure, which is itself handled with the GITT. To allow for critical comparisons against the results obtained by means of this alternative solution path, we have also proposed a more direct solution involving a pseudotransient term, but with the aid of a classical Sturm-Liouville eigenvalue problem. The fully converged results confirm the adequacy of this single domain approach in handling conjugated heat transfer problems in microchannels, when axial diffusion effects must be accounted for.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091402-091402-8. doi:10.1115/1.4024426.

Temperature gradient focusing (TGF) is a highly efficient focusing technique for the concentration and separation of charged analytes in microfluidic channels. The design of an appropriate temperature gradient is very important for the focusing efficiency. In this study, we proposed a new technique to generate the temperature gradient. This technique utilizes a microchannel filled with liquid-metal as an electrical heater in a microfluidic chip. By applying an electric current, the liquid-metal heater generates Joule heat, forming the temperature gradient in the microchannel. To optimize the temperature gradient and find out the optimal design for the TGF chip, numerical simulations on four typical designs were studied. The results showed that design 1 can provide a best focusing method, which has the largest temperature gradient. For this best design, the temperature is almost linearly distributed along the focusing microchannel. The numerical simulations were then validated both theoretically and experimentally. The following experiment and theoretical analysis on the best design also provide a useful guidance for designing and fabricating the liquid-metal based TGF microchip.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091403-091403-7. doi:10.1115/1.4024461.

Unexpected gas bubbles in microfluidic devices always bring the problems of clogging, performance deterioration, and even device functional failure. For this reason, the aim of this paper is to study the characterization variation of a valveless micropump under different existence conditions of gas bubbles based on a theoretical modeling, numerical simulation, and experiment. In the theoretical model, we couple the vibration of piezoelectric diaphragm, the pressure drop of the nozzle/diffuser and the compressibility of working liquid when gas bubbles are entrapped. To validate the theoretical model, numerical simulation and experimental studies are carried out to investigate the variation of the pump chamber pressure influenced by the gas bubbles. Based on the numerical simulation and the experimental data, the outlet flow rates of the micropump with different size of trapped gas bubbles are calculated and compared, which suggests the influence of the gas bubbles on the dynamic characterization of the valveless micropump.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091501-091501-8. doi:10.1115/1.4024464.

This work is a statistical study of the broadband light absorption in thin film solar cells, enhanced by metallic surface nanotexturing. We consider optimum grating structures on the surface of amorphous silicon solar cells obtained by inverse optimization, and study the joint statistics of the resulting absorption enhancement/spectra in the presence of time and structural variants, such as fabrication error and year around changes in the solar irradiance, as well as the angle of incident. We adopt yearly data for solar irradiation at individual hours. In conjunction with the data for light absorption spectra at various incident angles and random samples of the fabrication error vector, we evaluate the real world performance of optimized solar cells. The resulting conclusions serve as a sensitivity/time analysis for better understanding the limits of performance and robustness of thin film cells and optimal light trapping mechanisms.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091502-091502-6. doi:10.1115/1.4024459.

A tunable metamaterial is proposed by combining a thermochromic oxide with a fishnet structure. The reflection and transmission coefficients are calculated by finite-difference time-domain (FDTD) method. The effective electromagnetic parameters of the metamaterial are retrieved on the basis of these data. The results reveal that an effective negative refractive index is obtained by this proposed structure and the wavelength region with negative refractive index can be self-regulated by simply tuning the temperature, which is of importance to extend the applications of negative refractive index materials. The effects of structural sizes on the negative refractive index are discussed. The size-dependence indicates that the wavelength range in which the apparent refractive index is negative can be tuned to be located at the desired position by dexterously tailoring the structural parameters.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091503-091503-6. doi:10.1115/1.4024466.

Thermal and optical engineering applications of electromagnetic wave scattering from rough surfaces include temperature measurement, radiation heating process, etc. Most of the surfaces have random roughness and are often with coating material different from the substrate. However, the understanding of radiative properties of coated rough surfaces is not well addressed at this point. This paper presented a novel hybrid partial coherence and geometry optics (HPCGO) model to improve the generic geometry optics (GO) prediction by incorporating a previously developed partial coherence reflectance equation. In this way, HPCGO expands the applicable region of GO model and largely reduces the computation time of integrating different wavelength results in the regular hybrid model that considers coherence effect only. In this study, the HPCGO model is first compared with the more rigorous Maxwell equations solvers, the finite-difference time-domain (FDTD) method, and integral equation (IE) method. Then, the HPCGO model is applied to study the coherent effect of directional-hemispherical reflectance from coated rough surfaces. It is found the roughness of coated rough surface can cause partially coherent or noncoherent scattered light even if the incident light source is coherent. It also shows the reflected electromagnetic wave's coherence effect reduces with increased coating thickness and surface roughness, besides the previously recognized incident wave-number bandwidth. The effect of reduce coherence in scattered wave is quantified. Finally a regime map, even limited in the roughness and coating thickness dimensionless parameter ranges, provides the region of validity of the HPCGO model.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091504-091504-6. doi:10.1115/1.4024468.

Spectral and directional control of thermal emission holds substantial importance in different kinds of applications, where heat transfer is predominantly by thermal radiation. Several configurations have previously been proposed, like using gratings, photonic crystals (PCs) and resonant cavities. In the present work, we investigate the thermal radiative properties of a microstructure consisting of a SiC grating on a photonic crystal. The emissivity of the microstructure is calculated with the rigorous coupled-wave analysis (RCWA) algorithm as a function of the angular frequency and the emission angle. The results reveal that thermal emission from the microstructure can exhibit very novel feature compared to those previously studied. Especially, significantly enhanced thermal emission can be achieved in a broad spectral band due to excitation of surface photon polaritons (SPhPs), PC modes, magnetic polaritons (MPs) and the coupling between them. We show that it is possible to flexibly control the thermal emission feature by adjusting the microstructure's dimensional parameters properly.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091505-091505-9. doi:10.1115/1.4024469.

Spectral and directional control of thermal emission is critically important for applications such as space cooling and energy harvesting. The effect of magnetic polaritons (MPs) on spectral modulation has been analyzed in metallic grating structures with a dielectric spacer on a metallic film. It has been predicted that the spectral emission peaks exhibit omnidirectional characteristics when MPs are excited. The present work provides an experimental demonstration of coherent thermal emission from several microfabricated grating structures in the infrared region from room temperature to elevated temperatures. The emittance at elevated temperatures is directly measured using an emissometer, while the room-temperature emittance is indirectly obtained from the reflectance measurement. The rigorous coupled-wave analysis and an LC-circuit model are employed to elucidate the mechanisms of various resonant modes and their coupling effect, taking into consideration the temperature-dependent electron scattering rate of the metals.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Heat Transfer. 2013;135(9):091601-091601-5. doi:10.1115/1.4024462.

Gas flow through arrays of rectangular nanofins is modeled using the linearized free-molecular drag and heat transfer equations. These are combined with the one-dimensional equations for conservation of mass, momentum, and energy, and the ideal gas law, to find the governing equations for flow through the array. The results show that the pressure gradient, temperature, and local velocity of the gas are governed by coupled ordinary differential equations. The system of equations is solved for representative arrays of nanofins to find the total heat transfer and pressure drop across a 1 cm chip.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(9):091602-091602-4. doi:10.1115/1.4024465.

The paper reports calculations of friction pressure gradient for the special case of laminar annular flow condensation in microchannels. This is the only flow regime permitting theoretical solution without having recourse to experimental data. Comparisons are made with correlations based on experimental data for R134a. The correlations differ somewhat among themselves with the ratio of highest to lowest predicted friction pressure gradient typically around 1.4 and nearer to unity at high quality. The friction pressure gradients given by the laminar annular flow solutions are in fair agreement with the correlations at high quality and lower than the correlations at lower quality. Attention is drawn to the fact that the friction pressure gradient cannot be directly observed and its evaluation from measurements requires estimation of the nondissipative momentum or acceleration pressure gradient. Methods used to estimate the nondissipative pressure gradient require quality and void fraction together with equations which relate these and whose accuracy is difficult to quantify. Quality and void fraction can be readily found from the laminar annular flow solutions. Significant differences are found between these and values from approximate equations.

Commentary by Dr. Valentin Fuster