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The electron beam (e-beam) in the scanning electron microscopy (SEM) provides an appealing mobile heating source for thermal metrology with spatial resolution of ∼1 nm, but the lack of systematic quantification of the e-beam heating power limits such application development. Here, we systemically study e-beam heating in LPCVD silicon nitride (SiNx) thin-films with thickness ranging from 200 to 500 nm from both experiments and complementary Monte Carlo simulations using the CASINO software package. There is good agreement about the thickness-dependent e-beam energy absorption of thin-film between modeling predictions and experiments. Using the absorption results, we then demonstrate adapting the e-beam as a quantitative heating source by measuring the thickness-dependent thermal conductivity of SiNx thin-films, with the results validated to within 7% by a separate Joule heating experiment. The results described here will open a new avenue for using SEM e-beams as a mobile heating source for advanced nanoscale thermal metrology development.

We present a new mounting design for thin crystal optics with cryogenic cooling compatibility. We design a crystal geometry with two symmetric strain-relief cuts to mitigate the distortion from mounting. We propose to sputter gold onto the crystal and the holder to ensure excellent thermal contact and sufficient mechanical bonding. The system is analyzed and verified by finite element analysis to have an acceptable level of strain due to mounting. The thermal performance of this mounting scheme is validated in an example cryogenic cooling system and the results indicate a tolerance of power density up to ∼1 kW/mm2.

With dramatically improved brightness and repetition rate, the thermal load for crystal optics in x-ray free-electron laser applications has also significantly increased. To mitigate the thermal load, one effective method is cryogenic cooling. However, the emerging nondiffusive heat transfer phenomenon at cryogenic temperature may cause design failure if overlooked. To evaluate the optical performance of thin crystal optics under thermal load at cryogenic temperature, an integrated numerical tool is presented and applied to characterize the thermal load on thin crystals with nondiffusive phenomena accounted. Significant thermally induced distortion of the rocking curve is observed from numerical simulation, leading to potential seed power reduction in hard x-ray self-seeding application. Cryogenic cooling is confirmed necessary by simulation to handle the thermal load at high repetition rate operation.

Thermal load has been a haunting factor that undermines the brightness and coherence of the self-seeded X-ray free-electron laser. Different from uniformly pulsed mode, in pulse train mode a thermal quasi-steady state of the crystal monochromator may not be reached. This leads to a dynamic thermal distortion of the spectral transmission curves and seed quality degradation. In this paper, the pulse-to-pulse thermal load effects on the spectral transmission curves and seed quality are shown, and some instructive information for the tuning process is provided.

Water bridge dynamics between an atomic force microscopy (AFM) tip and a flat substrate is studied by using a multibody dissipative particle dynamics (MDPD) model. First, the numerical model is validated by comparing the present results of droplet contact angles and liquid bridges with those reported in the literature. Then, the ability of MDPD to capture the meniscus shape and behavior for different operating conditions and geometric parameters is examined for both static and dynamic cases. Hence, several parametric studies and analyses of the AFM tip configuration and its operating conditions are reported. It is found that a critical capillary number of about 0.001 is calculated based on 5% change on the force measurements between the static and dynamic results. It is also demonstrated that the hysteresis behavior in the capillary force exerted on the AFM tip can be successfully predicted by using the MDPD model when the tip approaches or retracts from the substrate. Moreover, there is an excellent agreement in the results of breakup distance for different water bridge volumes between the predictions of the MDPD model and the theory. Also, the hysteresis of capillary force exerted on an AFM tip composed of multibody design is studied. The prediction on the transition of the capillary force vs distance between the AFM tip and the substrate is in good agreement with the experimental results. Therefore, we demonstrate a validated MDPD model which can successfully capture liquid bridge dynamics. This model can be used as a powerful design tool for meniscus manipulation technology, such as dip-pen nanolithography, as well as for studying dynamic, e.g., tapping mode AFM tip, interactions with a liquid bridge.

In this paper, we propose a microscale liquid oscillator using electrowetting-on-dielectric (EWOD). Specifically, a mesoscale liquid bridge (LB) between two horizontal surfaces with EWOD is considered. When EWOD is applied, the solid surface becomes more hydrophilic, and hence the contact angle (CA) is reduced. Following the activation of EWOD, the LB can remain connected or it can break into either symmetric or asymmetric shapes depending on the initial liquid volume and wettability of the two surfaces. The LB dynamics activated by EWOD is studied using the multibody dissipative particle dynamics (MDPD) method. Our numerical results show that the behavior of an LB under EWOD can be interpreted via three modes. In the first mode, the LB does not break after applying EWOD. In the second mode, the LB breaks and does not reform. The third mode happens when, depending on the interplay of the volume of the liquid and CA manipulation, the LB continuously breaks, recoils, and reforms. For asymmetric cases, it was observed that the LB may completely detach from one surface and may not reform. It was also observed that decreasing the wettability of a surface, for cases with a continuous breaking and reformation behavior, increases the connecting time interval and decreases the breaking time interval in one break-reform cycle. The results provided in this investigation facilitate fundamental understanding of LB dynamics and their application for the design of microscale liquid oscillators using EWOD.

Thermal management on lithium-ion batteries is a crucial problem for the performance, lifetime, and safety of electric vehicles (EVs) and hybrid electric vehicles (HEVs). Fire and explosions can be triggered by thermal runaway if the temperature of the lithium-ion batteries is not maintained properly. This work describes a minichannel cooling system designed at the battery module level and the investigation on its efficacy on the mitigation of thermal runaway. Nail penetration was employed to simulate the internal short circuits, which in reality may be caused by vehicle collisions and/or manufacturing defects. Two integrated models were utilized to study thermal runaway: the conjugate heat transfer model and the reaction kinetics model. Numerical simulations were conducted to understand the thermal runaway process and the effects of flow rate, thermal abuse reactions, nail penetration depth, and nail diameter. It is concluded that minichannel cooling at cell level cannot cease thermal runaway in a single cell, but it can prevent battery fratricide due to thermal runaway propagation between cells.

Lithium-ion batteries are widely used for battery electric (all-electric) vehicles (BEV) and hybrid electric vehicles (HEV) due to their high energy and power density. An battery thermal management system (BTMS) is crucial for the performance, lifetime, and safety of lithium-ion batteries. In this paper, a novel design of BTMS based on aluminum minichannel tubes is developed and applied on a single prismatic Li-ion cell under different discharge rates. Parametric studies are conducted to investigate the performance of the BTMS using different flow rates and configurations. With minichannel cooling, the maximum cell temperature at a discharge rate of 1C is less than 27.8 °C, and the temperature difference across the cell is less than 0.80 °C using flow rate at 0.20 L/min, at the expense of 8.69e-6 W pumping power. At higher discharge rates, e.g., 1.5C and 2C, higher flow rates are required to maintain the same temperature rise and temperature difference. The flow rate needed is 0.8 L/min for 1.5C and 2.0 L/min for 2C, while the required pumping power is 4.23e-4 W and 5.27e-3 W, respectively. The uniform temperature distribution (

Many studies have revealed that Fourier’s law becomes invalid in the study of heat transfer in micro/nanoscale systems due to phonon-mediated nondiffusive heat transport. It is necessary to develop high-fidelity non-Fourier models and numerical methods to study nondiffusive heat transfer for both experimental data analysis in nanothermometry and thermal design in micro-/nano-systems. Starting from the phonon Boltzmann transport equation (BTE), we develop an enhanced Gray (EG) model by considering the second-order terms in Taylor expansion. In the proposed enhanced Gray BTE (EG-BTE), two parameters associated with inherent material properties, i.e., the diffusive relaxation time and the ballistic mean free time, are used to characterize the diffusive heat conduction and nondiffusive heat conduction, respectively. While the diffusive relaxation time is calculated directly from the thermal conductivity, the ballistic mean free time of nonmetallic materials can be determined from transient grating experiments. An implicit lattice Boltzmann method (ILBM) is developed to solve the EG-BTE, which is unconditionally stable. The EG-BTE model is validated by comparing predictions of thermal decay processes with transient grating experiments. After being validated, the EG-BTE model is applied to study heat conduction across thin films from ballistic to diffusive regime at room temperature as well as thermal wave propagation in crystal at low temperatures. The numerical results demonstrate that the EG-BTE provides a unified model to describe multiscale heat conduction covering both nondiffusive and diffusive regimes.

High-speed cold spraying of micro-particles, at temperatures well below their melting point, impinging on desired substrates is a useful emerging additive process. One principal element believed to dramatically influence the bonding strength and final coating quality of many metallic cold-spray deposits is the native oxide layer on the surface of the impinging particles and substrate. Here, smoothed-particle hydrodynamics (SPH), a mesh-free, particle-based method, is used to simulate cold-spraying, including the action of oxide layers in the deposition and bonding processes. The effect of increasing particle impact velocity on oxide breakup and subsequent bonding is studied. Particle/substrate bonding is quantified via metallic interaction at the interface and found to continuously improve with increasing velocity well beyond the critical velocity. The coefficient of restitution is also calculated for different oxide layer thicknesses on the particle and the substrate. The effects of particle diameter and oxygen content on the impact process are reported and compared to results from the literature. The particle-based simulation approach is found to reproduce experimentally characterized trends in oxide content effect and provide insight into the mechanisms of oxide breakup during bonding.

Non-uniform thermal load causes performance degradation of crystal X-ray optics. With the development of high-brightness X-ray free-electron lasers, the thermal load on X-ray optics becomes even more severe. To mitigate the thermal load, a quantitative understanding of thermal effects on the optical performance is necessary. We derived an analytical model for monochromator performance under a non-uniform thermal load. This analytical model quantitatively describes the distortion of the rocking curve and attributes different contributions to different factors of thermal load. It provides not only monochromator design insights and considerations, but also a quick estimation of the rocking curve distortion due to thermal load for practical situations such as pump-probe experiments.

Li-ion based energy storage devices have highly temperature dependent characteristics such as performance, life-cycle, efficiency and safety. Large temperature gradient within a cell results in thermal stresses and nonuniform current density leading to accelerated degradation. This adversely affects the life cycle of the cell due to capacity and power fade. There are similar issues due to large temperature variation within a battery pack. Operation of Li-ion cell outside the desirable temperature range also leads to lower efficiency, degradation and safety related issues. Different thermal management approaches have been proposed and demonstrated in past. The present work focuses specifically on minichannel based liquid cooling for conducting a parametric study. Minichannels have been found effective in various thermal management applications due to their simple construction and high convective heat transfer. In past, minichannels have been proposed and used in battery thermal management. However, designing of such systems has been somewhat arbitrary without considering various factors and trade-offs involved. There is a lack of rigorous studies for determining various parameters related to thermal management system that would result in adequate thermal management in a cost-effective manner. In the present work, a comprehensive parametric study has been carried out on the minichannel based liquid cooling for thermal management of Li-ion battery pack. A simplified computationally efficient numerical simulation-based approach has been used to conduct parametric study for optimizing the design and operating parameters of the thermal management system.

Optical trapping and manipulation have emerged as a powerful tool in the biological and physical sciences. In this work, we present a miniature optical tweezers device based on multifocal optical vortex metalens (MOVM). The MOVM is capable of generating multiple focal fields with specific orbital angular momentum at arbitrary position. The optical force of the vortex field exerted on both high-refractive-index particle and low-refractive-index particle are analyzed. The simulation results show that the two kinds of dielectric particles can be trapped simultaneously. Besides, it is also feasible to manipulate plasmonic nanoparticles even under the resonant condition, which is realized by constructing a 4Pi focusing system with metalenses. Moreover, the metalens can be made into an array format that is suitable for trapping and manipulating various nanoparticles with diverse motion behaviors. The work illustrates the potential of such optical tweezers for further development in lab-on-a-chip devices, and may open up new avenues for optical manipulation and their applications in extensive scientific fields.

The detachment of droplets from cylindrical fibers is of fundamental importance for both scientific research and engineering applications. Due to the challenges to determine dynamic contact angles on the fiber surface, the process of the droplet detachment from a fiber is not well understood. In this paper, a multibody dissipative particle dynamics (MDPD) method, a particle-based mesh-free method that can automatically capture the dynamic contact angles through direct modeling of liquid-solid particle interactions, was applied to study the detachment process of a liquid microdroplet from a cylindrical solid fiber pulled by an atomic force microscopy (AFM) tip under a constant velocity. After the validation of the numerical results through comparison with experiments in a benchmark case, the same numerical tool was applied to analyze the droplet detachment mechanisms. Based on the slope of the time history curve for the displacement of the droplet mass center, the detachment process can be divided into six stages. The change of slope in each stage can be explained from the change of surface energy. The results can greatly advance the fundamental understanding of the detachment process of microdroplets from cylindrical fibers.

Manipulation of nanoparticles in solution is of great importance for a wide range of applications in biomedical, environmental, and material sciences. In this work, we present a novel plasmonic tweezers based on metahologram. We show that various kinds of nanoparticles can be stably trapped in a surface plasmon (SP) standing wave generated by the constructive interference between two coherent focusing SPs. The absence of the axial scattering force and the enhanced gradient force enable to avoid overheating effect while maintaining mechanical stability even under the resonant condition of the metallic nanoparticle. The work illustrates the potential of such plasmonic tweezers for further development in lab-on-a-chip devices.

Significant influence from coking (aggregation and formation of a non-desirable carbonaceous porous layer on a solid metal surface) on the heat transfer in a cooling system of hypersonic aircraft with hydrocarbon fuel as the coolant is observed. In the present work, an explanation was proposed to understand the influence mechanism of coking on the heat transfer in cooling channel. In addition, a numerical model was further established to verify the explanation by comparing the heat transfer in cooling channel with different thicknesses of coke layer. The results indicated that critical thicknesses of coke layer with respect to effective thermal conductivity ( δ cl , λ c ) and heat transfer ( δ cl , h c ) existed. More specifically, the heat transfer was noticeably enhanced when δcl δ cl , h c , while the heat transfer is deteriorated as δcl > δ cl , h c . A transition thickness with length of ( δ cl , h c – δ cl , λ c ) existed between δ cl , λ c and δ cl , h c under the combined influences of convection of core flow and coke layer. By comparing the heat transfer in cooling channel after coke deposit at different inlet Reynolds number, it was further found that δ cl , λ c and δ cl , h c decrease with increasing the turbulent intensity. This implied that the influence of inlet Reynolds number on the coke deposition rate and turbulent heat transfer were dissimilar. This study is expected to provide further insight into the optimization of cooling channel for the purpose of reducing the heat transfer deterioration caused by coke.

Thermal runaway is the top safety concern for Li-ion electrochemical energy storage systems. There are multiple abuse conditions that may cause thermal runaway in Li-ion cells. Although thermal runaway has been extensively studied, the runaway mechanisms due to external short circuit and ultra-high discharge rates (>10C) are relatively less studied. In the present work, an analytical thermal runaway model is developed to predict thermal runaway in prismatic and pouch Li-ion cells due to either external short circuit or ultra-high discharge rates. The heat generation data corresponding to the ultra-high discharge rates considered in the thermal runaway model have been obtained separately using an electrochemical-thermal coupled model. The analytical thermal runaway model is validated against experiments as well as COMSOL, a finite element-based commericial solver. Using the analytical model, the effects of key parameters on cell safety have been analyzed. Finally, the analytical model has been used to evaluate effectiveness of a thermal runaway prevention strategy based on boiling in minichannels of a water-cooled minichannel based battery thermal management system.

A hybrid phonon gas model was developed to describe heat pulse propagation in dielectric crystals. In this model, heat energy is described as a mixture of longitudinal and transverse phonon gases. The novel aspects of this model include: (1) the distinction between longitudinal and transverse phonon excitations is taken into account; (2) transitions between ballistic and second sound transport, ballistic and diffusive transport, and second sound and diffusive transport are successfully captured. For the first time, benchmark cases of heat-pulse experiments in NaF at low temperatures in ballistic, ballistic–diffusive, and diffusive regions have been completely reconstructed in numerical simulations, which demonstrate the validity of the new model. It was elucidated how heat pulses are transmitted by longitudinal, ballistic transverse and dispersive transverse phonons at low temperatures. The numerical results not only yield new insight in physics of transient ballistic–diffusive heat conduction with effects from different phonon scattering processes, but also provide numerical tools to study similar transient ballistic–diffusive heat conduction in nanoelectronics and modern optoelectronics.

While there are intensive studies on the coalescence of sessile macroscale droplets, there is little study on the coalescence of sessile microdroplets. In this paper, the coalescence process of two sessile microdroplets is studied by using a many-body dissipative particle dynamics numerical method. A comprehensive parametric study is conducted to investigate the effects on the coalescence process from the wettability gradient, hydrophilicity of the solid surface, and symmetric or asymmetric configurations. A water bridge is formed after two microdroplets contact. The temporal evolution of the coalescence process is characterized by the water bridge's radii parallel to the solid surface $({W}_{m})$ and perpendicular to the solid surface $({H}_{m})$. It is found that the changes of both ${H}_{m}$ and ${W}_{m}$ with time follow a power law; i.e., ${H}_{m}={\ensuremath{\beta}}_{1}{\ensuremath{\tau}}^{\ensuremath{\beta}}$ and ${W}_{m}={\ensuremath{\alpha}}_{1}{\ensuremath{\tau}}^{\ensuremath{\alpha}}$. The growth of ${H}_{m}$ and ${W}_{m}$ depends on the hydrophilicity of the substrate. ${W}_{m}$ grows faster than ${H}_{m}$ on a hydrophilic surface, and ${H}_{m}$ grows faster than ${W}_{m}$ on a hydrophobic surface. This is due to the strong competition between capillary forces induced by the water-bridge curvature and the solid substrate hydrophobicity. Also, flow structure analysis shows that regardless of the coalescence type once the liquid bridge is formed the liquid flow direction inside the capillary bridge is to expand the bridge radius. Finally, we do not observe oscillation of the merged droplet during the coalescence process, possibly due to the significant effects of the viscous forces.

The effects of accelerated crucible rotation technique (ACRT) and dynamic translation on liquid phase diffusion (LPD) growth of SixGe1−x single crystals have been separately investigated numerically in earlier works and were found to have a very positive impact on the LPD growth process. Building upon these findings, in this paper, we study the consequences of imposing both ACRT and dynamic translation on this growth technique. Time-dependent, axisymmetric numerical simulations using moving grid approach have been carried out using finite volume code Ansys Fluent. Crucible translation effect is simulated using dynamic thermal boundary condition. Results are compared to the case in which this growth system is subjected to ACRT only. It is predicted that by combining ACRT with dynamic pulling, excellent axial compositional uniformity can be achieved and growth rate can be improved substantially without significantly compromising on the benefits of employing ACRT. The results show that it is advantageous to utilize the combination of ACRT and dynamic translation during LPD growth rather than using them independently for producing relatively uniform composition SixGe1−x single crystals in a shorter span of time.

A new treatment of pressure boundary conditions for the DSMC method is proposed for flow prediction in microchannels. Validity and accuracy of the new method are verified by comparing to the analytical solutions of the micro-Poiseuille flow under slip condition. The new method shows better convergence compared with previous boundary treatments. This advantage becomes more remarkable as the geometry of the microchannel becomes more complex. A study on a microchannel with sudden expansion is demonstrated using the new DSMC method. Wall temperature in the expanded region of the microchannel independently varies from 200 to 800 K to study the effects on the pressure distribution, velocity, mass flow rate, and heat flux of the microchannel flow. The results show that the wall temperature in the expanded region significantly affects the microchannel flow. Some unique phenomena are observed to be quite different from those of the macroscopic flow and the mechanism of these interesting phenomena is discussed.

With the development of technology, in smaller and smaller length scale, emerging non-diffusive heat transfer has been challenging the traditional Fouriers law of heat conduction. Consequently, lots of models have been constructed. In this paper, a novel two-parameter non-diffusive heat conduction (TPHC) model was applied to decode the physics in nanoscale transient grating experiments. Better fitting with the experiment was shown in the numerical simulation and newly discovered collective diffusive effect was observed by this TPHC model.Copyright © 2015 by ASME

Single-cell analysis techniques have been developed as a valuable bioanalytical tool for elucidating cellular heterogeneity at genomic, proteomic, and cellular levels. Cell manipulation is an indispensable process for single-cell analysis. Digital microfluidics (DMF) is an important platform for conducting cell manipulation and single-cell analysis in a high-throughput fashion. However, the manipulation of single cells in DMF has not been quantitatively studied so far. In this article, we investigate the interaction of a single microparticle with a liquid droplet on a flat substrate using numerical simulations. The droplet is driven by capillary force generated from the wettability gradient of the substrate. Considering the Brownian motion of microparticles, we utilize many-body dissipative particle dynamics (MDPD), an off-lattice mesoscopic simulation technique, in this numerical study. The manipulation processes (including pickup, transport, and drop-off) of a single microparticle with a liquid droplet are simulated. Parametric studies are conducted to investigate the effects on the manipulation processes from the droplet size, wettability gradient, wetting properties of the microparticle, and particle-substrate friction coefficients. The numerical results show that the pickup, transport, and drop-off processes can be precisely controlled by these parameters. On the basis of the numerical results, a trap-free delivery of a hydrophobic microparticle to a destination on the substrate is demonstrated in the numerical simulations. The numerical results not only provide a fundamental understanding of interactions among the microparticle, the droplet, and the substrate but also demonstrate a new technique for the trap-free immobilization of single hydrophobic microparticles in the DMF design. Finally, our numerical method also provides a powerful design and optimization tool for the manipulation of microparticles in DMF systems.

Thermal transport across interfaces can play a critical role in nanosystems for thermal management and thermal energy conversion. Here, we show the dependence of the thermal boundary conductance (G) of the interface between a 70-nm Al transducer and a Si substrate on the size of a laser pump diameter (D) in the time-domain thermoreflectance (TDTR) experiments at room temperature. For D ≥ 30 μm, G approaches to a constant where diffusion dominates the heat transfer processes. When D decreases from 30 μm to 3.65 μm, G decreases from 240 to 170 MW/m2K due to the increasing nonlocal effects from nondiffusive heat transport. This finding is vital to our understanding of the thermal boundary conductance: it depends not only on inherent interfacial conditions but also on external heating conditions, which makes the accurate measurements and theoretical predictions of thermal transport across interfaces in micro/nanosystems more challenging.

Heat transfer in the vicinity of nanoscale hot-spots is qualitatively different from that in the macroscale, which effect stems from the breakdown of Fourier law due to phonon nondiffusive transport. In this work, we validate a recently proposed alternative, high-fidelity phonon transport model, the unified nondiffusive-diffusive (UND) model, which takes into account the mixed ballistic-diffusive nature of heat transport, as well as reduces to the Fourier law as a limiting case. In the UND model, the nondiffusive phonons are treated using the Boltzmann transport equation, while the diffusive phonon gas is treated by the Fourier law. The numerical results of Maznev et al. for the geometry and spatial dependence of variables corresponding to the transient gratings experiments of Johnson et al. are used for validation of the model.

A numerical approach is presented to describe how ultrafast acoustic wave is generated by laser pulse in multilayer thin films. Transient surface displacement obtained through simulation is confirmed by the experiment measurement data. Analyzing results with the two temperature model, it is able to determine the interfacial thermal resistance. Optical reflectance changes are obtained by combining the contributions of thermal and acoustic effects, which agree with the experiment well.

An appropriate heat conduction model is indispensable for experimental data analysis in nanothermometry in order to extract parameters of interests and to achieve a fundamental understanding of phonon-mediated heat transfer in nanostructures and across interfaces. Recently, nanoscale periodic metallic gratings are used as a group of distributed heaters as well as transducers in nanothermometry. However, in this technique, there are coupled hotspot-size-dependent effective thermal conductivity (ETC) and hotspot-size-dependent thermal interface resistivity, which posts a challenge for experimental data analysis using Fourier’s law that fails to extract both ETC and thermal interface resistivity simultaneously. To overcome this challenge, a novel two-parameter nondiffusive heat conduction (TPHC) model, which has been successfully applied to data analysis in different types of pump-probe experiments, is applied to analyze laser-induced nondiffusive heat transfer in nanoscale metallic grating experiments. Since the hotspot-size-dependent ETC is automatically captured by the TPHC model, the hotspot-size-dependent interface resistivity becomes the only parameter to be determined from experiments through data fitting. Thus, the hotspot-size-dependent thermal interface resistivity can be determined from experiments without the impact from the hotspot-size-dependent ETC. Currently, there is a lack of a criterion to predict when Fourier’s law breaks down in nanoscale heat transfer. To fill this gap, a criterion based the TPHC model is identified to predict the valid range of Fourier’s law, which is validated in both theoretical analyses and nanoscale metallic grating experiments.

We present a continuum hybrid phonon gas model to describe transient ballistic-diffusive heat transport. In this model, heat energy is carried by a mixture of longitudinal and transverse phonon gases so that the distinction between longitudinal and transverse phonon excitations is taken into account. This new model is validated by the successful reconstruction of benchmark cases of heat-pulse experiments in NaF, which have never been completely reconstructed before. It is elucidated how thermal pulses are transmitted by longitudinal and transverse phonon gases. This model not only helps us yield new insight in transient ballistic-diffusive heat conduction mechanisms but also provides numerical tools to study transient ballistic-diffusive heat conduction in nanoelectronic and modern optoelectronics.

In this paper, we reported a new ballistic-diffusive model for heat pulse propagation in dielectric materials. The internal energy and heat flux are split into ballistic part originating from the boundaries and diffusive part originating from inside medium. The ballistic part is modeled based on analytical solutions while the diffusive part is described by Guyer-Krumhansl equations. To validate this model, heat pulse propagation in pure NaF at low temperature is studied. The observed longitudinal waves, transverse waves, second sound waves, and diffusive waveforms from the experiments conducted in early 1970s are numerically reconstructed. There is qualitative agreement between numerical results and experimental observation.Copyright © 2012 by ASME

Recent advances in nanotechnology create a demand for greater scientific understanding of the transient ballistic phonon transport at the nanoscale. It is believed that ballistic phonons may travel for long distances without destruction, but it is unclear how far they can travel. Here, a numerical model is developed to study phonon transport in silicon nanofilms. It is elucidated how thermal pulses are transmitted in silicon nanofilms by longitudinal, ballistic transverse and dispersive transverse phonons. It is found that both ballistic longitudinal and ballistic transverse phonons are highly dissipative so they can only travel for short distances, while dispersive transverse phonons at lower frequencies are less dissipative and can travel for longer distances. There exists a similarity parameter (Knudsen number) in thin-film heat conduction with different thicknesses.Copyright © 2012 by ASME

In this paper, heat pulse propagation in pure NaF at low temperature is studied by using hydrodynamic equations of a weakly interacting phonon gas. The observed longitudinal waves, transverse waves, second sound waves, and diffusive waveforms from the experiments conducted in early 1970s are numerically reconstructed. In the numerical simulations, the phase velocities of the longitudinal waves and the transverse waves are based on the experimental results, while two relaxation times for normal scattering process (τN ) and resistive scattering process (τR ) respectively are adjusted to fit the arrival time of second sound or to fit the observed waveforms observed in the experiments. The waveforms based on numerical results can fit the observed waveforms quite well. It is elucidated how heat pulse is transmitted by different waves in the dielectric crystal from ballistic to diffusive regions.Copyright © 2011 by ASME

High heat dissipation is generated in Traveling-wave-tube circuit for ultrawideband high-power submillimeter-wave generation due to e-beam power loss during transport. Active cooling is required to ensure the device can physically handle the high heat flux without damage. An efficient thermal management solution is provided for cooling the high-power vacuum device based on microchannel technology. A thermal conduction model is developed in Ansys Workbench® to calculate thermal resistance in the vacuum device while a conjugated thermal conduction and convection model is developed in Fluent for the design of microchannel heat sink. The performance of microchannel heat sink is investigated to ensure a safe operational condition (maximum temperatures 250 °C) for the high-power vacuum device with 95% e-beam transport efficiency.

Carbon nanotube (CNT) forest/cluster synthesized by a thermal CVD process has millimeter growth height, large porosity and nano level pore size, plus high thermal conductivity of individual CNT, thus it is potentially a good wick structure material in developing micro heat transfer devices. However, thermal properties, including effective thermal conductivity (ETC) of a bulky CNT layer, may not be as good as the common metallic wick materials. In this paper, a Netzsch DSC 404 C Pegasus is used for measurement of the CNT heat capacity. CNT volume density is obtained by measuring the ratio of a bulky CNT weight and volume. Both the laser flash and 3-omega measurement methods are employed to measure ETC for CNT wick structures synthesized by the thermal CVD processes. For the laser flash method, measurement deviations caused by reflective silicon and thin substrate are corrected by surface treatment and increased sample thickness. Measurement results of the laser flash indicate that a 600μm thick CNT layer has ETC varying from 0.7–1.2W/m.K. For the 3-omega approach, the measurement system is validated on a quartz substrate. However, the test results yield larger ETC on 250μm CNT samples. Geometric and one dimensional thermal conduction analysis indicate that the bulky CNT thermal properties are tied to CNT synthesis processes. ETC of bulky CNT layer can be enhanced by straightening CNT growth and increasing CNT growth volume density.Copyright © 2009 by ASME

Reliability of microfluidic designs is an extremely important issue that defines the range of applicability of micro-devices. The reliability design of micro-channels presents new complications that require extensions of today's method to incorporate reliable prediction methodologies. In this paper, using the improved Direct Simulation Monte Carlo, a prediction method was proposed for the reliability design of micro-flow channels. The Direct Simulation Monte Carlo method appears to be useful in micro-flow simulations because the corresponding Knudsen numbers are often beyond those that can be simulated by continuum approaches. Based on the assumption of certain pressure distributions with the second-order on the boundaries in the micro flow cells, the improved DSMC method was used to obtain the corresponding pressure and viscous forces of the micro-flow. For all boundaries of the microstructure interacting with the fluid experienced loads from the flow, the load was expressed as the sum of the corresponding pressure and viscous forces obtained from the DSMC method. Using the Finite Element Method, the stress distribution and the deformation of the microstructure under the loads can then be given. Considering failure criteria of the material and structure, the reliability parameters were further estimated and analyzed. In order to demonstrate the effects of the method in optimizing the design of microfluidic devices, the performance and reliability predictions are made for a microchannel under backward-facing flows. The results show that the method is necessary and effective to predict weak spots of micro devices and prevent the extreme deformation in design of microchannels.

Micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS) have become the research focuses which attract a great deal of attention in recent years. The fluidic and thermal behaviors are usually different from those of the macro devices. In this paper, the heat and mass transfer characteristics of the rarefied nitrogen gas flows in microchannels are investigated using DSMC method. In order to study the effects of the wall temperature (T w ) on the mass flux and wall heat flux in the microchannels, the temperature of the incoming gas flow (T ∞ ) is set constant at 300 K, and the wall temperature varies from 200 K to 800 K. For all of the simulated cases, majority of wall heat flux stays within the channel entrance region and drops to nearly zero when it reaches the middle region of the channel. When T w ≪ T ∞ , with the restriction of the pressure driven condition and continuity of pressure, the number density of the flow has to decrease along the flow direction eventually after a short increase at the entrance region. When T w ≫ T ∞ , the number density of the flow drops rapidly near the inlet, and the temperature of the gas flow increases. As the T w increases, the flow becomes more rarefied, the mass flux decreases, and the resistance at the entrance region increases. Furthermore, when T w ≫ T ∞ , sudden jump in heat transfer flux and temperature are observed at the exit region of the channel.

An unsteady microfluidic T-form mixer driven by pressure disturbances was designed and investigated. The performance of the mixer was examined both through numerical simulation and experimentation. Linear Stokes equations were used for these low Reynolds number flows. Unsteady mixing in a micro-channel of two aqueous solutions differing in concentrations of chemical species was described using a convection-dominated diffusion equation. The task was greatly simplified by employing linear superimposition of a velocity field for solving a scalar species concentration equation. Low-order-based numerical codes were found not to be suitable for simulation of a convection-dominated mixing process due to erroneous computational dissipation. The convection-dominated diffusion problem was addressed by designing a numerical algorithm with high numerical accuracy and computational-cost effectiveness. This numerical scheme was validated by examining a test case prior to being applied to the mixing simulation. Parametric analysis was performed using this newly developed numerical algorithm to determine the best mixing conditions. Numerical simulation identified the best mixing condition to have a Strouhal number (St)of 0.42. For a T-junction mixer (with channel width = 196 μm), about 75% mixing can be finished within a mixing distance of less than 3 mm (i.e. 15 channel width) at St = 0.42 for flow with a Reynolds number less than 0.24. Numerical results were validated experimentally by mixing two aqueous solutions containing yellow and blue dyes. Visualization of the flow field under the microscope revealed a high level of agreement between numerical simulation and experimental results.

Erosion results from interactions between the pipe surface and fluids traveling along the surface. Fluid-structure interactions have a profound influence on the erosion that takes place. The location, rate and extent of thinning or loss of a protective surface film depend strongly on the nature of the flow regime and interactions. Erosion-corrosion involves the modification, thinning and removal of protective films composed of corrosion product or scale deposits from a susceptible metal surface by fluid shear stress under high turbulence conditions. In the paper, multi-scale simulation of fluid-structure interactions between the flow and the protective films on the pipe surface is presented. The fluid shear stress and pressure of the flow in a pipe with a step is obtained by macro-fluid dynamic analysis. Viscous forces and the system’s pressure impose forces to the surface of the pipe. Using micro-simulation method, the fluid-structure interactions between the flow and the protective films is modeled. The deformation of the protective films is shown and changed with the different velocity and flow regime. Using the multi-scale simulation of fluid-structure interactions, the location, rate and extent of the erosion on the pipe surface can be predicted. The results are proved by the actual instances.Copyright © 2008 by ASME

A novel micro mixer was designed and optimized by using numerical simulations. The performance of the mixer was tested by mixing two aqueous solutions under the microscope and the flow field was visualized using two different dyes. Efficient mixing was achieved by using pressure disturbances. Optimal mixing conditions were found by parametric studies using numerical simulations. Based on results of numerical simulation, more than 82% mixing can be finished within a mixing distance of less than 3.0mm for flow rates ranging from 1.2mul/min to 20mul/min. The mixing results were validated by comparing the flow patterns between numerical results and experimental results. There was a strong agreement between numerical results and experimental results

We describe a prototype micro-laboratory for rapid genetic identification of bacterial pathogens from infected human body fluids. The core module of the detection platform is a microfabricated electrochemical sensor array. Picomolar amperometric detection is achieved through formation of a DNA sandwich between capture and detector probe pairs and bacterial 16S rRNA, coupled with an oxidoreductase cnzymatic transducer. Using the sensor array functionalized with a panel of species-specific DNA oligonucleotide probes, detection of bacterial urinary pathogens have been demonstrated in a pilot clinical validation study demonstrating 98% sensitivity for Gram-negative pathogen detection, using conventional urine culture as the standard. Genotypic species identification is achieved within 45 minutes, compared to 1-2 days needed for standard bacterial culture technique. A biofilter has also been fabricated and validated with clinical urine specimens. Efficient concentration of the pathogens is demonstrated and replaces the need for conventional centrifugation. A microfluidic mixer to facilitate reagent mixing is also under development as part of sample preparation module. Preliminary experimental data indicate good correlation with computational simulation. These are critical milestones towards our development of an integrated point-of-care platform for pathogen detection in body fluids.

Nondiffusive heat transfer has attracted intensive research interests in last 50 years because of its importance in fundamental physics and engineering applications. It has unique features that cannot be described by the Fourier law. However, current studies of nondiffusive heat transfer still focus on studying the effective thermal conductivity within the framework of the Fourier law due to a lack of a well-accepted replacement. Here, we show that nondiffusive heat conduction can be characterized by two inherent material properties: a diffusive thermal conductivity and a ballistic transport length. We also present a two-parameter heat conduction model and demonstrate its validity in different pump-probe experiments. This model not only offers new insights of nondiffusive heat conduction but also opens up new avenues for the studies of nondiffusive heat transfer outside the framework of the Fourier law.

Based on a phonon hydrodynamic equation, we derive an analytical model to predict the effective thermal conductivity along dielectric thin films or nanowires with smooth wall surface. The model contains only two parameters: bulk thermal conductivity and Knudsen number, no other fitting parameter included. The predictions from the simple model agree approximately with available experimental data in silicon nanostructures and it outperforms other analytical models based on the Boltzmann transport equation.