Chang-Lin Tien

Molecular Dynamics Study of Solid Thin-Film Thermal Conductivity

This study uses the molecular dynamics computational technique to investigate the thermal conductivity of solid thin films in the direction perpendicular to the film plane. In order to establish a benchmark reference, the computations are based on the widely used Lennard-Jones argon model due to its agreement with experimental liquid-phase data, its physically meaningful parameters, and its simple two-body form. Thermal conductivity increases with film thickness, as expected from thin-film experimental data and theoretical predictions. The calculated values are roughly 30 percent higher than anticipated. Varying the boundary conditions, heat flux, and lateral dimensions of the films causes no observable change in the thermal conductivity values. The present study also delineates the conditions necessary for meaningful thermal conductivity calculations and offers recommendations for efficient simulations. This work shows that molecular dynamics, applied under the correct conditions, is a viable tool for calculating the thermal conductivity of solid thin films. More generally, it demonstrates the potential of molecular dynamics for ascertaining microscale thermophysical properties in complex structures.

Molecular dynamics investigation of thickness effect on liquid films

This work applies the molecular dynamics simulation method to study a Lennard-Jones liquid thin film suspended in the vapor and to explore the film thickness effect on its stability. For the accurate estimation of local pressure distributions in the film, an improved method is proposed and used. Simulation results indicate that profiles of the local surface tension distribution vary widely with film thickness, while surface tension values and density profiles show little variation. As the film gets thinner, the two liquid–vapor interfacial regions begin to overlap and liquid-phase molecules in the center region of the film experience larger tension in the direction parallel to the film surface. Such interface overlapping is believed to destabilize the film and the occurrence of film rupture depends on the system temperature and the cross-sectional area of the computational domain.This work applies the molecular dynamics simulation method to study a Lennard-Jones liquid thin film suspended in the vapor and to explore the film thickness effect on its stability. For the accurate estimation of local pressure distributions in the film, an improved method is proposed and used. Simulation results indicate that profiles of the local surface tension distribution vary widely with film thickness, while surface tension values and density profiles show little variation. As the film gets thinner, the two liquid–vapor interfacial regions begin to overlap and liquid-phase molecules in the center region of the film experience larger tension in the direction parallel to the film surface. Such interface overlapping is believed to destabilize the film and the occurrence of film rupture depends on the system temperature and the cross-sectional area of the computational domain.

MARANGONI EFFECT IN MICROBUBBLE SYSTEMS

This work explores the application of the Marangoni effect in micro systems involving small gas or vapor bubbles in a liquid environment subjected to a temperature gradient. The Marangoni effect characterizes the variation of surface tension along the bubble surface resulting from the temperature gradient around the bubble, thus driving the bubble toward the higher temperature region. This phenomenon is more pronounced as the bubble becomes smaller and the temperature gradient becomes steeper, both of which can be achieved in microbubble systems. Potential applications based on the Marangoni effect include linear bubble actuators, dynamic microvalves, and hot-spot locators. The optimum bubble size for these applications is expected to be of the order of 10 mu m. A smaller bubble may be difficult to introduce into the working system and maintain its size. Presented for illustration is a feasibility analysis for both a noncondensable gas bubble and a vapor bubble situated above a microheater. The analysis y...

RECENT DEVELOPMENTS IN MICROSCALE THERMOPHYSICAL ENGINEERING

The past decade has registered a rapid expansion of research effort focused on microscale thermophysical engineering. The increased technological demand for the miniaturization of devices requires a comprehensive understanding of the fundamental phenomena that govern thermal transport at short length and time scales. A substantial amount of microscale research has been devoted to microelectromechanical systems (MEMS) and ensuing design challenges. Improved fabrication, processing, and measurement and analysis tools have been critical to new technological achievements. This paper reviews the recent progress in microscale thermophysical engineering as seen in fast-developing areas such as thermal transport phenomena, experimental and computational techniques, thermal microdevices and laser applications. Promising research directions in these areas are also highlighted.

Cavitation and bubble nucleation using molecular dynamics simulation

This article reports the first systematic study on cavitation and bubble nucleation using the molecular dynamics simulation method. It successfully simulates the hysteretic process of bubble collapse and nucleation as a numerical counterpart of the Berthelot tube cavitation experiment. For a unary molecule system, a stable bubble regime and minimum equimolar dividing radii of bubbles are obtained with respect to computational domain sizes. For a binary molecule system, the addition of foreign molecules to the solvent molecules stimulates the nucleation more effectively in comparison to that in the unary system. The affinity between the solute and the solvent molecules controls the inception of nucleation and results in different nucleation characteristics according to its value. For an attraction coefficient greater than unity, the solute molecules spread uniformly and attract the solvent molecules, which induces bubble nucleation readily. For the coefficient less than unity, the solvent molecules segrega...This article reports the first systematic study on cavitation and bubble nucleation using the molecular dynamics simulation method. It successfully simulates the hysteretic process of bubble collapse and nucleation as a numerical counterpart of the Berthelot tube cavitation experiment. For a unary molecule system, a stable bubble regime and minimum equimolar dividing radii of bubbles are obtained with respect to computational domain sizes. For a binary molecule system, the addition of foreign molecules to the solvent molecules stimulates the nucleation more effectively in comparison to that in the unary system. The affinity between the solute and the solvent molecules controls the inception of nucleation and results in different nucleation characteristics according to its value. For an attraction coefficient greater than unity, the solute molecules spread uniformly and attract the solvent molecules, which induces bubble nucleation readily. For the coefficient less than unity, the solvent molecules segregate themselves from the solvent molecules, which results in a void shell between the solute and the solvent molecules.

MOLECULAR DYNAMICS SIMULATION OF THERMAL CONDUCTION IN NANOPOROUS THIN FILMS

Molecular dynamics simulations of thermal conduction in nanoporous thin films are performed. Thermal conductivity displays an inverse temperature dependence for films with small pores and a much less pronounced dependence for larger pores. Increasing porosity reduces thermal conductivity, while pore shape has little effect except in the most anisotropic cases. The pores separate the film into local regions with distinctly different temperature profiles and thermal conductivities, and the effective film thermal conductivity is lowest when the pores are positioned in the center of the film. Such tunability by pore placement highlights new possibilities for engineering nanoscale thermal transport.

Molecular dynamics simulation of the meniscus formation between two surfaces

The molecular dynamics computational method is used to simulate meniscus formation around an asperity in a rough surface represented as a sinusoidal wave. Simulation results show that the meniscus formation depends on the interaction potential between the solid wall and the liquid atoms. For completely and partially dry substrates a meniscus cannot form around an asperity. For partially and completely wetting substrates the asperity helps to adsorb the fluid atoms and form a meniscus. These simulation results confirm that if the film thickness exceeds a critical value, the capillary pressure contributes strongly to stiction.

INTERFACIAL AMBIGUITIES IN MICRODROPLETS AND MICROBUBBLES

In the past two centuries , interfacial phenomena have been a subject of ( ) considerable research interest. The liquid-vapor including liquid-gas interface of microdrople ts and microbubble s is of particular importance in many industrial applications. One article in this issue introduces some applications of phase-change w x phenomena in a microsystem 1 . It emphasizes the actuation mechanism controlled by the microbubble formation in a metastable liquid. This mechanism is strongly dependent on interfacial phenomena through the macroscopic parameters such as superheat limit and nucleation rate. Interfacial phenomena undergo some major changes from the macroscale regime to the micro r nano-scale regime, which causes some well-defined macroscopic parameters to lose their physical significance. In the case of homogeneous nucleation, the radius of a critical embryo is usually 1 ; 10 nm, and the thickness ( ) of the density transition layer from one phase to the other interface thickness is w x about 1 nm 2 . The embryo shape usually has significant deviation from a sphere. ( ) ( ) Thus, the embryo size or radius is no longer well-defined i.e., size ambiguity . Similarly, if the whole embryo is under the influence of tensile stress, surface ( ) tension is no longer a surface property, its action location the surface of tension ( cannot be easily specified, and its magnitude differs from the bulk value. i.e., ) surface tension ambiguity . Most classical approaches have difficulties in dealing with nanoscale embryos w x 3 . They, including Gibbs thermodynamic theory and density functional theory ( ) DFT , either assume the core of the embryo to be at the bulk phase , or the embryo shape to be perfectly spherical, both of which may not be valid for ( ) nanoscale embryos. Molecular dynamics MD simulation does not have these assumptions , but its accuracy is limited by computational power. This perspective brings forth some ambiguous issues of size and surface tension and discusses their practical implication.