Bing-Yang Cao

Molecular Momentum Transport at Fluid-Solid Interfaces in MEMS/NEMS: A Review

This review is focused on molecular momentum transport at fluid-solid interfaces mainly related to microfluidics and nanofluidics in micro-/nano-electro-mechanical systems (MEMS/NEMS). This broad subject covers molecular dynamics behaviors, boundary conditions, molecular momentum accommodations, theoretical and phenomenological models in terms of gas-solid and liquid-solid interfaces affected by various physical factors, such as fluid and solid species, surface roughness, surface patterns, wettability, temperature, pressure, fluid viscosity and polarity. This review offers an overview of the major achievements, including experiments, theories and molecular dynamics simulations, in the field with particular emphasis on the effects on microfluidics and nanofluidics in nanoscience and nanotechnology. In Section 1 we present a brief introduction on the backgrounds, history and concepts. Sections 2 and 3 are focused on molecular momentum transport at gas-solid and liquid-solid interfaces, respectively. Summary and conclusions are finally presented in Section 4.

Equation of motion of a phonon gas and non-Fourier heat conduction

Heat conduction in solids is due to the motion of the phonon gas. A more general description of the heat transport in solids includes consideration of the mass, pressure, and inertial force of the phonon gas. The mass of the phonon gas refers to the equivalent mass of its energy based on Einstein’s mass-energy relation. The thermal vibration of the lattice creates the phonon gas pressure and the momentum change of the phonon gas results in an inertial force. The phonon gas velocity is directly proportional to the heat flux. These concepts are used to establish an equation of motion for the phonon gas including the driving, inertial, and resistant forces using Newtonian dynamics. This equation reduces to Fourier’s law of heat conduction when the inertial force can be neglected relative to the other terms so that heat conduction becomes pure diffusion. However, Fourier’s law of heat conduction no longer holds if the heat flux is very high, such that the inertial force of the phonon gas is not negligible. In...

Temperature dependence of the tangential momentum accommodation coefficient for gases

The temperature dependence of the tangential momentum accommodation coefficient (TMAC) is investigated by examining gas flows in a submicron channel using molecular dynamics simulations. The results show that the TMAC decreases with the increasing temperature following an exponential decay law, and is more sensitive to lower temperatures than to higher ones. The molecular trapping-desorption behaviors near the channel surface are found to be responsible for this dependence.

Generalized heat conduction laws based on thermomass theory and phonon hydrodynamics

The Fourier’s law of heat conduction is invalid in extreme conditions, such as the second sound in solids and anomalous heat conduction in nanosystems. The generalized heat conduction law with nonlinear and nonlocal effects is derived from both macroscopic thermomass theory and microscopic phononBoltzmann method in this paper. The coincidence between thermomass theory and phononhydrodynamics is also analyzed through their microscopic basis. The convective term in the momentum equation of the thermomass theory comes from the nonlinear terms of the distribution function, which is often neglected in previous phononhydrodynamics derivations. The Chapman-Enskog expansion leads to the Laplacian term, which is similar to the derivation of Navier-Stokes equation in hydrodynamics and inspires the introduction of a Brinkman extension in the thermomass equation. This comparison reveals how the nonlinear effects could be described by generalized heat conduction laws.

Thermal resistance between crossed carbon nanotubes: Molecular dynamics simulations and analytical modeling

A nonequilibrium molecular dynamics (MD) method is used to calculate the thermal resistance between crossed carbon nanotubes (CNTs). The thermal resistance is predicted to be of the order of 109–1011 K/W. The effects of the crossing angle, nanotube length, and initial nanotube spacing on the thermal resistance are studied in detail with the fixed boundary condition applied in the axial direction of each CNT. The thermal resistance is found to increase with the increasing crossing angle while decrease with the increasing nanotube length and converge to a constant eventually. An increase in the thermal resistance is observed for nanotubes with larger initial spacing and the increase becomes abrupt as the initial spacing is increased to the van der Waals diameter. Between the crossed CNTs the phonon transport is constricted through the contact. The thermal resistance between the crossed CNTs calculated by MD is found to be close to the ballistic constriction resistance, which indicates that the constriction ...

Polymer nanowire arrays with high thermal conductivity and superhydrophobicity fabricated by a nano-molding technique

High thermal conductivity is helpful for thermal control and management, and superhydrophobicity can benefit fluid friction reduction and liquid droplet control in micro-/nanodevices. We report on a nano-molding technique that can prepare polyethylene nanowire arrays with high thermal conductivity (more than 10 W/m-K) and superhydrophobicity (contact angle >150°). The thermal conductivities of the fabricated high-density polyethylene nanowire arrays with diameters of 100 nm and 200 nm, measured by a laser flash method, are about 2 orders of magnitude higher than their bulk counterparts. The estimated thermal conductivity of a single high-density polyethylene nanowire is as high as 26.5 W/m-K at room temperature, while the thermal conductivity of low-density polyethylene nanowire is a little smaller. The self-organized surfaces of polymer nanowire arrays are found to have micro-to-nanoscale hierarchical nanostructures, and have superhydrophobicity of greater than 150° contact angles for water. We also measure the wettability of organic liquids, including glycerin, ethanol, paraffin liquid, and methyl silicone oil. We find glycerin gives hydrophobic wettability, but the others give hydrophilic wettabilities. This technique is promising for fabrication due to the advantages of simple fabrication, high quality, low cost, and mass production.

Strain-assisted tunneling current through TbMnO3∕Nb-1 wt %-doped SrTiO3 p–n junctions

Simple oxide heterostructures have been fabricated by growing multiferroic TbMnO3 thin film on Nb-1 wt %-doped SrTiO3 substrate. In addition to the beneficial rectifying characteristics in a temperature range from 350 to 30 K, the intriguing observation is that at constant reverse bias the current increases with decreasing temperature below 275 K. By analyzing the band diagram, the anomalous increasing current with decreasing temperature was ascribed to the strain-assisted tunneling current through TbMnO3∕Nb-doped p–n junctions.

Thermal gradient induced actuation in double-walled carbon nanotubes

Molecular dynamics simulations are applied to investigate the thermal gradient induced actuation in double-walled carbon nanotubes, where a temperature difference can actuate the relative motion of double-walled carbon nanotubes. The thermal driving force calculated through a stationary scheme is on the order of pico Newtons for a 1 K nm(-1) temperature gradient. The driving force is approximately proportional to the temperature gradient, but not sensitive to the system temperature. For the outer tube longer than 5 nm, the thermal driving force is nearly constant. For the outer tube shorter than 5 nm, however, the driving force decreases with decreasing tube length. The motion trace is found to depend on both the chirality pair and system temperature. A critical temperature can be defined by the potential barrier perpendicular to the minimum energy track of potential patterns. When the system temperature is higher than the critical temperature, the motion shows random behavior. When the system temperature is lower than the critical temperature, the motion, translational and/or rotational, is confined within the minimum energy track, which is indicative of the feasibility of directional control.

A uniform source-and-sink scheme for calculating thermal conductivity by nonequilibrium molecular dynamics

A uniform source-and-sink (USS) scheme, which combines features of the reverse [F. Müller-Plathe, J. Chem. Phys. 106, 6082 (1997)] and improved relaxation [B. Y. Cao, J. Chem. Phys. 129, 074106 (2008)] methods, is developed to calculate the thermal conductivity by nonequilibrium molecular dynamics (NEMD). The uniform internal heat source and sink are realized by exchanging the velocity vectors of individual atoms in the right half and left half systems, and produce a periodically quadratic temperature profile throughout the system. The thermal conductivity can be easily extracted from the mean temperatures of the right and left half systems rather than by fitting the temperature profiles. In particular, this scheme greatly increases the relaxation of the exited localized phonon modes which often worsen the calculation accuracy and efficiency in most other NEMD methods. The calculation of the thermal conductivities of solid argon shows that the simple USS scheme gives accurate results with fast convergence.

Anomalous orientations of a rigid carbon nanotube in a sheared fluid

The nanoparticle orientation in fluid systems can be correlated with the rotational diffusion and is widely used to tune the physical properties of functional materials. In the current work, the controllability of the orientation of a single rigid carbon nanotube in a fluid is investigated by imposing a linear shear flow. Molecular dynamics simulations reveal three forms of anomalous behavior: (i) “Aligned orientation” when the nanotube oscillates around a particular direction which is close to the flow direction at a small angle of about 10° in the velocity-gradient plane; (ii) “Interrupted orientation” when the oscillation is interrupted by a 360° rotation now and then; (iii) “Random orientation” when 360° rotations dominate with the rotational direction coinciding with the local fluid flow direction. The orientation order is a function of the Peclet number (Pe). The results show that the correlation between Pe and the orientation order from the two-dimensional model does not apply to the three-dimensional cases, perhaps due to some anomalous behavior and cross-section effects. This work provides clear pictures of the nanoparticle movement that can be used to guide particle manipulation techniques.