James Chen

An advanced kinetic theory for morphing continuum with inner structures

Advanced kinetic theory with the Boltzmann-Curtiss equation provides a promising tool for polyatomic gas flows, especially for fluid flows containing inner structures, such as turbulence, polyatomic gas flows and others. Although a Hamiltonian-based distribution function was proposed for diatomic gas flow, a general distribution function for the generalized Boltzmann-Curtiss equations and polyatomic gas flow is still out of reach. With assistance from Boltzmanns entropy principle, a generalized Boltzmann-Curtiss distribution for polyatomic gas flow is introduced. The corresponding governing equations at equilibrium state are derived and compared with Eringens morphing (micropolar) continuum theory derived under the framework of rational continuum thermomechanics. Although rational continuum thermomechanics has the advantages of mathematical rigor and simplicity, the presented statistical kinetic theory approach provides a clear physical picture for what the governing equations represent.

A multiscale study of the boundary layer development for microfluidic system

Abstract The evolution and characterisation of the boundary layer are critical in many CFD applications. In microfluidic systems, the performance of microfluidic fuel cells is based on the boundary layer development of two fluid streams. In this article, the Micropolar theory (MPT) is used as a multiscale theory to study the boundary layer development for small scales. Different scale simulations are performed to study the molecule size effect in the boundary layer development. Different boundary conditions for gyration are examined and interpreted as surface roughness for microfluidic systems. Therefore, the effects of surface roughness on the boundary layer growth are explored. The multiscale nature of the MPT is used to study the energy transfer between the two scales in the theory.

Tunable optical extinction of nano-antennas for solar energy conversion from near-infrared to visible

We present a systematic study of tunable, plasmon extinction characteristics of arrays of nanoscale antennas that have potential use as sensors, energy-harvesting devices, catalytic converters, in near-field optical microscopy, and in surfaced-enhanced spectroscopy. Each device is composed of a palladium triangular-prism antenna and a flat counterelectrode. Arrays of devices are fabricated on silica using electron-beam lithography, followed by atomic-layer deposition (ALD) of copper. Optical extinction is measured by employing a broadband light source in a confocal, transmission arrangement. We demonstrate that the plasmon resonance in the extinction may be tailored by varying lithography conditions and is modified significantly by ALD. Most important, is the ability to control the gap spacing between the two electrodes, which, along with overall size, morphology, and material properties, modifies the plasmon resonance. We employ Finite-Difference Time-Domain simulations to demonstrate good agreement between experimental data and theory and use scanning electron microscopy to correlate plasmonic extinction characteristics with changes in morphology.