The increasingly fabricated micro/nano structures applied in Micro-electromechanical systems and composite materials have led to the demand for a better understanding of the novel physical phenomena occurring at small scales, for example, the rarefaction effects in micro/nano scaled gas flows and the thermal properties in nano structured materials. For better practical applications and also potential utilizations, a comprehensive investigation of the important rarefaction phenomena observed in micro devices is required; at the same time, a better simulation based design tool is highly desirable in the modeling and design of small structured materials.

Mechanical loadings induced by thermal non-equilibrium rarefied gas has attracted a significant amount of scientific interes...[ Read more ]

The increasingly fabricated micro/nano structures applied in Micro-electromechanical systems and composite materials have led to the demand for a better understanding of the novel physical phenomena occurring at small scales, for example, the rarefaction effects in micro/nano scaled gas flows and the thermal properties in nano structured materials. For better practical applications and also potential utilizations, a comprehensive investigation of the important rarefaction phenomena observed in micro devices is required; at the same time, a better simulation based design tool is highly desirable in the modeling and design of small structured materials.

Mechanical loadings induced by thermal non-equilibrium rarefied gas has attracted a significant amount of scientific interest over a long period of time, while there is still much which is unknown about the fundamental mechanism and new phenomena arising in micro systems. Using asymptotic analysis of the Boltzmann equation in the near continuum regime, the underlying mechanism of the Knudsen force in thermophoresis and heated microbeams is revealed, and the effects of system parameters are examined. In particular, it has been found that the force orientation on heated microbeams is highly sensitive to object shape, and it is feasible to tune force orientation via proper shape design. The discovered phenomenon could find its applications in novel methods for micro particle manipulation and separation. Secondly, the Knudsen torque acting on an asymmetrically located heated microbeam is also investigated. It has been found that by manipulating the system configuration, and the rotational direction of the torque can be changed. Two types of rotational motion of the microbeam have been identified: the pendulum motion of a rectangular beam with multiple stable positions, and the unidirectional rotation of a cylindrical beam. Further, the magnitude of Knudsen torque is shown to be much increased in the transition regime, thus it provides a feasible way to create rotational engine in micro systems.

It is a challenging task to understand and model the submicron thermal transport, especially for nanoscale superlattices and composites. Currently, heat transport in small scales is either modeled at the atomic level, which has a limitation on the problem size that can be handled; or investigated through transport of phonons, which are treated as classical particles governed by the Boltzmann transport equation (BTE). However, in the pure particle approach, the wave nature of the phonons is entirely ignored. Hence such an approach is only valid in the incoherent regime. Recent experiments on superlattices demonstrated that coherent phonon transport exists, due to the interference of phonons scattered from the interfaces. Thereby, for nanostructured materials, both coherent and incoherent phonon transport are likely to be important, and hence an efficient simulation method that can simulate both two effects would be highly desirable. In this work, a new mesoscopic simulation method is developed to model the phonon wave and particle behaviors simultaneously in the length scale larger than phonon wavelength. This is realized by incorporating the amplitude and phase information with a particle treatment, and combined with a BTE solver, the Monte Carlo method. Meantime, an interface model framework has been proposed, which involves both coherent and incoherent phonon scattering. By modeling the cross-plane heat transport problem in superlattices, this method shows the ability to give the same results as a pure particle based approach in the diffuse limit. While in the wave regime, the new method can also capture very well the experimentally observed wave interference effect.