Gas kinetic theory and molecular dynamics simulation of nanomaterial transport in dilute gases.

摘要

Drag force, diffusion coefficient, and electric mobility are equivalent attributes of particle transport. These properties are essential to the characterization of particle dynamics in a gas and particle size. Classical transport theories for micrometer sized particles are shown to be invalid in the nanoscale because of strong interactions resulting from the van der Waals or other potential forces between the fluid molecules and particle. In this work, the drag force, diffusion coefficient, and electric mobility of nanoparticles are investigated in the free molecule regime on the basis of gas kinetic theory. A set of analytical formulas for the transport of small, spherical particles are developed and their accuracy is compared with existing experimental data.; The gas kinetic theory analysis reveals that the collision between a gas molecule and a particle undergoes a transition from specular to diffuse scattering as the particle size increases from molecular to micron scales. To qualitatively explain the origin of this transition, molecular dynamics (MD) simulations are employed to give a molecular view of how a gas molecule collides with a nanoparticle. The MD simulations verify the existence of the transition and it is shown that the diffuse scattering is the consequence of molecular absorption on the particle surface.; The transport theory of nanoparticles is generalized to the entire Knudsen number regime through a semiempirical approach. It is shown that the generalized theory is valid for spherical particles of arbitrary sizes.; Thermophoresis of nanoparticles is of great interest in many applications. Analytical solutions of thermophoretic force and velocity of nanoparticles in the free molecule regime are derived. The classical Waldmann solution of thermophoresis is shown as a special case of the current theory.; The transport theory of nanoparticles is extended to nanotubes. The drag force formulation for nanotubes in dilute gases is developed. The formulation may be used to predict the drag force for a nanotube moving in a gas with arbitrary orientation if the potential interaction between the gas molecules and tube is known.; Finally, the drag force of aggregates in the free molecule regime is investigated through a combination of Monte Carlo and MD simulations.