Gas kinetic theories of nanomaterial transport
Keywords: Mechanical Engineering.

The study of nanomaterials has been growing rapidly in the past few decades because of their potentials in a broad range of new material functionalities. With their applications already widely demonstrated, interests in nanomaterial research are expected to continue to expand. The transport properties of nanomaterials play major roles in many areas of material synthesis, characterizations, and applications. In this dissertation research, gas-kinetic theory analyses are employed to obtain a more generalized transport theory for nanoparticles and nanocylinders in dilute gases. A substantial emphasis is placed on gas-surface interactions and their effects on momentum transfer during gas-particle collision. Aiming to advance a theory for generalized transport properties of nanocylinders undergoing drift in dilute gases, a comprehensive gas-kinetic analysis is made for the drag force on nanosized cylinders of large aspect ratios. The potential energy of interactions between the gas molecule and cylinder is considered, thus enabling a non-rigid body treatment of the underlying gas-surface interactions. The momentum exchanges between gas and cylinder are considered in the limits of specular and diffuse scatterings. A momentum accommodation coefficient is introduced to describe the transition between the limiting modes of momentum transfer. Upon a consideration of the effect of Brownian rotation, drag force expressions are proposed with and without the cylinder end effect. The alignment of the cylinder with respect to the drift velocity is considered. The resulting generalized transport theory of aerodynamic drag, electric mobility and diffusion for slender bodies in free molecule regime is examined for its validity using available mobility data of carbon nanotubes in air at room temperature. The generalized transport theory is combined with a potential function proposed in this dissertation work to make predictions for the binary diffusion coefficient of long-chain molecules in dilute gases. The approach resolves a long-standing difficulty in modeling the diffusion coefficient of long-chain molecules using the Chapman-Enskog theory with spherical potential functions. In order to examine the applicability of the theory, experiments are carried out through collaboration with researchers at NIST to determine the diffusion coefficients of several normal alkanes in nitrogen and helium over the temperature range of 300 to 600 K using reversed-flow gas chromatography. The new diffusion coefficients are shown to provide substantially better predictions for the extinction strain rates of n-decane and n-dodecane non-premixed flames. The importance of incorporating the Soret effect for large molecules in these flames is also emphasized. To unravel the physics behind gas-particle momentum accommodation, comprehensive molecular dynamics (MD) simulation have been performed for two model systems: Ar-buckminsterfullerene (C60) and silver particles in nitrogen. It is found that in agreement with an earlier study, the cause for diffuse scattering is decidedly surface adsorption of the gas. A statistical theory is proposed with the aim of describing the probability of gas trapping adsorption and hence the momentum accommodation coefficient of gas-particle collisions. Parameters needed to evaluate this probability are obtained through MD simulations. The rotational effect of diatomic molecules on gas-particle collisions is discussed. The validity of the statistical theory is examined by comparing its predictions against experimental mobility data of nanoparticles of several materials. The kinetics and equilibrium of physisorption of gas molecules on particle surface is examined by MD simulation. The impact of surface adsorbates on gas-particle momentum accommodation coefficient is discussed.