AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Diffuse Vs. Specular Algorithms to Explain Electrical Mobility in Diatomic Gases
Carlos Larriba-Andaluz, CHRISTOPHER HOGAN JR., University of Minnesota
Abstract Number: 605 Working Group: Nanoparticles and Materials Synthesis
Abstract Electrical Mobility measurements of polyatomic ions are ubiquitously used as capable ways of separating molecular entities by cross section and charge. Collision Cross Sections(CCS) are a measure of drag and include not only the Physical Cross Sections(PCS), but the enhancement effects of gas-molecule scatterings and/or electrostatic potentials. Often, CCS are mistaken by their more available counterpart PCS and in general this leads to poor agreement between calculations and experiments, mostly because simulated molecular structures used for calculations focus in matching potential energy surfaces while disregarding bulk physical properties. In this work, we will carefully define MD structures to describe the scattering process as accurately as possible. Room Temperature Ionic Liquids (RTIL), e.g. EMIBF4/ EMI-N(CN)2, are used as candidate structures. The reason behind this choice is that RTIL are non-volatile liquids that form spheres with nearly no exception. These test subjects are guaranteed to achieve their correct size by comparing the density of a single large uncharged MD structure to the bulk density of the material. The structure is subsequently reduced to match the bulk density and the reduction ratio is employed equally in the rest of cases, so that all structures achieve their correct volume fraction that might or might not coincide with the void fraction of the bulk material. A novel and efficient set of momentum transfer algorithms for diatomic gases is invoked on such fixed structures to explore the validity of specular vs. diffusive scattering laws. It will be shown that specular models fail to correctly describe the necessary scattering process that would have to take place in order to achieve experimentally observed enhancement factors. Furthermore, specular scattering cannot predict the constant enhancement factor that appears in mobility experiments in diatomic background gases. In all, diffuse scattering laws must be applied to correctly define electrical mobility measurements.