10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
A Field Dependent and Orientation Dependent Mobility Calculator: The Next Generation of Electrical Mobility Calculations
Behram Kapadia, Tianyang Wu, CARLOS LARRIBA-ANDALUZ, IUPUI
Abstract Number: 1543 Working Group: Aerosol Physics
Abstract The ability to predict the electrical mobility of a charged entity from a molecular model is becoming of increasing interest as high resolution mobility measurements are more readily available. Most aerosol generated nanoparticles (and molecular ions and nanoclusters) are not spherical and require more than just a particle size to understand them or separate them. This asphericity may also be sought for as it may provide the particle with unique transport, optical and physical properties. Therefore, the commonly employed mobility diameter, i.e. the equivalent spherical diameter based on mobility measurements, while important in particle characterization, it is an ill conditioned parameter when dealing with molecules, nanocluster or particles with strikingly different aspect ratios. As such, we previously developed a very efficient parallelized state of the art algorithm, IMoS, that calculates collision cross sections (CCS) and mobilities of ion, clusters and nanoparticles within the free molecular regime from all-atom structures or coarse grain models. The CCS is equivalent to the mobility diameter but establishes the possibility of alluding to shape and orientation. It also has the advantage, unlike electrical mobility, that is not dependent a priori on the reduced mass of the gas-ion pair. For these reasons, it is the parameter of choice in IMoS to compare different nanoparticles/clusters.
The existing proposed algorithms however have their limitations. Among a few of such limitations, one can name 1) the calculations always assume that all orientations are equally probable ,2) that the mobility is independent of the electric field employed (true for E/N<<0), 3) that all atoms/coarse grained structures are assumed fixed which requires accommodation and reemission laws to be assumed and 4) that the ion/nanocluster has no rotational speed when the momentum exchange occurs.
Here we propose a novel MD/MonteCarlo algorithm that calculates the ion/nanoparticle as if it were in a real gas and subject to an arbitrary field in three dimensions. The ion/nanoparticle can freely rotate and its orientation, angular velocity and drift velocity are a function of the gas/ion collisions, the reduced mass, the strength of the field, and the position of the charges within the ion. It is the effect of the collisions and the particle orientation together with the electric field which now establishes the equilibrium drift velocity, preferred orientation, if any, and angular velocity. This equilibrium drift velocity, together with non-linearized theory establishes the relation between the field and the mobility which in turn yields the true mobility of the ion. This algorithm will not only provide new insights into mobility calculations never produced before but will also provide a unique understanding in multiple applications which involve charging efficiency, instrument sensitivity, diffusion losses or even collisional heating. The algorithm is parallelized and provides results within a reasonable amount of time. The code may also be extended out of the free molecular regime and into the transition regime when certain considerations are taken into account.