3-D Modelling of Brownian Motion of Nano-Particles in Aerodynamic Lenses

Abstract

A computer code for analyzing nano-particle motions in an aerodynamic particle beam focusing system was developed. The effectiveness of the focusing system consisting of several lenses, nozzle and downstream tube of the nozzle was analyzed. The code included an accurate 3-dimensional model for the Brownian diffusion of nano-particles in sharply varying pressure field in the aerodynamic lens system. Lagrangian particle Trajectory analysis was performed assuming a one-way coupling model. The particle equation of motion used included drag and Brownian forces. Trajectories of different size nano-particles in an aerodynamic lens were analyzed, and the particle beam focusing process was studied. The numerical results of 3-D model for particle beam diameter, penetration efficiency and beam divergence angle were compared with axisymmetric model and discussed. The importance of the accuracy of the computational model for the simulation of the Brownian diffusion for predicting the focusing performance of the aerodynamic lenses was discussed. The simulation results showed that for particle diameters less than 50 nm in helium, the Brownian force could significantly affect the beam focusing and particle collection efficiency. Furthermore, the nano-particle trajectories in this range are three-dimensional and an axisymmetric model may not correctly capture the features of particle motions in aerodynamic lenses.