Three-Dimensional Simulation 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 code uses an accurate three-dimensional model for the Brownian diffusion of nano-particles in strongly 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. The prediction of the 3-D model for penetration efficiency, beam divergence angle, beam diameter and radial cumulative fraction were evaluated and were compared with those of the axisymmetric models. The simulation results showed that for particle diameters less than 30 nm in helium gas, the Brownian force could significantly affect the beam focusing and particle penetration efficiency. Some potential errors in the naive usage of axisymmetric model were discussed. It was shown that the earlier axisymmetric models lead to the incorrect mean square radial displacement of Brownian particles. The present 3-D approach, however, leads to the correct value of the radial mean square displacement of 4Dt. The effect of the inlet orifice and relaxation region on the performance of the lens system was also investigated. It was shown that the major losses of the 4 to 10 nm particles occur in the inlet orifice and relaxation region walls. For 15 to 30 nm particles, the main losses occur at the inlet orifice walls. Some alterations of the shape of the inlet orifice were examined and a new design is suggested to reduce the loss of the particles at the inlet flow control orifice.
Document Type: Research Article
Publication date: 2009-03-01