Computational study of electrostatic focusing of aerosol nanoparticles using an einzel lens

Abstract

This study computationally explores the possibility of focusing charged aerosol nanoparticles using electrostatics, similar to focusing of electrons and ions. A non-dimensional electrostatic focusing parameter χe, defined as the ratio of electrostatic potential energy to the kinetic energy of an aerosol nanoparticle, significantly determines focusing performance. The focusing device considered here is a 3-electrode electrostatic (“einzel”) lens. The average focal length of the lens is seen to have an inverse power relationship with χe. For low values of χe ∼3 in this study, the particles are seen to cross the lens axis once, while at higher χe multiple axis cross-over points appear. Similar to electron and ion optics, nanoparticle focusing is also limited by spherical aberration and beam divergence due to finite spread of particles in the inlet cross section of the lens and spatial non-uniformity of the focusing electric field. Other factors that influence focusing performance such as the electrostatic lens geometry, and the distribution of velocity and kinetic energy of the particles at the inlet of the lensing region are recognized, but not considered here for simplicity. In vacuum, good focusing performance (i.e.) a narrow beam of nanoparticles with minimum spherical aberration and small divergence angle is theoretically possible if χe<1 and if spread of particles in the inlet is confined to 20% of radius of the cylindrical lens. The effect of gas pressure is also probed to understand the degradation of focusing performance due to particle-gas interactions. It is seen that, for particles of specified size and density, a certain maximum pressure exists beyond which the device can no longer be efficiently used to focus nanoparticles. Likewise, below a certain pressure, the focusing performance is nearly independent of gas pressure, thereby enabling the selection of an operating pressure for such devices.

Publication Title

Journal of Aerosol Science

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