Opposed Migration Aerosol Classifier (OMAC)

Abstract

A new differential mobility classifier is described. This instrument classifies aerosol particles in a channel flow between porous (or screen) electrodes. The aerosol enters the channel parallel to the porous electrodes, while a larger, particle-free cross-flow enters through one of the porous electrodes and exits through the opposing porous electrode. A potential difference between the electrodes causes charged particles to migrate upstream, against the cross flow. Particles whose migration velocity, v_m exactly balances the cross-flow velocity, v, are transmitted directly along the length of the channel with the minor aerosol flow. Particles whose migration velocities differ from the cross-flow velocity deposit on or pass through the porous electrodes. In the limit of nondiffusive particles, the probability of transmission of particles with migration velocities different from v* _m = –v matches the triangular transfer function of the classical differential mobility analyzer (DMA). Monte Carlo simulations of the transmission of diffusive particles reveals that the resolution of this Opposed Migration Aerosol Classifier (OMAC) remains close to the same ideal, nondiffusive limit of the DMA to much lower voltages than those required by the DMA to achieve equivalent resolution. This extended range enables development of a Nano Opposed Migration Aerosol Classifier (nOMAC) with the same dynamic range of mobilities as the DMA but in a much smaller package. The lower voltage operation also enables operation at low pressure without the loss of dynamic range that DMAs suffer and at higher peak resolutions than are possible with DMAs. Furthermore, the classification method can be applied to gravitational, centrifugal, thermophoretic, and other separation fields, or to separations of particles in liquids.