Rate constants of particle charging by electrospray nanodroplets

An analysis is presented of the attachment of electrospray nanodroplets by larger particles in a grounded metallic tube where both are carried by a laminar air flow. Droplets and particles that get charged by attaching droplets migrate toward the tube wall under the action of the electric field induced by their charges, but they do so at different velocities owing to the disparity of their electrical mobilities. The electric currents they cause in the tube wall originate therefore in different regions of the tube, and can be separately measured by splitting the tube into two insulated segments. It is shown that, in two limiting conditions, the cross-section for droplet–particle collisions can be extracted from these measurements.