Growth dynamics of granular films produced by electrospray.

Particulate coatings produced by electrospray deposition (ESD) of ethyl cellulose particles are found to widen over time. We hypothesize that during the ESD process, the electrospray expands due to repulsion caused by accumulated electrostatic charge in the film. The radial profiles of film thickness, mass density (per unit area), and porosity (gas volume fraction) have been determined as a function of several factors which influence electrostatic charging, namely, collection time, relative humidity, and deposition flux. The mass density has been determined from the local film thickness attained upon thermal annealing, which is largest near the center of the spray. The local porosity is lowest at the film center (at between 0.5 and 0.6), whereas across the film, it is roughly uniform (between 0.6 and 0.7) for deposition in a dry ambient, and slightly higher and more variable for humid ambient deposition. The granular films are compact (with low fractality) as expected for ballistic deposition (large Peclet number).