COALESCENCE AND AGGLOMERATION OF DROPLETS SPRAYED ON A SUBSTRATE

The coalescence of droplets of a highly viscous liquid (87 wt% glycerin in water) sprayed onto a solid surface was studied. Experiments were done on the merger of two droplets deposited sequentially on a flat surface and also on liquid droplets sprayed onto a surface. Two unequal sized droplets were deposited on a steel plate, with the smaller one overlapping the larger. The different curvatures of the two droplets produce different capillary pressures in them, driving them to merge. The smaller droplet was pulled into the larger one; the greater the difference in size between the droplets, and the larger their separation, the more rapid the droplet motion. Experiments were also done in which liquid was sprayed onto a transparent surface and the motion of the impacting droplets photographed from below. Measurements of the wetted surface area were done using image analysis. The wetted area first increased as liquid was deposited. Once the spray was turned off the area decreased as smaller droplets merged with larger ones. Droplet coalescence prevented the formation of a uniform film and liquid accumulated in separate patches.