On the adhesion of drops and particles on impact at solid surfaces. II

Abstract

The results of a study of the oblique impaction of aerosol drops onto a solid surface may be interpreted on the hypothesis that there is a range of velocities of impact in which adhesion on contact is not certain and that the drops leave the surface at or near the point of impact. A high-speed photographic study of the oblique impact of large drops onto a solid surface has revealed that there is an elastic reaction on impact and ( 1) the area of contact depends upon the velocity of impact and the drop size; ( 2) when a drop strikes a surface, the area of contact might be relatively small for a considerable time; and ( 3) the real area of contact might be very small, presumably owing to air trapped between the liquid and solid. Using Hertz's theory of elastic impact and introducing a qualitative correction for the possibility that the real area of contact may be less than the apparent led to a deeper understanding of the experimental expression previously presented for the lack of adhesion of aerosol drops and particles in filtration and other experiments. The lack of adhesion is attributed to the air drag which acts throughout the contact process and aided by the tendency to bounce removes drops by a necking out process and particles by a peeling action. At low velocities the air drag is not large enough to detach drops and particles on impact, and at large velocities the real area of contact is large enough to ensure adhesion. This accounts for there being a range of velocities in which adhesion on contact is not certain, whereas adhesion occurs above and below this range.