Studies at phase interfaces. I. The sliding of liquid drops on solid surfaces and a theory for spray retention

The surface properties of the spray liquid/solid combination are among the most important factors controlling the retention of spray liquids on solid surfaces. The effect of these properties on the retention of pesticide sprays has been examined by a study of the sliding of drops of water and solutions of surfactants on wax and cellulose acetate surfaces. A theory, supported by experimental results, has been evolved to explain the movement of drops in terms of the size of the drop, the angle of tilt of the surface, the air/liquid surface tension, and the advancing and receding contact angles. This theory has been developed to predict the volume of spray liquid that will be retained on a solid surface. The retentions found in practice agree well with the theoretical figures for most of the spray/solid combinations examined under comparable conditions of spray droplet impaction. A slight modification of the theory permits the correction of some anomalous results obtained with easily wetted surfaces. A retention factor F for any spray liquid/solid combination may be calculated to predict the degree of retention of the liquid on the solid. This factor may be simplified, without undue loss of accuracy, to the form: F = θM[γAL(cos θR - cos θA)/ρ]12, where θA is the advancing contact angle; θR is the receding contact angle; θM is the arithmetic mean of θA and θR; γal is the air/liquid surface tension; and ρ is the density of the spray liquid. The use of such a retention factor facilitates the study of the formulation of spray fluids.