Effect of Surface Roughness on the Dependence of Contact Angles on Drop Size

Abstract

Absence of drop size dependence of contact angles of sessile drop systems is sometimes observed in experiments. The contact angle data sometimes fluctuate periodically about a horizontal line. Moreover, in cases where a drop size dependence of contact angles exists, the contact angle data often scatter significantly. These fluctuations may be caused by surface roughness. In this paper, two idealized rough surface models are developed. The mean contact angle of a sessile drop in each rough solid surface model is calculated. The fluctuations of the drop size dependence of contact angles produced by these models resemble those obtained experimentally and the fluctuations may therefore be a consequence of the roughness on solid surfaces. It is also concluded that the apparent absence of drop size dependence of contact angles does not necessarily imply zero or extremely low line tension.