Revisiting Cassie equation: Contact angle measurements on smooth and chemically heterogeneous surfaces

Abstract

The Cassie equation has been used for many decades, but the limitation of the Cassie equation is still being investigated via various experimental and theoretical studies. Here we measured water contact angles for flat, chemically heterogeneous surfaces (silica surfaces with hexagonal array circular patches of 20 nm thick trichloro- (1H, 1H, 2H, 2H-perfluorooctyl) silane layers). Various patch size and spacing has been utilized to investigate the static contact angle changes due to chemical heterogeneity. We found that (i) the deviation between the measured contact angles and the Cassie equation increase with the patch diameter, (ii) the standard deviation of measured contact angles increase with the patch diameter, (iii) as the patch diameter increases, the water droplet no longer maintains truncated sphere structure and become deformed, (iv) when the ratio of the droplet base diameter and patch diameter is greater than ~0.17, contact angles deviate significantly from the Cassie equation.