Contact angle hysteresis for wetting of hydrophobic, anisotropically curved surfaces

Our study is focused on the influence of solid surface geometry on contact angle hysteresis, specifically on the values of the advancing contact angle θ_A and receding contact angle θ_R. Recent studies show that when mm-sized spheres coated with polydimethylsiloxane (PDMS) were partially immersed in an air–water interface, θ_R decreased monotonically with increasing deviatoric curvature D, defined as half the difference between the two principal curvatures. θ_A remained unchanged. Here we report on experiments with two different geometries. First, we focused on pendant water droplets on PDMS-coated flat glass plates and cylindrical glass rods with mm-scale diameters. We find that the quasi-static θ_R increases monotonically from about 85° to about 94° and the quasi-static θ_A remains unchanged as the dimensionless product of deviatoric curvature and contact-line diameter increases from 0 to 0.9. To further explore the effect of anisotropic interface curvature, next we studied a PDMS coated glass rod partially immersed into an initially flat water-air interface. With the rod held at an angle from the vertical, we inserted it and withdrew it and measured θ_A and θ_R on the left and right sides. These angles change when the rod tilt angle varies between 0 and 60°. We will present all of these curvature-dependent advancing and receding angle data and discuss possible mechanisms. This work may provide new insights into the origin of contact angle hysteresis.