Contact angle hysteresis of a droplet on a hydrophobic, anisotropically curved surface

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 focus on a water droplet suspended from PDMS-coated flat glass plates and cylindrical glass rods with mm-scale diameters. Our droplet volume ranges from 2-24μL, with air-water-solid contact line diameters from about 1.7-2.7mm. In our experiment, we find the quasi-static θ_R decreases from about 84° to about 70° when D increases from 0 to 0.04/mm. The quasi-static θ_A decreases from about 103° to about 99° when D increases from 0 to 0.17/mm. Therefore, the contact angle hysteresis increases from about 19° to about 33° as D increases from 0 to 0.04/mm. In addition, we find that the contact line recedes more along the azimuthal direction than along the axial direction. This work may provide new insights into the origin of contact angle hysteresis and offer a route to calculating droplet shapes.