Dynamic contact angle beating from drops impacting onto solid surfaces exhibiting anisotropic wetting

This paper reports an experimental investigation of low Weber number water drops impacting onto solid surfaces exhibiting anisotropic wetting. The wetting anisotropy is created by patterning the solid surfaces with unidirectional parallel grooves. Temporal measurements of impacting drop parameters such as drop base contact diameter, apparent contact angle of drop, and drop height at the center are obtained from high-speed video recordings of drop impacts. The study shows that the impact of low Weber number water drops on the grooved surface exhibits beating phenomenon in the temporal variations of the dynamic contact angle anisotropy and drop height at the center of the impacting drop. It is observed that the beating phenomenon of impacting drop parameters is caused by the frequency difference between the dynamic contact angle oscillations of impacting drop liquid oriented perpendicular and parallel to the direction of grooves on the grooved surface. The primary trigger for the phenomenon is the existence of non-axisymmetric drop flow on the grooved surface featuring pinned and free motions of drop liquid in the directions perpendicular and parallel to the grooves, respectively. The beat frequency is almost independent of the impact drop Weber number. Further experimental measurements with solid surfaces of different groove textures show that the grooved surface with larger wetting anisotropy may be expected to show a dominant beating phenomenon. The phenomenon is gradually damped out with time and is fully unrecognizable at higher drop impact Weber numbers.

(a) Beating phenomenon observed in the temporal variation of dynamic contact angle anisotropy, Δθ (=θ⊥ − θ||) during the impact of water drop with Do ≈ 2.55 mm and Uo ≈ 0.5 m/s onto a grooved surface, and multiple beats seen in the temporal variation of Δθ (b) and drop apex height, z/Do (c).Figure optionsDownload as PowerPoint slideHighlights
• Drop impact on grooved surfaces studied experimentally.
• Non-axisymmetric drop flow resulted from drop impact on grooved surfaces.
• Beating phenomenon is observed in the temporal variations of drop impact parameters.
• Beating phenomenon becomes less pronounced as impact We increases.
• Beating phenomenon is more dominantly seen for drop impact on grooved surfaces with higher wetting anisotropy.