Dynamics of drop impact and contact line motion on micro-pillared surfaces

Physical texturing creates patterned and pillared surfaces that display superhydrophobicity in drop spreading and drop impact studies. Often, such surfaces are accompanied by large hysteresis since the three-phase contact line may get trapped over and within the pillars. In this context, wetting characteristics of a water drop spreading over a micro-pillared surface of copper are investigated. Apart from drop spreading on a bare pillared surface, two companion studies where the pillars are fully and partially coated using superhydrophobic and hydrophobic coatings have been carried out. The Weber number and Reynolds number based on the drop diameter and impact speed are varied over the range 1–41 and 440–2870, respectively, while the Bond number remains constant, ∼1.03. Imaging sequences show spreading behavior that is distinctive of the surface chosen. For a fully coated surface, the drop is seen to jump-off upon impact while a residual drop remains over uncoated and partially coated pillars during the receding phase. These observations are compared against three-dimensional numerical simulations that resolve the pillar shapes. Simulations are seen to be qualitatively in good agreement with experiments. Simulations additionally probe the contact line movement over coated and uncoated pillars for comparison with experiments. The filling of the interpillar space with liquid and the resulting interface deformation are examined. Jointly, the emptying of the interpillar gap is also discussed. Experiments and simulation show a jump in drop footprint when the contact line leaves the pillar, and the associated velocity becomes large.