Bi-axially Morphing Droplet Shape by an Active Surface

Drop morphology can be manipulated by designing localised solid/liquid interactions to create a favourable interfacial energy equilibrium. A topographical surface with hierarchical roughness can be harnessed to generate complex drop morphologies, enhance uni-axial and anisotropic spreading, in a designable fashion. Here, we propose using an active surface with a responsive roughness (wrinkle patterns) under uniaxial compression/stretching, to morph droplet shape bi-axially in a continuous and reversible manner. The keys to achieve biaxial drop shaping are the in-plane confinement from lattice hole patterns and the programmable formation of roughness, to pin and guide contact line movement in both in-plane directions. The complex interplay between wetting and the patterns are elucidated by both experiments and numerical analysis. Our results enrich the current understanding of shaping droplets by managing the contact line pinning/movement on an engineered elastic substrate, and providing insights for emerging applications in the areas such as droplet micro-fluidics, liquid robotics, ink-jet printing, 3D printing and healthcare.