Curvature-Driven Droplet Transport and Spontaneous Motion Using Concentric Ring Structures

Directional motion of droplets following impact is often attributed to gradients in surface wettability, but such motion can also emerge from geometric asymmetry on microstructured surfaces with a fixed pitch. Using high-speed imaging and goniometric analysis, we show in this study that 2.6 mm water droplets impacting concentric rings with pitch values of 25 and 100 μm at Weber numbers of 1.6-31.6 exhibit a net inward motion. This occurs despite the absence of any topographic or chemical gradient. Goniometric analysis revealed nonuniform wetting properties arising from fabrication-induced variations, while a simple model of the contact area demonstrated geometric asymmetry in curvature and wetting interaction at the end of the spreading phase. These combined effects produce "gradient-like" directional behavior on nongradient structures. Our findings highlight the underappreciated role of radial curvature in enhancing droplet transport and offer new avenues for fluid management and system design.