Self-selected motion of overlaid droplets on heterogeneous surfaces via competitive wetting

Droplets would move following the designed wettability gradient on the surface, toward the position with high surface energy, which is vital for various applications like ink printing, biological detection, and water collection. Anomalously, here we have found that the core of the overlaid droplet shows anti-wettability-gradient movement on the wettability boundary. The core droplet would perform different directional motions after the impact on the same established surface, depending on the impact dynamics and the composition ratio of the overlaid droplet. The macroscopic motion preference of the overlaid droplet is related to the wetting preference of the overlayer on the heterogeneous surface. Competitive wetting/dewetting of different components of the overlayer droplet induces a special shuttle motion of the core droplet. The phenomenon would provide new insight into wetting and droplet control, being promising for broad application scenarios.