Enhancing Directional Droplet Transport via Surface Charge Gradient: Insights from Molecular Dynamics Simulations.

The phenomenon of spontaneous droplet transport has a wide range of implications in water collection, microfluidic manipulation, oil-water separation, and various other fields. Achieving efficient and controllable spontaneous droplet transport is therefore of paramount importance. This study investigates the potential of surface charge manipulation to enhance spontaneous droplet transport through comprehensive molecular dynamics simulations. Our findings reveal that the surface charge of the substrate significantly influences its wettability, reducing the contact angle of the droplet and increasing both the contact area and interaction energy. Moreover, we introduce a novel approach to enhance droplet mobility by creating a surface charge gradient on the substrate. By introducing bands with varying charges along a specific direction of the substrate, the droplet experiences a force directed toward regions of increasing charge, thereby facilitating its movement. Importantly, the driving mechanism of droplet motion is well explained by combining classical electrowetting theory with the analysis of the droplet's advancing and receding contact angles, which demonstrates that a more pronounced surface charge gradient generates greater force and enhances droplet mobility. These findings offer valuable insights into the design of microfluidic systems and related applications based on electrowetting.