Directed droplet transport is crucial for applications, such as microfluidic manipulation, targeted water harvesting, and efficient thermal management. However, strategies relying on a single driving mechanism are often limited by either insufficient propelling force or high energy consumption, making it challenging to achieve both high-speed and long-distance transport. In this study, we propose a simple yet effective dual-gradient synergy strategy that couples a surface wettability gradient with a surface charge gradient to enable rapid and long-range droplet transport, as demonstrated through molecular dynamics simulations. A charged wedge-shaped hydrophobic pattern was constructed on a superhydrophobic substrate, subjecting the droplet to both an unbalanced Young's force and asymmetric electrostatic interactions during its motion. Our results show that the wettability gradient dominates the initial acceleration of the droplet, whereas the electrostatic driving force becomes progressively dominant as the wettability-induced force decays, thereby sustaining the droplet motion over significantly extended distances. The effects of the wedge apex angle and surface charge density on the droplet driving force, velocity, and acceleration are systematically investigated, with the underlying transport mechanism elucidated through detailed force analysis. Finally, curved and path-selectable dual-gradient surfaces are designed to demonstrate the high design flexibility and practical applicability of the proposed approach. This work provides novel design concepts and a theoretical foundation for potential applications in droplet manipulation.
Jia et al. (Fri,) studied this question.