Self-propelling droplet on polyelectrolyte-grafted soft interfaces

Self-propelling droplets are widely useful for a variety of applications, including self-cleaning, water harvesting, drug delivery, and biological sample analysis. However, state-of-the-art methods of droplet transport face limitations due to a trade-off between achieving high velocity and ensuring a longer transport range. In this work, we, for the first time, introduce a novel technique of patterning the surface with a soft polyelectrolyte layer of variable thickness. Such nanoarchitecture ensures a longitudinal gradient in surface charge, inducing an asymmetric electric force that can be utilized to drag the droplet along the functionalized path, letting us achieve higher velocities for longer range. We numerically investigate the efficacy of the proposed approach along with a simplified analytical model. Subsequently, the effects of polyelectrolyte charge and grafting thickness gradient on droplet transport behavior are examined. The proposed approach can not only showcase higher droplet velocity along with a long-distance transport but could also open up the possibility of tuning the droplet motion through modulating polyelectrolyte charge through changes in solution composition, thereby unraveling a new way of controlling droplet motion with the use of soft materials.