Lubricant-induced tunability of self-driving nanodroplets on conical grooves

Droplet manipulations are ubiquitous in a broad range of applications, including microfluidics, bio-analysis, and cargo transport. With the increased complexity and requirements of the applications, it is necessary to seek tunability for smart control of droplets. Hence, in the present work, a strategy based on conical grooves was proposed to realize a combination of tunable control and self-driving nanodroplets. The present idea should be an integration of the inspiration of multi-species, including butterfly wings, cactus spines, and pitcher plant leaves. Droplets can move spontaneously on the proposed surface driven by surface Laplace pressure differences, and the direction of movement can be reversed only by the addition of a lubricant layer. The wetting and transition of nanodroplets on the conical grooves were investigated using Molecular Dynamics simulation, and it was demonstrated that three types of self-transport mechanisms can be summarized, and this should be the origin of the tunability. Moreover, a complete frame was established based on the coupling effects of the groove morphology and interface tension. Thus, it was concluded that the reversible motion was induced by a transition of the self-transport mechanism triggered by a new-formed lubricant-water interface. It is believed that the tunability shown in the present work can help explore the space of the smart control policy of nanodroplets.