Dynamic spreading of a water nanodroplet on a nanostructured surface in the presence of an electric field

Abstract

Molecular dynamics simulations are implemented to investigate the statics and dynamics of wetting for a water nanodroplet on a nanostructured surface in the presence of a vertical electric field. The results show that the electric field induces electro-stretching, electro-wettability, modified solid-liquid interfacial tension, and pinning at the triple line, and they jointly affect the spreading exponent and static contact angle of the nanodroplet. Under an upward electric field, the spreading is always hindered, and the static contact angle monotonously increases with the field strength. Interestingly, under a downward electric field, the spreading is first hindered and then is promoted as increasing the field strength, leading to a first increased and then decreased static contact angle. The increased solid-liquid interfacial tension and the enhanced pinning are found to be two main mechanisms for the slowing down of spreading and the increase in static contact angles for both electric fields at low field strengths. The same trends are observed at high field strengths under the upward electric field; however, exactly the reverse trends occur under the downward electric field, leading to the acceleration of spreading and the decrease in static contact angle at high field strengths. Moreover, it is found that the enhancement in intrinsic wettability of the surface can suppress the breaking of hydrogen bonds, thereby reducing the solid-liquid interfacial tension and accelerating the spreading. The enhancement in intrinsic wettability can also reduce the pinning at the triple line as well as the energy barrier of the Cassie-Wenzel transition, which contributes to the reduced static contact angle.