Wetting Behaviors of a Nano-Droplet on a Rough Solid Substrate under Perpendicular Electric Field.

Fenhong Song,Jing Fan,Ben Q Li,Long Ma,Lihui Zhang,Qicheng Chen

doi:10.3390/nano8050340

Abstract

Molecular dynamic simulations were adopted to study the wetting properties of nanoscale droplets on rough silicon solid substrate subject to perpendicular electric fields. The effect of roughness factor and electric field strength on the static and dynamic wetting behaviors of a nano-droplet on a solid surface was investigated at the molecular level. Results show that the static contact angle tends to decrease slightly and show small difference with the increase of roughness factor, while it shows an obvious increase for the ramp-shaped surface because the appearing bottom space reduces the wettability of solid surface. Additionally, under the electric field, a nano-droplet was elongated in the field direction and the equilibrium contact angle increases with the increase of electric field strength. The nano-droplet was completely stretched to be column-shaped at a threshold value of the field. Besides, accompanied by the shape variation of water droplets, the molecular dipole orientations of water molecules experience a remarkable change from a random disordered distribution to an ordered profile because of the realignment of water molecules induced by electric fields.

Highlights

The wetting of a water droplet on a solid surface plays a critical role in describing the characteristics of solid surfaces in a broad range of technological applications
Niu et al [18] investigated the static properties of a water droplet on a solid surface and the results showed that a water droplet on a pillared silicon surface could transform from the Wenzel state to the Cassie state following vibration of the rough surface
In the absence of an electric field, the static contact angles of a nano-droplet on different rough solid surfaces were analyzed and it was found that contact angle showed a slight decrease and a small difference with the increase of roughness factor

Summary

Introduction

The wetting of a water droplet on a solid surface plays a critical role in describing the characteristics of solid surfaces in a broad range of technological applications. Wettability of liquid on a solid surface is determined by numerous factors, such as chemical composition, material properties (e.g., surface energy) and roughness or heterogeneities of the solid substrate, impurities in liquid, and applied external forces (e.g., electric field force) [7]. Some researchers have conducted simulations and experimental studies on the effect of an electric field on the spreading of droplets on solid substrate and found novel micro phenomena, such as contact angle saturation [20], asymmetrical spreading and asymmetry-to-symmetry transition [21,22,23]. The molecular dynamic simulation method is carried out to explore the static and dynamic wetting behaviors of a nano-droplet on a rough silicon solid surface subjected to perpendicular electric fields. Contact angles and some interfacial properties are taken to describe the effect of electric field strength and roughness factors on the complicated electro-wetting

Molecular Simulation Model

Effect of Roughness on Static Contact Angles

Distribution of Water Molecular Parameters

Conclusions