Experimental Study On Droplet Spread Dynamics On Superhydrophobic Pillar-arrayed Surfaces

Droplet impact on solids surfaces is a common phenomenon in nature.Controlling the spreading is closely related to many industrial and agricultural scenarios during droplets impacting the surfaces.For bioship or liquid metal printing,it’s desirable to control the spreading profiles of droplets.And it is necessary to increase the contact area between droplets and surfaces during the process of pesticide spraying to improve the utilization rate.It was reported that droplets impact rough surfaces presenting polygonal contour,which provides a basis for the regulation of the spreading profile of the droplets.However,only a few studies have focused on the regulation of droplet spreading profiles on the superhydrophobic Pillararrayed surfaces.Therefore,this study performs experimental research on droplet spread dynamics on the pillar-arrayed surface,including the following aspects.Based on the idea of anisotropy spreading on the pillar-arrayed surfaces,experiments of droplets impinging on the surfaces with various topographies were performed,then the spreading profiles variations and inherent mechanisms were studied.Surfaces with arrays structure of submillimeter scales were fabricated by laser etching and template method.The dynamics process of droplets impacting on the surfaces were recorded by a high-speed camera to analyzing the variations of the spreading profiles.It was found that the spreading profile was circle at low Weber number.Faceted spreading and retraction of droplets were observed at high Weber number,and square-,pentagon-,and hexagon-shaped droplets were registered.A theoretical model was established according to the spreading velocity.It is verified that during spreading,two main stages were recorded as kinematic stage and the viscous stage.The viscous stage,in turn,appeared as a consequence of two substages governed by various time scaling laws.Cassie-Wenzel transitions start from the area adjacent to the axis of droplets,and the wetting area increases with the Weber number.The anisotropy spreading mechanism is revealed.That is,the droplet immerses into the microstructure during spreading on pillararrayed surfaces.The anisotropy of posts arrangement leads to the anisotropy of the viscous dissipation,which in turn presents a polygonal profile.To reveal the effect of micstructure,viscosity and other parameters on the droplets spreading characteristics,an experimental study of multiphase droplets impacting on the pillararrayed surfaces was conducted.The droplets containing ethanol with different volume fractions impacted on the pillar-arrayed surfaces,and were compared with deionized water.The effect of volume fractions,Weber number,viscosity and surfaces topographies on spreading characteristics was analyzed.Results indicate that the larger the Weber number or center-tocenter spacing,the larger the kinetic energy and the smaller viscous resistance obtained during the droplets impinging surfaces,and the larger the spreading speed and maximum spreading diameter.The greater the viscosity or surface tension,the greater the viscous dissipation during the droplets spreading on the rough surface,and the smaller spreading area is sufficient to store the remaining energy.Therefore,under the same experimental conditions,droplets with low surface tension and viscosity are more likely to obtain a lager spreading velocity and diameter.Through the above study,the effects of surface topographies on the profiles and spreading characteristics of droplets is analyzed.This provides a theoretical and experiment basis for regulating the shape and spreading of droplets.