Investigation On The Singular Jet Of Viscous Droplets Impacting On A Superhydrophobic Surface

The phenomenon of droplets impacting the wall is widely found in additive manufacturing,epidemic prevention and control,spray cooling,anti-icing,pesticide spraying and surface cleaning,etc.A jet with very small diameter but high velocity,which is called the singular jet,might be occurred when a droplet impacting on the superhydrophobic surface.The previous study mainly focuses on the singular jet behavior when pure water droplets impacting on the surfaces.However,the working fluid in many applications is not limited to pure water,and the evolution of the internal flow field structure,velocity change and pressure distribution when the singular jet occurs has not been studied.Therefore,in order to explore the influence of viscous changes on the behavior of singular jets and the law of the internal flow field.Experimental and numerical simulation study were carried out in this paper.A visual experiment setup was established based on microscopic high-speed camera technology to capture the dynamic process and singular jet phenomenon of droplets impacting on the superhydrophobic surfaces.The superhydrophobic surface with a static contact angleθ ≈158° was prepared and contact angle hysteresis△θ<5°.We prepared various viscosities(from 0.9m Pa·s to 27.7m Pa·s)Newtonian working fluid by proportioning glycerol/water solution.Based on the finite element method,a numerical model of the droplet impacting the wall is constructed.We analyzed the effects of viscosity change on the flow field structure,pressure field,velocity vector and singular jet behavior of the droplets.The phase diagram for the occurrence of singular jet behavior of viscous droplets is summarized.The main research results are as follows.The experimental results show that when the pure water droplet hitting the superhydrophobic surface in the lower We number range(2.7≤We<18.8),the entrained bubbles will directly lead to the generation of strange jets.However,when the viscosity of the droplet is greater than 14.2 m Pa·s,even if the droplet impact velocity(We > 100),the singular jet phenomenon no longer appears.From the dimensionless phase diagram,it can be found that the singular jet mainly occurs in the region of Re number between 700 and 1000.Moreover,the range of We numbers for the singular jet is wider.However,no singular jet phenomenon occurs in the regions of Re<300 neither Re>1100.The numerical simulation results indicate that the singular jet is related with the formation of the cavity during the retraction stage of the droplet impacting.And when the singular jet occurs,there is a large pressure field at the center of the droplet.Altering the viscosity could change the interfacial morphology between the gas and the liquid at the bottom of the cavity inside the droplet.As the viscosity increases,the gas-liquid interface at the bottom of the cavity will change from an upwardly convex shape to a downwardly concave shape.Therefore,an upward jet cannot be formed and singular jet would not happen.The results show that changing the viscosity of working fluid will affect the singular jet behavior when the droplets impacting on the superhydrophobic surfaces.This study will provide a theoretical basis for the regulation of droplet dynamics.