Study On Mechanism Of Drop Impinging On Non-wetting Surface And Its Application

As a common capillary phenomenon,drop impacting on solid surface has attracted great attention in both basic scientific research and industrial engineering.In recent years,based on the development of materials in super-wetting surfaces which demonstrated great potential in applications of anti-fogging,anti-icing,drug spraying,inkjet printing,internal combustion engine combustion and liquid atomization,the phenomenon of drop impact on non-wetting surfaces has also been widely studied.However,due to the complex physical mechanism and diverse influential factors,it necessitates more research on the drop impact on the non-wetting surface to unveil potentially unknown phenomenon and mechanism,such as solid-liquid/liquid-liquid interaction during the impact process,the change of the surface wettability after droplet impact,the effect of air layer on the drop impact,and so on.Different from the previous studies which focused on controlling the impact and rebound of drops on non-wetting surfaces,this dissertation mainly studies the liquid-solid interaction during the drop impact on non-wetting surfaces by introducing new impact parameters and its applications in fields of microdroplet preparation and rainproof windows.The main research results and conclusions are as follows:(1)In this dissertation,a novel splashing phenomenon of two-component drop impinging on a superamphiphobic non-wetting surface is investigated,which promotes the splash,and the relevant mechanism is revealed by experimental and numerical simulation.It is generally believed that the drop splashing can be inhibited by increasing the viscosity of the impinging drops.However,by constructing water-glycerin twocomponent drops,we demonstrate a novel impingement phenomenon that the splashing and instability of low-viscosity water drops is promoted by the high-viscosity glycerin part.Different from single-phase drop impact,the two-component drop presents an asymmetric shape due to the different diffusion and contraction dynamics in water and glycerin during the impacting process.Through theoretical analysis and simulation,it is revealed that the interface deformation induced by shearing force can promote the drop splashing.It is worth noting that,unlike the phenomenon of splash suppression in previous studies,our work may have a wide range of applications in accelerating the splash process.The discovery is expected to be extended to other applications,including inkjet printing,pesticide deposition,and spray cooling processes.(2)In this dissertation,a method is proposed to induce the abnormal partial rebound phenomenon of drop impact on the superhydrophobic non-wetting surface by using the surface charge effect.Based on this method,the controllable preparation and manipulation of microdroplets on the superhydrophobic non-wetting surface is realized.Specifically,the surface adhesion is regulated by the surface charge generated by drop impact on the superhydrophobic surface.During the retraction phase of drop impact,the rising liquid column was pulled by the surface,thus generating microdroplets under the Rayleigh instability.The amount of surface charge generated depends on the impact energy of the drop.The electric field force generated by surface charge,liquid surface tension and Rayleigh instability will jointly determine the generation and size of microdroplets on the superhydrophobic surface.The advantage of this method lies in the utilization of regulated adhesion by surface charge on a non-wetting surface,where microdroplets with tens of microns can be obtained without additional precision equipment or reducing the size of the nozzle.In addition,the method can manipulate the resulting microdroplets due to the erasable capability of surface charge.The findings are expected to be applied in microdroplet reactors,self-assembly and on-demand sampling.(3)Based on the principle of wettability,by introducing 100% gas layer on the surface of a hydrophilic glass,it is demonstrated that the impacting droplets can rebound under the gas layer applied on the hydrophilic glass surface,so as to realize the nonwetting property on the hydrophilic glass.Specifically,a device for introducing gas layer is designed.By this device,the gas layer with controlled thickness and velocity is successfully introduced into the glass surface.Through a series of droplet impacting experiments and numerical simulations,it is demonstrated that the droplet impacting velocity,the velocity of gas layer,and the thickness of the gas layer finally determines the wettability of glass surface.The relationship of these three influence factors is also established.Furthermore,the experimental results show that this method can be well applied to rainproof windows.Through the experimental simulation on rainy days,it is proved that the introduction of gas layer at the solid-liquid interface can avoid the blind field of vision caused by the adhesion of raindrops on the side window glass and the rearview mirror.This method is expected to solve the hidden danger of driving safety caused by raindrop adhesion.