Research On Droplet Dynamics On Complex Surfaces

As a significant part of the interface science,the dynamics of droplet on solid surface has attracted great attention from researchers in recent years.The typical dynamics of droplet on solid surface are spontaneous wetting,impinging,evaporation,condensation,etc.So far,most studies,however,have been carried out by utilizing simple liquid droplets（water,ethanol,glycerin and their mixture）and solid surface prepared in lab and there has few research performed with the introduction of complicated surfaces,soft surfaces and complex liquids that are commonly applied in industry.Therefore,this dissertation combines experimental and theoretical analysis to deeply study the physical mechanism behind phenomena of spontaneous wetting,impinging and evaporation of droplets on complex surfaces.The main findings are as follows:1.In this study,the wetting processes and impinging phenomena of droplets on SiO₂ nanofilms before and after gamma irradiation are observed relying on the key project of The National Natural Science Foundation of China.The results show that the irradiation changes not only the surface morphology but also the chemical properties.Further analysis of the spanoustance wetting processes of droplets shows that the increase of surface free energy is the root cause of the wettability change.In addition,the influence of surface wettability on the dynamics of impinging droplet is studied.With the increase of the irradiation dose,the contact line of the impact droplet is pinned much easier,and the θ_C is smaller when the droplet reaches stable state.This study provides a theoretical basis for the degradation of SiO₂ nanofilm properties on optical components in high power laser devices,and explains the reason why a large number of optical components in laser devices are easy to be contaminated.2.The droplet impingement and atomization behavior on superhydrophobic mesh is studied.The results indicate that the atomization happens at both the recoil stage and the spreading stage when the droplets impinge on the superhydrophobic mesh.Through experimental and theoretical analysis,the results reveal that the spray formed during droplet recoiling is stimulated by the longitudinally symmetric air cavity collapse on the superhydrophobic mesh;and the water spray produced during droplet spreading is expelled under the action of the inertia-induced hydrodynamic pressure.By analyzing the spray coverage area,it is found that the correlation between its effective radius R_WS-S and the Weber number could be discriped by R_WS-S～We1/2.Neverthless,given the restriction of pore structure of the mesh,the effective radius R_WS-S of the spray coverage area barely changes when it reachs the critical value.The size of spray is also analyzed,and the results demonstrate a linear correlation of （?）/L～1.75 between the average spray size and pore diameter of the mesh.Besides,the distribution of spray size matches the Gamma distribution.As a typical multi-phase fluid phenomenon,the atomization behavior is widely applied in agricultural and industrial fields.This research deeply studies the correlation between the spray characteristics and the droplets impinge conditions,so it will be helpful for the application of atomization in associate feilds.3.The evaporation of blood droplets on surfaces with different moduli is studied.Compared to other liquids,the evaporation process and patterns formed after evaporation of blood droplets on surfaces with different moduli are more intricate.The results indicate that 1)As for blood droplet with a size less than capillary length,its pattern formed after the evaporation is only dependent on the surface modulus and not correlated with its size.2)Under the double effects of evaporation flux and the surface viscoelasticity,blood droplets can finally form unique cake and straw-hat patterns on the surface with different moduli.3)The evaporation process of blood droplets can be divided into pre-gelation,gelation and desiccation stages.4)The surface modulus obviously affects the contact line and contact angle of the blood droplets during evaporation.By analyzing the contact line and the contact angle of blood droplets over time,the critical time of gelling can be found.As an inter-disciplinary topic of biology and physics,this research can provide the reliable data to support the development of biochips.