Research On Cross-scale Interfacial Flow Dynamics Of Impinging Drops

The drop impact on solid surface is one of the most common mesoscopic physical phenomena in nature,which has a wide range of applications in both agriculture and industry.However,the existing researches on drop impact are on the focus of the relatively macroscopic phenomena,such as deposition,rebound,and the maximum spreading radius.The cross-scale dynamics during impact process is less analyzed.This dissertation combines experiment,theoretical analysis and numerical simulation to study impact behaviors ranging from millimeter scales to micrometer scales.The understanding of the cross-scale behavior helps to have a comprehensive knowledge on the mechanism of the drop impact dynamics and promotes its applicaition development.The main results of this dissertation are as follows:(1)We combined experimental observations and numerical simulations to perform a systematic study on the spreading dynamics of diverse viscous droplets on different wettable surfaces.Experimentally,we spelled out the coupling effect of liquid viscosity,surface wettability and impact velocity on droplet configuration evolutions,droplet-surface contact line dynamics,and variations of dynamic contact angles.Numerically,we not only reproduced the spreading characteristic of diverse impinging droplets,but also recovered their energy budgets,which further allowed us to understand the failure of the existing theoretical model based on energy conservation to predict the maximum spreading factor in the experiments.We also performed analyses of the residual flow fields in the impinging droplets at the maximum spreading,where circular flows could be formed when the impact velocity was sufficiently high,and we experimentally visualized such flows.(2)The jetting dynamics triggerd by the drop impact and its instability were analyzed.Based on the above results on the spreading process from macroscopic view,the research was then extended to the receding process,where jetting occurs.The effect of surface wettability on jet droplet was first explored.It is found that the jet formation is robust on non-wetting surfaces.Then a systematic study on water drop impact on superhydrophobic surfaces was conducted and two jetting mechanisms were obverved.At low Weber numbers,the axial squeezing flow during the cavity collapse caused the singular jetting which results in V_j(?)R_j^-1.However,under relatively higher Weber numbers,the jet emission originates from the focusing of the capillary wave and thus jetting velocity follows V_j(?)R_j^-0.5.In the further rising,Rayleigh-Plateau instability is stimulated and thus a jet droplet is produced.Through dimensionless analysis of the thinning process of the jet and the mass conservation,a general equation describing the inertial dominated thinning process and the relation between sizes of the jet and the jet droplet were obtained.Moreover,combining momemtum conservation for the free surface flow and the spreading dynamics,the size of the jet droplet and Weber number were correlated.The effect of liquid property on jet droplet was further investigated and phase figures as function of liquid properties for the jet droplet generationtion were obtained.Finally,a handy macrodrop-impact-mediated fluid microdispensiong was proposed and its potential applications were demonstrated.(3)The spreading and retraction dynamics of the impinging oil-in-water compound drops on superlyophobic surfaces were explored.Based on the above results of monophase impinging drops,the impact dynamics was extended to multiphase drops.New impact phenomena were observed in compound drop impact such as the water droplet entrapment in the oil core.It is found that the small oil core could inhibit the bubble entrapment and singular jetting formation.The water droplet entrapment in oil core,which is the consequence of the competition between the pressures in water and oil,was observed for compound droplets with large oil volume fractions and within a certain range of Weber numbers.Within the scope of our research,the spreading dynamics is not significantly affected by the oil cores.However,the oil cores accelerate the receding process,which in turn reduce the contact time up to 20%.At last,based on the water entrapment inside the oil core,a new method to produce the micro water-in-oil compound drop was proposed.