Nonequilibrium Interfacial Behaviors Of Water Droplets

Water droplet as a unit of water is common in nature as rain drops or dewdrops and important in industrial as well as agricultural processes including spray cooling,printing,lubrication,and irrigation.If a water droplet is deposited onto a solid substrate,its contact angle dominated by surface tensions reflects equilibrium interactions between water droplet and the solid substrate.If a water droplet is deposited onto a liquid reservoir of high mobility,the liquid surface presents various transient phenomena such as splash and coalescence.These transient phenomena usually finish within seconds driven by surface tension.As an efficient container for mass and heat transfer,nonequilibrium behaviors of water droplet is worth studying.For situations in which water droplets are deposited onto liquid and solid surfaces,we choose water droplets on miscible liquid film and water droplets on charged solid surfaces in aerosols to study,respectively.We discover that aqueous droplets exhibit spherical caps upon contact with a fully miscible liquid film of lower surface tension,rather than flatten within seconds,where the continuous liquid surface is similar to the partial wetting on a solid substrate,despite the spontaneous mixing of the two liquids.The droplet gradually flattens and collects film liquid before merging into the film after minutes.Its apparent contact angle can be higher than 50°for a water droplet on ethanol thin films and its lifespan can be up to minutes.Analysis with the help of numerical simulations show that benefiting from large viscous forces in thin film due to spatial confinement,the Marangoni surface flow is substantially suppressed.Therefore,the surface tension gradient responsible for this phenomenon is maintained because the normal diffusion of film liquid into the droplet can timely dilute film liquid supplied by up-hill Marangoni flow at the droplet surface.We obtain positively and negatively charged surfaces via changing temperature of Li Ta O₃ crystals and observe that a large net surface charge up to 0.1 C/m² can nominally change the contact angles of pure water droplets comparing to the same uncharged surface.However,even a small amount of surface charge can efficiently increase the water contact angle in the presence of aerosols as the exposure continues.Our results indicate that such surface charges can hardly affect the morphology of the macroscopic droplet,while progressive adsorption of a small amount of organic contaminant from aerosols with the help of Coulomb attraction can notably decrease the wettability of solid surface.In this thesis,we investigate the nonequilibrium behaviors of water droplets when deposited onto a miscible thin liquid film of lower surface tension and when deposited onto solid substrates with surface charges in aerosols.Our findings remove the conventional cognition that droplet coalescence is prompt on fully miscible liquid surfaces,and provide a fundamental understanding of the interactions between charged surfaces and water in real aerosol environments.These results are expected to benefit the design of new types of microfluidic devices and control of wettability in real environments.