Phase Transitions Of Liquid Drops On Solid Substrates:Film Boiling And Solidification

When a drop impacts on solid substrates,phase transition may happen depending on the temperature of the solid substrate.In this thesis,we study phase transitions of a drop on substrates,including the liquid-vapor phase transition and liquid-solid phase transition.We mainly consider about two aspects,one is the film boiling of liquid drops on superheated substrates.Another one is the solidification of a drop on the supercooled substrate.Firstly,for the drop on the superheated substrate,a vapor layer is formed below the drop,isolating it from direct contact with the solid substrate and,thus,constituting the socalled Leidenfrost state.Leidenfrost drops may have two final fates,including explosion and takeoff,which may result from the contamination within the drop.Therefore,we study the evaporation dynamics of suspension drops containing micro-particles,and find the suspension drops tend to explode at the end.The reason for the explosion is that the drop locally contacts the heated substrate,then the liquid at the contact area rapidly boils and induces the drop explosion.The radius of the drop before the explosion is determined by the initial drop size and initial concentration of particles.Then we can predict the final fate of a Leidenfrost drop through its initial condition.Secondly,we study the boiling process of a ternary ouzo Leidenfrost drop.Due to different volatilities and mutual solubilities of three components,the multicomponent drop experiences very rich evaporation dynamics.The initial drop is miscible and transparent.The preferential evaporation of the most volatile component,ethanol,triggers nucleation of the oil droplets in the remaining drop,which,consequently,the drop becomes an opaque oil-in-water microemulsion.The tiny oil droplets subsequently coalesce into a large one,which,in turn,wraps around the remnant water.Because of the very low evaporation rate of the encapsulating oil layer,the drop can no longer produce enough vapor for its levitation,and,thus,falls and contacts the superheated surface.The direct thermal contact leads to vapor bubble formation inside the drop and consequently drop explosion in the final stage.Thirdly,we study the impact of a Leidenfrost drop on inclined superheated substrates.Due to the vapor layer between the drop bottom and the substrate,the motion of the drop is hardly affected by the drag force from the substrate.The spreading dynamics of the drop is mainly determined by the vertical component of the impact velocity to the substrate,and the sliding distance is mainly determined by the parallel component of the impact velocity to the substrate.In the end,we develop a hybrid numerical method based on the volume of fluid(VOF)method and the immersed boundary method(IBM).Considering the density difference between the solid and liquid phases,surface tension and gravity,the numerical method can be used to simulate the solidification problem with three phases containing two moving interfaces,such as the freezing process of liquid films and drops on solid substrates.The theoretical solution and experimental result are used to check the accuracy of the method.In addition,we experimentally and numerically investigate the effects of the conductivity of the surrounding medium on drop freezing dynamics.It is found that the surrounding medium with high conductivities can be regarded as another cold source,which can help to cool down the outer surface of the drop and induce an ice shell formation.The ice shell will limit the dilatation of the encaptured water phase,resulting in the change of the final shape of the frozen drop.