Investigation Of Liquid Film Deposition And Contact Line Dynamics During Two-phase Displacement

The phenomenon that two immiscible fluids displace each other has important applications in nature and industry,and it is also an important issue of interfacial fluid mechanics.In the process of displacement,the interface changes from a stable meniscus to an unstable wetting transition state,which involves the dynamics of moving contact line and the three-dimensional effect of the solid substrate.The exploration of its mechanism can contribute to the rapid improvement of process technology.In this paper,the problem of two-phase displacement on substrates with complex geometries is experimentally studied.The main work and results are given as follows:(1)The wetting transition in a partially wetting horizontal circular capillary is experimentally investigated,focusing on the critical displacement speed,the change of contact line velocity and the influence of displacement speed on bubble length and liquid slug length.When the displacement speed is lower than the critical value,the gas-liquid interface keeps a stable meniscus structure and moves forward at a constant speed.When the speed exceeds the critical value,the contact line speed suddenly drops,and a layer of liquid film is pulled out on the tube wall.Close to the contact line,the liquid film behaves as a ridge-shaped bulge,and its volume increases,eventually leading to a pinch off and generation of Taylor bubbles.This process is repeated continuously,resulting in a periodic bubble structure.As the displacement speed increases,the length of the bubbles gradually increases,while the length of the liquid slug between the bubbles gradually decreases,and finally tends to a stable value.The experimental results of bubble velocity and length are in good agreement with the theoretical results of Bretherton(1961a)and Gao et al.(2019).(2)The gas-liquid displacement of a partially wetting liquid(with a contact angle of about 100°)in a square capillary is experimentally studied.We mainly focus on the wetting transition phenomenon of two-phase displacement,contact line dynamics and the movement of bubbles.With the continuous increase of the displacement speed,there are four typical flow regimes:(?)stable meniscus;(?)the thin liquid filaments on both sides of the corners fall off and break into droplets;(?)thick liquid filaments in the corners are generated and spontaneously retract;(?)the corner filaments and the liquid film on the wall coexist,producing Taylor bubbles.The experiment has verified that the cross-section of the bubble in the square tube is approximately circular,and an empirical formula for predicting the thickness of the liquid film is proposed.On this basis,formulas for calculating bubble length and velocity consistent with the experimental results are proposed.(3)We experimentally study the gas-liquid displacement problem in a vertical square capillary with partial wetting(the contact angle is about 49°).The main concern is the movement of the moving contact line,the evolution of the inner wall liquid film,and the dynamics of bubbles.It is found that the gas-liquid displacement in the partially wetting square tube exhibits five interface modes:(?)stable meniscus;(?)thin liquid filaments are pulled out on both sides of the corners and break into small droplets;(?)wetting transition first occurs at the inner corner,pulling out the thick liquid filament,and the end of the thick liquid filament is connected with two thinner liquid filaments;(?)wetting transition occurs on the wall,and the first rim structure is unstable,leading to the second rim;(?)only one rim is produced,and it is directly closed to generate bubbles.The experiment gives the critical displacement speed of the five modes,and also shows the variation of the retraction speed of the liquid filament and film.As a supplement,the vertical rise of bubbles in the square tube was further studied.It is found that there is a critical bubble length,beyond which the bubble rising speed in the square tube does not change.The experimental results of the rising speed of the bubble with the size of the pipe are given,and are in good agreement with theoretical results.(4)The experiment is performed to study the morphology of the droplet sliding on an inclined solid corner of the partial wetting(contact angle is about 49°).There are different shapes of droplets depending on the angle of inclination.We mainly focus on the droplet evolution,the movement of the moving contact line and the geometric shape of droplet evolution.When the angle of inclination is small,the droplet falls very slowly,and the shape of the droplet can be described as a pearl.When the tilt angle is moderate,a trail is formed at the tail of the droplet.When the inclination angle goes up further,the liquid filament dragged out by the droplet appears a capillary shock,connecting a thick filament and a thin one,and the thin liquid filament eventually breaks up.The experiment gives the critical angle of inclination between the three modes,as well as the variation of the contact line velocity with the angle of inclination.The morphology of the liquid filaments drawn from the corners is further studied.