The Lattice Boltzmann Simulations On Two-Phase Flows With Moving Contact Lines On The Surface Of Solid Material

As the numerical investigations into two-phase flows enter the mesoscopic level,the ratio of area to volume increases so rapidly that the traditional CFD?Computational Fluid Dynamics?method gradually appears insufficient.He-Chen-Zhang model is one of the most widely used LB models in multiphase flow simulation.It can be well simulated for the small scale two-phase flow with moving contact lines on the surface of solid material with HeChen-Zhang model.In this context,a circular droplet slipping along the filament under the gravity field is studied with the two-phase flow lattice Boltzmann method,and the influence of geometric factors on the dewetting dynamics of liquid film on the surface of the rough structure are discussed.The specific work is shown as follows.1)A calculation sketch of a droplet slipping along the filament in the gravity field is designed based on the He-Chen-Zhang model.Here the contact angle hysteresis for non-ideal surface,prescribed by advancing angle ?_A and receding angle ?_R,is taken into account.The numerical results show that the contact angle hysteresis and the transition points on the edge of the filament can hinder the movement of the contact line,so that the slipping droplet will exhibit different motion modes.The contact angle hysteresis tends to prevent the movement of contact lines,but gravity would drive the droplet to slip accompanied by its surface deformation.Accordingly,four dynamic modes for droplet slipping along the wetting filament have been identified for the first time.For the non-wetting filaments,there are only two kinds of dynamic modes during the process of droplet slipping.The droplet tends to adhere to the surface of the filament when it slips along the wetting filament,that is,the contact lines either stay on the surface of the filament,or are pinned at transition points on the edge of the filament.The droplet will not be completely separated from the filament,and will only be suspended on the surface of filament,or partially remained on the filament after the droplet breaking up.As for the non-wetting filament,the slipping droplet tends to detach from the filament,and may not adhere to the surface of filament to reach a static equilibrium,i.e.,the droplet either keeps pinned at the edge of the filament,or detaches from the filament completely without the fracture of droplet.In this paper,the effects of Bond number,Ohnesorge number and contact angle hysteresis on the motion modes of droplet slipping are studied carefully.The phase diagram of six typical slipping modes in the Bo-Oh plane and the Bo-?_R plane are given in the paper.If the wettability of filament is fixed,Bo number will be the key factor determining the slipping mode of droplet;Oh number can greatly affect the dynamic process of the droplet slipping and change its motion mode to some extent.2)A calculation sketch of the dewetting dynamics of a thin liquid film on the surface of rough structure in the Wenzel state is designed based on the axisymmetric HeChen-Zhang model.The influence of geometric factors on the dewetting dynamics of liquid film has been discussed in details,containing the Pillar structure and the Mushroom structure.The numerical results show that there are three motion modes of the liquid film dewetting on the Pillar structure in Wenzel state,but only one motion mode on the Mushroom structure.According to the residual liquid phase in the groove,the dewetting process of liquid film can be divided into no residual mode,partial residual mode and complete retention mode.Here the phase diagram of three typical dewetting modes with the plane of h₃-h₂ are given for the Pillar structure.It is indicated that the liquid phase immersed in the groove is less likely to be retention,as the Pillar height h₂ deceases and the Pillar spacing h₃ increases.For the Mushroom structure,it is interesting to found that the Mushroom height h₂ no longer affects the dewetting dynamics of liquid film.The barbs structure of mushroom may make it pinned for contact lines,and the liquid film will be more and more thinner during the process of interface receding until it is cut off by the transition points.In this case,the velocity of liquid film dewetting mainly depends on the Mushroom spacing.Specifically,the liquid film will spend more time in the groove during the dewetting process,as the spacing of Mushroom increases.What's more,the dewetting dynamic of liquid film on the Mushroom in the special state?partial Wenzel and partial Cassie?is also studied in details.