Surface Roughness Effects On Contact Line Motion With Small Capillary Number

The wetting process on a solid surface is ubiquitous in nature.It is probably one of the most notorious and intransigent tasks in fluid dynamics involving interfaces to describe the underlying physics of moving contact line,as the conventional NavierStokes equations run into a non-integrable stress singularity arising from the motion of the contact line.On the other hand,the wetting process becomes more sophisticated as the solid surface is intrinsically rough.Therefore,the dynamics of the three phase contact line have been of great interest in both science and technology as it has a rich phenomenon in nature.Therefore,exploring the physical mechanism of the contact line behavior on a rough surface has great theoretical significance.In the dissertation,an energy-consist phase-field method,coupled with NavierStokes equations is used to simulate the wetting process on an adaptive triangular mesh.The stress singularity is evaded as the method has an intrinsica diffusion length on the no-slip wall.The mesh is refined at the interface to fully resolve the diffuse interface.Meanwhile,Finer mesh is also adopted at the rough surface to resolve the topological structures.In addition,some experimental investigates are conducted to explore the wetting failure.The contents of this dissertation are organized as follows.1)Moving contact line on the sphere surface with small capillary number.Water entries of hydrophobic spheres with gas viscosity artificially increased are numerically investigated,which can circumvent the difficulties caused by limited interface thickness in numerical simulations.The viscous effects on the water entry in the early stage are studied.The water entry cavity formation,air film rupture and contact line dynamics on a curved solid surface are explored.First,the influence of the Weber number,wettability and the Bond number on the shape of the cavity are considered,and the four regimes of cavity type are found to be dependent of the Weber number and the Bond number.The Rayleigh-Besant equation is applied to predict the cavity expansion in large Reynolds number.And the simulation results are consistent with the theoretical model,which imply that the viscous effect works only in the vicinity of the moving contact line.Then,the numerical results show that the increase of viscosity ratio will enlarge the life time of the gas film,and the life time can be predicted by viscous squeezing flow model qualitatively well.After the gas film ruptures at the bottom of the sphere,a contact line is generated.It retracts along the sphere's surface,and the retracting speed fulfills U_MCL?T_c?-1?2? law,which is independent of the viscosity ratio.2)Surface roughness effects on contact line motion with small capillary number.How surface roughness influences contact line dynamics by simulating forced wetting in a capillary tube is conducted.The tube wall is decorated with microgrooves and is intrinsically hydrophilic.According to the numerical results,a criterion is proposed to judge whether the grooves are entirely wetted or not at vanishing capillary numbers.When the contact line moves over a train of grooves,the apparent contact angle exhibits a periodic nature,no matter whether the CassieBaxter or the Wenzel state is achieved.The oscillation amplitude of the apparent contact angle is analyzed and found to be inversely proportional to the interface area.The contact line motion can be characterized as stick-jump-slip in the CassieBaxter state and stick-slip in the Wenzel state.By comparing to the contact line dynamics on smooth surfaces,equivalent microscopic contact angles and slip lengths are obtained.The equivalent slip length in the Cassie-Baxter state agrees well with the theoretical model in the literature.The equivalent contact angles are,however,much greater than the predictions of the Cassie-Baxter model and the Wenzel model for equilibrium stable states.On the macroscopic level,the wetting dynamics on a rough surface can be approximated by those on an equivalent smooth surface.Finial,qualitative analysis of different roughness types and sizes are presented.3)Air entrainment for vertical water entry of rough plate.The wetting failure phenomena by vertically plunging a rough plate into the liquid with a large range of viscosity through experimental investigations,lubrication theory analysis and numerical simulations are carried out.When the velocity of the solid plate is over a threshold value,the air is entrapped,that is the wetting failure is occurred.Noted that the wetting failure phenomenon is not only dependent on the liquid phase,but also has an important contribution by the gas phase.The threshold velocity is weakly dependent with the wettability of solid plate when the advancing contact angle is small than the neutral angle.The dependence of the critical speed in the viscosity ratio,range from 5×10^-5 to 10^-2,meets a power law Ca_C???_g/?_l?^-2/3,which is independent of the roughness of the solid plate.Furthermore,by comparing the results between experimental investigations,theoretical analysis and numerical results,the wetting dynamics on a rough surface can be approximated by those on an equivalent smooth surface at the wetting failure state on the macroscopic level are derived.