Investigation Of Contact-line And Film Dynamics In Dip Coating

The problems of moving contact line widely exist in nature,daily life and industrial applications,in which dip coating is a typical moving contact line problem.In the study of dip coating,it is necessary to consider not only the multi-scale feature and stress singularity of the moving contact line,but also the chemical heterogeneity and three-dimensional effect of the actual physical surface,which are challenging to the research of such problems.In this thesis,numerical simulations and experiments are employed to study this classical dip coating problem.The results and conclusions are briefly given as follows:(1)The dynamics of moving contact line and film deposition on a chemically hetero-geneous plate withdrawn from a liquid bath is numerically studied.The plate is patterned with vertical stripes characterized by alternating wettabilities.When the withdrawing velocity is small,a three-dimensional stable meniscus is formed.If the velocity exceeds a threshold,liquid films and droplets are produced on the more wettable region.When the width of the more wettable region is small,the contact line always loses its stability at the center of these stripes.For sufficiently large width of the more hydrophilic region,a trapezoidal film can be observed,in analogy to the wetting transition on a uniform plate of finite width.However,the onset of meniscus instability is found to occur close to the stripe boundaries,a location independent of the stripe width,indicating a three-dimensional mecha-nism of wetting transition.Furthermore,the dynamic evolution of the liquid film is also analyzed.(2)The dynamics of moving contact line on an inhomogeneous substrate,which con-sists of horizontal stripes with different wettabilities and is withdrawn from a liquid bath,is numerically studied.When the width of stripes is very large,four typical states of contact line dynamics are observed with the increase of with-drawing velocity.In particular,when the withdrawing velocity is smaller than the critical value of wetting transition on the more wettable surface,the contact lines move periodically between the equilibrium positions of the more wettable and less wettable regions,and the 'stick-slip' process can be observed.An anal-ysis of 'stick-slip' motion is carried out according to the Voinov-Cox theory,and the relaxation time of 'slip' process is predicted.When the width of stripes is small,it is found that the evolution of the contact line and liquid film is similar to that on an equivalent homogenous substrate,whose contact angle equals the prediction of Cassie theory.(3)The dip coating of chemically homogenous and heterogeneous plate is investigated experimentally,in which the chemically heterogeneous plate is patterned with vertical stripes characterized by different contact angles.It is confirmed that the wetting transition for a homogenous plate with finite width occurs at a capillary number well below the theoretical prediction.And the critical wetting transition velocity is equal to the dewetting velocity of the thick film relative to the vertical plate.By quantitatively comparing the results of chemically homogenous plate with those of heterogeneous plate,it is shown that the edge effect can promote the formation of liquid film.Further,the inverted trapezoid liquid film is observed experimentally on chemically heterogeneous surface,and it is found that with increasing the plate speed,the dimple connecting the bottom of the inverted trapezoidal liquid film with the liquid bath becomes unstable,leading to fingering films,which is similar to the linear instabilities of advancing contact lines.(4)Dynamics of contact line and liquid films when dip coating an in-plane inclined plate are experimentally studied.The effect of plate width and oblique angle,which is between the edge of the plate and the vertical withdrawing direction,on the critical velocity of wetting transition and the receding velocity of contact line are given.It is found that the asymmetric trapezoidal liquid film will be deposited on the surface after wetting transition.The dependence of angles between the inclined contact line and the horizontal plane on the relative velocity between the contact line and the plate,oblique angle and the plate velocity is theoretically given,and is in good agreement with the experimental results.Further,a tail structure at the intersection of the horizontal contact line and the inclined contact line is studied,and it is found that,when the plate velocity is high and the oblique angle is large,the tail structure is easy to lose stability and produces droplets.