Dynamics Of Drop Impact Onto Complex Surface And Their Interaction

Interaction between drops and complex substrates is not only commonly seen in nature,but relevant to our daily lives and industrial processes.Despite of the success of previous research in this field,the effect of complex geometry on the impact dy-namics and dynamic wetting on the flow-structure interaction remain not clear yet.In this thesis,the related mechanisms will be studied using a diffuse-interface immersed boundary method and theoretically analyzed.The contents of the thesis include:(1)Drop impact onto a sphere is numerically investigated at moderate Reynolds and Weber number.It is naturally expected that the aspect ratio of the sphere to the drop,λr,would make a big difference to drop spreading and retraction on the sphere,compared with drop impact onto a flat surface.With the help of numerical simulations,we identify the key regimes in the spreading and retraction,analyze the results by scaling laws,and quantitatively evaluate the effect of λr on impact dynamics.We find that the thickness of the liquid film spreading on the sphere can be well approximated by hL,∞(1+3/4λr-3/2),where hL,∞ represents the film thickness of drop impact on a flat substrate.At the early stage of spreading,the temporal variation of the wetted area is independent of λr when the time is rescaled by the thickness of the liquid film.Drops are observed to retract on the sphere at a roughly constant speed,and the predictions of the theoretical analysis are in good agreement with numerical results.Furthermore,we develop our theoretical analysis to the concave spherical surface,and the film thickness can be approximated by hL,∞/(1+1/4λr-1)which differs from the convex surface.If taking the sign of the curvature of the concave surface into account,the film thickness increases with the curvature no matter whether the surface is concave or convex.In this sense,the spreading dynamics can also be applied to the concave surface by the comparison with convex surface.(2)The flow regimes in the drop impact onto a sphere are numerically investigated.By altering the Weber number and aspect ratio of sphere to drop,λr,four different flow regimes have been identified,namely the drop recoiling regime,sphere pene-trating,film wrapping and film detaching,respectively.We find that the contact line pinning on the spherical surface for small hydrophobic spheres.Through the analyses of force balance at the contact line,the position where the contact line is pinned can be correlated to the Weber number and contact angle.The regime transitions between different flow regimes have been theoretically predicted and compared well with the numerical results.(3)We numerically study the head-on collision between drop and sphere at relatively low Weber number and high Reynolds number,with focus on the effect of the mass ratio of sphere to the drop,λm,on the dynamic processes.We identify two flow regimes in the parameter space considered,drop attachment and drop detachment.For the drop detachment,the time scales for the drop spreading and retraction can be measured by scaling laws.The influence of drop deformation on the variation of sphere momentum is measured numerically.Theoretical prediction of the transition between the two flow regimes agrees well with our numerical results.In the end,we analyze the speed variation of the sphere and theoretically predict the distance of sphere descending at the occurrence of drop detachment.