The Interaction Between Droplets And Hydrophobic Or Super-Hydrophobic Solid Surfaces

Inspired by unique property of self-cleaned lotus leaves, the study of super-hydrophobic surface has been an active research field in recent years. Interactions between liquid and solid surface are determined by the surface property, and it is closely related to our daily life such as the industrial manufacture, food production and so on.In this study, we use the Lattice Boltzmann method to simulate the interaction between fluids and the hydrophobic and super-hydrophobic surfaces. Super-hydrophobic surface is characterized by a contact angle larger than 150° and the slidding angle less than 5°. Since the biggest contact angle is 120°or a smooth surface, the super-hydrophobic surface can be obtained by coating nano or micro-structured materials on the hydrophobic surface.Our study on the interaction between fluids and the hydrophobic and super-hydrophobic surfaces includes the following parts:(i) Firstly, we study the impacting process of a microdroplet onto a super-hydrophobic surface. In the simulation process, a downward velocity is attached to the droplet to make it impact the surface, and we find some new interaction manner between the droplet and the solid surface. The detailed conclusions are given as follows:when impacting on the surface with homogeneously distributed roughness, the droplet will bounce off or locate on the surface in the conventional Cassie or Wenzel state, and more importantly, we find that the droplet can locate on the surface in another two new wetting states, for which we call the Mid-embed state and Side-embed state. The two states are somewhat like the Wenzel state. In the Wenzel state, the interacting area between liquid above the interface and the substrate, Sup, is equal to that between the substrate and penetrated liquid, Sdown, in these new wetting states, Sup>Sdown, the penetrating liquid filling the intervals of the surface structures is continuous in the Mid-embed state, whereas for the Side-embed state, only intervals in the out-most are full of liquid and that in the middle is empty,(ⅱ) We studied the control of rebounding orientation of microdroplet impact onto a surface with roughness gradient. When impacted on surface with roughness gradient, we find the drop could not only bounce off following the roughness gradient, but also could against the gradient or rebound off vertically, and this phenomenon has never been found in the former study,(ⅲ) Finally, we also simulate the capillarity descend, when a hydrophobic capillary tube is inserted in the liquid tube, liquid in the tube will descend. In this part, we simulate and compare the descent height with the well-known Lucas-Washburn model, and point out the defect of the model.