Simulation And Experimental Study On Droplet Dynamic Process On Special Wettability Surface

Wettability of a solid surface is an inherent property determined by both the surface energy and the geometric structure.With the development of bionics,the researchers have developed a variety of materials with special wettability by imitating the biological surface structure,so as to realize the control of fluid dynamic process and then enhance the heat transfer.At present,the special wettabilities are mainly experimentally studied,and few investigations are focused on the droplets dynamic process on the surfaces.Generally,the available approaches require multiple processing steps and are carried out with high expense,resulting in high experimental costs and poor applicability.This dissertation demonstrated simple methods to prepare special wettability on copper substrates,and droplet dynamic process was theoretically investigated and experimentally tested.Firstly,the lattice Boltzmann method was introduced to establish the flow model of wettability gradient surface.Based on this model,the dynamic process of droplets on varying vertex angle gradient surfaces were investigated.The results show that the initial velocity and acceleration of the droplet are proportional to the vertex angle(?),and the displacement is inversely proportional to the ?.For a gradient surface with ?=15°,the displacement is 16.05 mm and the initial velocity is about 390 mm/s.While for the surface of ?=35°,the displacement and the initial velocity are 11.25 mm and 630 mm/s,respectively.Secondly,the wedge wettability gradient surfaces were prepared,and the dynamic process was studied experimentally to verify the accuracy of the flow model.The results show that the displacement deviation between the simulation and the experiment is large at the initial stage of t <0.2 s.With the increase of the vertex angle,the displacement deviation is gradually reduced.For the gradient surface with ? >=28°,the simulations agree well with the experiment results,and the deviation can be controlled within 10%.Finally,the sticky superhydrophobic surfaces were prepared by a remarkably straightforward method,and the droplet transportation was realized.The fabrication used electroless galvanic deposition to coat the copper substrates with a textured layer of silver.Subsequent immersion of the coated substrates in the surface modifier covers the dual-scale hierarchical structure with a low surface free energy monolayer and renders it hydrophobic.It is found that the lag angle is inversely proportional to the deposition time,and samples with shorter deposition times have sticky superhydrophobic properties.For instance,the surface with a deposition time of 15 s exhibits the superhydrophobity with a contact angle(CA)of 154.8°,and the effective stickiness with a contact angle hysteresis(CAH)of 45.1°.The prepared sticky superhydrophobic surfaces are finally shown in the application of droplet transportation,it is possible to transfer 3.5 ?L of droplets from the smooth superhydrophobic surface to the hydrophilic surface.