Computational Fluid Dynamics Simulations To Investigate The Effect Of Surface Microstructure On Droplet Infiltration

Surface wetting is a central issue in surface science research,especially the internal wetting state of droplets on different surface microstructures has an important impact on fields such as nano-frictional power generation,bionic material preparation,and microfluidic device fabrication.In this paper,computational fluid dynamics simulations are used to systematically investigate the regulation of the behavior of droplets with primary surface microstructures and secondary surface microstructures on the infiltration state,spreading length,and apparent contact angle of droplets on their respective surfaces by a total of four models,explore their laws,and establish and elaborate the relevant infiltration theoretical models.The specific studies include:（1）The simulation reveals that the infiltration of droplets on the surface will show the coexistence of multiple infiltration states,and the droplets on the primary surface microstructure will appear in three states,Cassie,Transition,and Wenzel.Droplets on secondary surface microstructures may appear in five states,Cassie,IFUV,IMNTH,Presence of bubble,and Wenzel.The curved column microstructure will make it more difficult for the droplet to achieve the Wenzel state and easier to achieve the Cassie state,having a larger range of Transition states.The addition of the secondary structure makes it more difficult for the droplet to achieve the Wenzel state on the surface.（2）The apparent contact angle and spreading motion of droplets are influenced by the microstructure morphology and the infiltration state.The more the droplet tends to be in the Wenzel state on the surface,the greater the variation of infiltration length with velocity,and the more the droplet tends to be in the Cassie state,the smaller the variation of infiltration length with velocity.The more the droplet is in the Cassie state on the surface,the more the dynamic spreading contraction oscillation is obvious.The droplet has a larger maximum spreading factor on the curved column than on the square column under the same conditions,and the maximum spreading factor decreases with the increase of the secondary structure.Compared with the square structure,the apparent contact angle of droplets on the curved column microstructure is small,and with the addition of secondary structure,droplets will obtain a larger apparent contact angle.The contact angles of droplets on rough surfaces are different from the predicted values of the ideal equation.（3）The different infiltration states between the gaps of a single group of microstructures are related to the forces at the gas-liquid interface.The liquid phase is in the Cassie state when>₍（2）>,in the Transition state when₍（2）>>,and in the Wenzel state when₍（2）>>.The coexistence of multiple immersion states on the surface microstructure is related to the changes of liquid phase density,pressure field and velocity field when the droplets hit the surface.The morphology of the microstructure and the number of secondary structures on the microstructure have an important influence on the immersion state of droplets on the surface,the apparent contact angle and its dynamic spreading process.The simulation results of droplet immersion on the four microstructure models can be helpful in the fields of surface self-cleaning and droplet immersion regulation.