Experimental And Numerical Studies On Dynamic Characteristics Of Droplet Impacting On Carbon Nanotube Arrays

The droplet impacting on solid surface is a common phenomenon in diverse areas,such as industrial production,pesticide spraying,biomedicine and aerospace,and the behavior characteristics and laws of droplet spreading,rebounding or breakup in the process of collision have great significance for practical applications.In this thesis,carbon nanotube(CNT)arrays fabricated on silicon substrates with different hydrophilicity and hydrophobicity were obtained by the plasma treatment firstly,and then the droplet impacting with CNT arrays surface was carried out to investigate the effects of changing the wetting conditions of the nano-textured surface on the impacting characteristics.Secondly,COMSOL Multiphysics software was used to simulate the dynamic characteristics of liquid droplet impacting on smooth solid surface based on the modified Level Set method.The comparison between the experimental and numerical results was made and a new hybrid dynamic contact angle model was proposed eventually,which provides certain guiding significance for some related research.The main conclusions are drawn as follows:(1)The dynamic evolution of droplet impacting on surface of superhydrophobic CNT arrays(static contact angle is 155°)under different Weber number(We)was systematically investigated by using high-speed camera.As We increased from 0.3 to416.7,the droplet showed different characteristics of adhesion,complete rebound,fracture rebound and fragmentation.According to the range of We,the dynamic evolution of droplet impingement was classified,which provided a reference for predicting the behaviors of droplet under superhydrophobic conditions.When We was in the range of 1.4 to 109.6,no fragmentation was observed during the process of droplet bouncing,and the contact time(t_c)was independent of droplet velocity,while there was a linear relationship between t_c and initial diameter of droplet.(2)The superhydrophobic CNT arrays were modified by plasma surface treatment technology to obtain different wetting surfaces with static contact angle of 29°,84°,105°,135°and 148°,respectively.The experiments of droplet impacting on the above surfaces were implemented with different Weber numbers in the range of 1.4-298.It was found that the dynamic evolution of droplet was affected by the Weber number,and surface wettability.The behavior distribution map of the droplet impacting on the CNT array was proposed as well,which could further provide a reference for predicting the characteristics of droplet impacting on different wetting surfaces.(3)The dynamic characteristics of droplet impinging on 155°superhydrophobic surface was numerically simulated by software of COMSOL.The improved Level Set method was utilized,and the Kistler model and Blake model,which expressing the change of dynamic contact angle,were respectively introduced.By comparing the simulation results with experimental data,it could be found that at two different stages of droplet spreading and receding,the simulation results based on Kistler model and Blake model were closer to the experimental data respectively.Thus,a Kistler-Blake hybrid dynamic contact angle model which combined the advantages of the above two models was proposed.The reliability and accuracy of hybrid model were verified by comparing the experimental data with simulation results.(4)The hybrid dynamic contact angle model was used to simulate the impingement of droplet on different wettability surfaces.Droplet Weber numbers were 1.4,6.6,and 33.2,respectively,and surface contact angles were 29°,84°,105°,135°,and 148°,respectively.By comparing simulation results with the experimental data,it was found that the calculation results of hybrid model was in good agreement with the droplet impacting behaviors on the hydrophilic(29°and 84°)and hydrophobic(148°)surfaces.For surface contact angle between moderate hydrophobic and superhydrophobic(105°and 135°),the simulation results were consistent with the experimental data only at spreading stage,there existed certain deviations during the receding stage.The dynamic contact angle of hybrid model oscillated around the static contact angle regardless of droplet oscillating or rebounding,and the amplitude of oscillation was inversely proportional to the static contact angle and proportional to Weber number.