| Droplet evaporation is a common and complex diffusion phenomenon driven by the concentration gradient difference of gas-liquid interface.The process of liquid wetting,spreading and evaporation on the surface of different functional materials is widely used in daily life and industrial production,which has a guiding role in understanding the performance and application of various functional materials.In this paper,the static and dynamic characteristics of the fixed droplets on the hydrophilic / hydrophobic surfaces of different materials are studied by the combination of experiment and theory.The visual experimental study of the droplet profile is carried out by using the contact angle measuring instrument,and the complete pinning phenomenon and special stick slip phenomenon in the evaporation mode are analyzed theoretically.The main work and conclusions are as follows:(1)Firstly,the static experiment of solid hydrophilicity / hydrophobicity on droplets was carried out.The static contact angle,forward contact angle and backward contact angle of droplets were measured by contact angle measuring instrument,and the wettability of droplets on different hydrophilicity and hydrophobicity surfaces was analyzed.The results show that the smaller the static contact angle is,the easier it is to wet the solid surface.The smaller the difference between the forward contact angle and the backward contact angle is,the easier the three-phase contact line of the droplet is to move and the easier it is to wet the solid surface.(2)After the initial understanding of the wettability of different materials,the dynamic experimental research of droplets on the surface of different hydrophilic / hydrophobic materials is carried out.The dynamic contact angle and contact radius curve with time are obtained by the contact angle measuring instrument.The evaporation mode of the fixed droplets is summarized,and the influence of hydrophilic / hydrophobic and thermal conductivity on the complete pinning process is analyzed and discussed.The results show that: on the hydrophilic surface,the evaporation mode of the fixed droplets is constant bottom radius mode,constant contact angle mode and mixing mode;on the hydrophobic surface,the evaporation mode of the fixed droplets except for the typical three modes,the hydrophobic PTFE coating surface and the silicone oil coating surface have three or four periodic stick slip modes respectively.The three-phase contact line of liquid drop on the hydrophobic surface is more difficult to move,which is more conducive to nailing.On this basis,the evaporation of droplets on the solid surface with different thermal conductivity is studied.It is found that the evaporation mode is the same,the solid surface with higher thermal conductivity is not conducive to nailing,and the solid surface with lower thermal conductivity is conducive to nailing.(3)After exploring the common mode of complete pinning,the periodic stick slip phenomenon on the surface of special hydrophobic silicone oil was further analyzed,and the reason for the sudden change of stick slip phenomenon was explained.The theoretical results were compared with the experimental results.The results show that the additional resistance increases and the surface free energy of the system decreases in each cycle of the four stick slip phenomena,and when the sudden change of the three-phase contact line of the droplet reaches a new equilibrium,the additional resistance of the three-phase contact line decreases and the surface free energy of the droplet increases.Due to the decrease of contact angle in the process of complete pinning,when the contact angle reaches the back contact angle,the corresponding additional resistance is the largest,the contact angle continues to decrease,the force of the three-phase contact line is difficult to balance,and the three-phase contact line shrinks;and the theoretical back contact angle and radius are basically the same as the experimental results. |
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