Study On Wetting And Motion Behaviors Of Droplets On Micro/Nanoscale Structured Surface

In nature,the surfaces of many animals and plants are composed of micro/nanoscale hierarchical roughness.Different sizes,spacings and arrangement patterns of the microstructure lead to various kinds of unique wettability.Based on the design of bionic engineering,the micro/nanoscale structured surfaces which are artificially prepared also show excellent superhydrophobicity and superhydrophobic stability.However,the internal mechanism is still unclear.In this paper,taking micro/nanoscale structured surfaces into account,the wetting states,wetting and dewetting transition,and the motion behaviors of droplets on wetting gradient surfaces have been investigated theoretically and experimentally.The main research works are as follows:?1?A thermodynamic approach is applied to analyze all the wetting states?including four stable wetting states and five transition states?of water droplets on the surface with micro/nanoscale hierarchical roughness,and the corresponding free energy expressions and apparent contact angle equations are deduced.Then,the influence of micro/nanoscale roughness on the wetting states has been analyzed in detail,and the wetting phase diagram is depicted.Finally,the stable wetting states are confirmed by the principle of minimum free energy,and the surfaces with micro/nanoscale hierarchical roughness are prepared through micromachining technology.The results prove the validity of the thermodynamic model.?2?For the wetting transition of water droplets on two kinds of typical micro/nanoscale structured surfaces?with smooth and structured sidewalls?,the influences of morphology of sidewall on free energy and energy barrier are quantificationally analyzed in the process of wetting transition and the effects of smooth and structured sidewalls on superhydrophobic stability are compared.?3?The wetting and dewetting transition on submersed micro/nanoscale structured surfaces are investigated.Firstly,the wetting transition based on the thermodynamic theory is systematically investigated and the influence of trapped air and micro/nano hierarchical structures on superhydrophobic stability is quantificationally analyzed.The collapse pressure is also predicted based on mechanics analysis,which is in good agreement with previous experimental results.Then two necessary thermodynamic conditions for dewetting transition are presented and the corresponding thermodynamic phase diagram of reversible transition is depicted.Finally,mechanics criterion for dewetting transition is analyzed and the equation of the reversible pressure of dewetting transition is given.?4?The motion behaviors of droplets on the gradient wetting surface are investigated.The superhydrophobic coating surface is prepared by silane modified TiO₂ nanoparticles.Then the wetting gradient is obtained on the superhydrophobic coating surface using a photolithography process.By changing surface tension of droplets to regulate driving force and hysteresis force,the movement distance of droplets on gradient wetting surface is effectively controlled.Finally,the wetting gradient is applied to the fluid control,such as self-motion of liquid on oneway channel,mixing and transport of droplets.