Droplet Impact Experiments And Drag Reduction On Superhydrophobic Surfaces

Due to its advantages of drag reduction and self cleaning,the superhydrophobic surface has a wide application prospect in industry,agriculture and daily life.The dynamic process of droplet impacting on solid surface has been the focus of research.An experimental apparatus has been built up for a droplet with a diameter of 2-3mm impacting on superhydrophobic surface from a height in the range of 1cm to 20 cm.The images of the impact process were successfully collected using high-speed CCD camera,which were then treated by Matlab.Two typical dynamic behaviors of droplets on superhydrophobic surfaces were observed,namely,the rebound behavior of low-speed impact and the crushing behavior of high-speed impact which was accompanied by the breakup of liquid bridges and the formation of satellite droplets.The mechanism of the two behaviors was explained.The low-speed rebound behavior of the droplets was analyzed quantitatively,which was divided into two stages: the contact stage with the superhydrophobic surface and the stage of the bounce.For the contact stage,the maximum droplet spreading coefficient increases with the impact velocity increasing,the maximum time for the maximum spreading coefficient decreases first and then increases with the impact velocity increasing.For the bounce stage,with impact velocity increasing,the maximum bounce height increases first and then decreases.The restitution coefficient decreases with impact velocity increasing.In addition,a certain theoretical formula for the maximum spreading coefficient,Re and We has been deduced.The micro morphologies of superhydrophobic surface have a significant influence on the fluid flow characteristics of the surface.The commercial software FLUENT has been used to simulate the flow characteristics of superhydrophobic surfaces with different microstructures under the laminar flow.The relationship has been analyzed between the drag reduction rate,the wall slip velocity,slip length and the microstructure type,the shear free area ratio,the cycle length of morphology.The simulation results show that the air is stored between the cylindrical microposts and inside the cylindrical micropits.The velocity is not zero at the gas-liquid interface,and there is an apparent velocity slip.The microstructure type has a significant effect on the drag reduction.The drag reduction rate of cylindrical microposts superhydrophobic surface is much higher than cylindrical micropits superhydrophobic surface,and the maximum drag reduction rate of the former is 24%.The slip length,the shear free area ratio,the cycle length of morphology conform to a certain theoretical formula.In this thesis,the Micro-PIV method is used to observe the velocity field of the superhydrophobic and superhydrophilic microchannels of trapezoidal cross-section with a flow rate of 4.5 ?L/min.We have found that the velocity curve of superhydrophilic microchannel is more convex than the velocity curve of superhydrophobic microchannel.That is,the maximum flow velocity in the superhydrophilic microchannel is larger than that in the superhydrophobic microchannel.In addition,the velocity curve rises near the wall.Because the wall tilts,the fluorescent particles only exist on one side,which leads to the inaccuracy of the experimental speed and the speed there is too large.In addition,several ideas for further debugging and improvement of the Micro-PIV experiment are also put forward.