Wettability And Drag Reduction Of The Superhydrophobic Surfaces

In the past decade,drag reduction of superhydrophobic surfaces,as one of the hot topics of drag reduction methods,has attracted an increasing number of researchers’ attention.This cheap and easy processing technique could be efficiently applied on the ocean engineering applications as well as some other high technology fields.The superhydrophobic surfaces,composed of micro/nano structures and low surface energy materials,will sandwich air bubbles between the fluid and the solid wall,which makes the flow near the superhydrophobic surface extremely complicated.Both the wettability and surface roughness has to be considered when investigating the drag reduction property of the superhydrophobic surfaces,which is a great challenge for both the numerical and experimental methods up to now.In this work,the wettability,as well as the drag reduction property of the superhydrophobic surfaces have been investigated both experimentally and numerically.The effect of the micro structures on the slip and drag reduction of superhydrophobic surfaces has been analyzed by investigating the global drag of the surfaces,flow field near the wall,slip velocity at the air-water interface,etc.The main results and innovations are as follows:(1)Superhydrophobic surfaces with regular micro patterns have been fabricated using soft lithograph,and other two fabrication methods of superhydrophobic surface were presented: roughening Teflon surface,and adding micro particles into low surface energy coatings.The effect of the micro structures on the contact angle of these surfaces was examined experimentally and theoretically.The manipulation of droplets,including droplet motion control and splitting,was performed using the anisotropic wettability of the solid surfaces.(2)The flow past superhydrophobic surfaces has been studied using simulation methods.Superhydrophobic surfaces were modeled with four types of micro structures: parallel grooves,perpendicular grooves,posts and cavities.The slip length,slip velocity,and drag reduction using these surfaces were analyzed as changing the micro texture types,structure scales,shear free ratios.(3)A superhydrophobic micro channel as well as an external drag reduction measurement system was fabricated.The effect of the micro structure scale,morphology,and length of the model on the slip length and drag reduction has been analyzed.It was also explained why the drag reduction of the superhydrophobic surfaces would decrease as increasing the length of the model.(4)A multifunction PIV system has been fabricated.The fluorescent particles and optical filter have been combined to get rid of the light reflection at the superhydrophobic walls.The velocity profile within the boundary layer of an external flow has been measured and it has been found that the slip velocity would be smaller at the tail area than at the front area of the superhydrophobic plate,which is in good agreement with the experiment and simulation results.(5)Using a continuous laser,microscope lens and high speed camera,a modified micro-PIV system was bulit,applying which the slip at the millimeter-sized air-water interface was measured.It shows that the slip velocity at the air-water interface on the parallel grooves is much larger than the one on the perpendicular grooves.A novel phenomenon has been found in our experiments that the reverse slip exists at some area of the air-water interface.