Fabrication And Drag Reduction Mechanism Of Super-hydrophobic Surfaces With Micro-nano Structures

Super-hydrophobic surfaces have been widely applied in modern life and industrialproduction because of their excellent properties. Super-hydrophobic surfaces have aself-cleaning property. On super-hydrophobic surfaces, liquid could keep as a dropletand easily scroll through the surfaces. In a liquid flow in a channel covered withsuper-hydrophobic surfaces, there is a significant velocity slip near the wall of thesurfaces, which is different from the situation of flows on normal surfaces. In the flowwith a micro-level characteristic length, researchers have already observed the slip ofthe velocity. In recent studies, it was found that the flow drag on super-hydrophobicsurfaces is smaller than that on normal surfaces, that is, there is a flow drag reduction onsuper-hydrophobic surfaces.Currently, most researchers in this field believe that there are mainly twoconditions for the formation of super-hydrophobic surfaces. The first condition is theexistence of a layer of hydrophobic material on the surface, which can reduce thesurface energy. The second condition is the existence of micro and nano structures onthe surface. Today there are a lot of methods which could prepare super-hydrophobicsurfaces with these two conditions. Chemical vapor deposition, chemicaletching, sol-gel method and photolithography, are often used to preparesuper-hydrophobic surfaces. Although there are many researches related to the dragreduction on super-hydrophobic surfaces, there is not yet a clear explanation andconclusion about the mechanism. Many researchers have already observed thedrag reduction in laminar flow on super-hydrophobic surfaces. However, thereare only a few researches which focus on drag reduction in turbulent flow. Inaddition, most of the super-hydrophobic surfaces using in these experimentsonly have one-scale micro structures on the surfaces. Super-hydrophobicsurfaces, which are more familiar with real lotus leaves, with dual-scalemirco-nano structures are seldom prepared and used in experiments.In this study, we used two novel methods to prepare super-hydrophobic surfaceswith dual-scale micro-nano. We measured the pressure drop in channels withsuper-hydrophobic surfaces and compare it to the pressure drop in normal channels. It is found that in Laminar condition, the rate of drag reduction is upto36.3%. In Turbulence condition, the drag reduction rate is about53.3%whichhas a more obvious than that in Laminar condition.In the exploratory experiments on drag reduction in channels withsuper-hydrophobic surfaces, we proved that the main reason of drag reduction isthat the existence of the gas-liquid interfaces between the micro structuresreduces the viscous stress.In this study, we used Micro-PIV technology and PIV technology tomeasure the velocity field in channels with super-hydrophobic surfaces,analyzed the impact of velocity fluctuation on drag reduction and compare it tovelocity field near normal surfaces. In Laminar condition, the velocityfluctuation near super-hydrophobic surfaces is as much as the velocityfluctuation near normal surfaces. As a result, their Reynolds stress produced bythe momentum transport is close to each other. So in Laminar condition, themain reason of drag reduction is the reduction of Reynolds stress caused by thegas-liquid interfaces. On the other hand, in Turbulence conditions, thesuper-hydrophobic surfaces could both reduce the Viscous stress and Reynoldsstress thus make the drag reduction in Turbulence more obvious.