Experimental Study On The Stability Of Air-liquid Interface On Superhydrophobic Surface

With the rapid consumption of energy and serious environmental pollution problems,it is urgent to find ways to use energy more efficiently.By reducing the surface friction of underwater vehicles,energy consumption and carbon dioxide emissions can be effectively reduced.Scientists began to study superhydrophobic surface(SHS)with excellent drag reduction after they got inspiration from the surface structures of lotus leaf.SHS generally have a contact angle greater than 150 °,which is the result of the combined effect of the low surface energy of the material and the micro-structure.Related experiments show that the air in the SHS's micro-structure brings about the slip boundary condition,the surface friction is reduced.However,due to the environmental conditions,such as surface structure and other factors,the drag reduction can be maintained for a limited time,which limits its application in engineering practice.For the occurrence of this phenomenon,the drag reduction,longevity and destruction process of SHS were experimentally studied,and the mechanisms of air layer destruction were theoretically based on Fick's law.In order to study the evolution of the air-liquid interface on the SHS,a circulating water channel with a replaceable part of the bottom wall was designed.In the chanel,the SHS with micro-grate was tested for flow drag reduction,air layer coverage change,and air layer damage mechanism.The drag reduction of the SHS is positively correlated with the air layer coverage.Obtained the maximum drag reduction on the SHS with a grate width of 30 ?m,which is 31.88%.The SHS shows better drag reduction at high Reynolds number,but the longevity is shorter;the drag reduction and duration of drag reduction effect are reduced with the increase of grate width.The morphology of the air-liquid interface has a profound effect on drag reduction and longevity.Through the combination of electron microscope and industrial microscope,the destruction process of the surface air layer,i.e.the process of instability of the air-liquid interface,is studied.The air layer destruction has 3 mechanisms:vertical diffusion destruction,shrinkage destruction and bubble destruction.Vertical diffusion destruction is a process in which the air-liquid interface is under the combined effect of pressure balance and air diffusion,the air-liquid interface gradually degenerates from the pinned state and depinned.On the basis of predecessors,based on Fick's law and Henry's law,the mechanism analysis of the vertical diffusion destruction of the SHS with trapezoidal micro-grate structure has been perfected.The mechanism of shrinkage destruction and bubble destruction was newly discovered in the experiment.The shrinkage failure mechanism is in the direction parallel to the grate,affected by the Laplace pressure pointing to the air side,the partial pressure of dry air in the grate,the hydrostatic pressure and the partial pressure of dry air in the atmosphere.The resultant force directed to the air side,under the action of this force,the air-liquid interface moves toward the air side,and shrinkage destruction occurs.Bubble destruction is the fastest destruction process,and it is the bubble nucleation phenomenon caused by the interaction of the air in the micro-grate and the low pressure points formed by turbulence.It's easier for bubble destruction at larger volume of air in the micro-grate and Reynolds number.