Preparation and Study on Bionic Elastic Slippery Coatings
In nature,plenty of plants and animals have evolved special surfaces for drag reduction to thrive in harsh natural environments.The surfaces are usually classified into three categories,including superhydrophobic surfaces(SHSs)inspired by Lotus leaves,slippery liquid-infused porous surfaces(SLIPSs)inspired by the Nepenthes pitcher plant and shark skin,and elastic surfaces inspired by Delphinus delphis skin.For SHSs,air bubbles are trapped in the hierarchical microstructures covered by epicuticular wax forms,forming a continuous air layer repelling water with a water contact angle of more than 150°.Once water droplets contact the air layer,they can easily roll off the surfaces with a sliding angle lower than 10°.Therefore,the frictional drag caused by water is much smaller.However,water can be embedded into the hierarchical microstructures if the air layer is destroyed.Consequently,the friction drag of an air-liquid interface changes into a solid-liquid interface,possessing high friction drag.Different from SHSs,the air layer on SLIPSs is replaced by lubricants such as water and oils with low surface energy,leading to a new intermediate interface with low friction drag.But,the lubricants slowly escape the surfaces,as they should be replenished.An elastic surface can reduce drag by controlling the turbulent boundary layer.But it can be easily destroyed because of its poor mechanical strength.In the work,elastic and slippery surfaces with excellent friction drag reduction were developed inspired by SHS,SLIPS,and flexibility.Briefly,an elastic silica gel coating was coated on a clean Al plate via spinning,forming an elastic coating(simulating Delphinus delphis skin).Then hydrophobic SiO2 nanoparticles were sprayed on the elastic coating,forming an elastic SHS coating with hierarchical microstructures(simulating Delphinus delphis skin and lotus leaves).Lastly,dimethyl silicon oil inpoured the porous structures,forming an elastic SLIPS coating(simulating Delphinus delphis skin and shark skin).At 200 r/min,the rag reduction was 23.2％,18.4％and 10.2％for the elastic SLIPS coating,the elastic coating,and the elastic SHS coating,respectively.At a high speed,the elastic coating and elastic SLIPS coating had a rag reduction of 4.0％and 5.6％,while the elastic SHS coating possessed a high friction drag(rag reduction of-15.0％).In the forward current direction,the coating transformed the liquid-solid surface into a liquid-liquid surface.The viscous dissipation and pinning effect of the external flow surface on the surface could be reduced.In addition,the velocity gradient was decreased and the deformation of the coating was weakened.In the radial direction,the coating decreased the pressure,reduced the turbulence,and increased the thickness of the boundary layer.Therefore,the drag reduction rate of the elastic SLIPS coating is higher than that of the elastic coating and the elastic SHS coating.It has great reference values for reducing the friction rag for marine vehicles.
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