Preparation And Drag Reduction Properties Of Low Surface Energy Superhydrophobic Composite Surfaces

Generally,the marine vehicle will be subjected to differential pressure resistance,wave-making resistance and frictional resistance in the process of traveling,among which the frictional resistance is the biggest resistance.Most of the energy of these vehicles in the process of traveling is consumed in overcoming the resistance of overcoming water,so the urgent need for the vehicle to solve in the work is to reduce the frictional resistance as much as possible.A large number of studies have shown that there are special micro-nano rough structures on the superhydrophobic surface that can trap air,form an air film,and play a certain barrier role,so that the effective contact area between droplets and the surface is reduced,and the contact between droplets and the surface is converted into gas-liquid contact with lower adhesion,so it has become one of the most potential methods for reducing surface resistance.However,when the superhydrophobic surface is applied to underwater drag reduction,there will be the limitation of poor mechanical stability,especially in the high-speed water flow,the rough structure of the superhydrophobic surface will be affected by the impact of the water flow,resulting in the disappearance of the air layer,and the hydrophobic performance will be reduced,which will affect its resistance reduction performance.Aiming at the above problems,this paper studies the superhydrophobic surface with low surface energy micro-rough structure,and further reconstructs the superhydrophobic surface to prepare a new surface,and studies the resistance reduction of different surfaces.The specific studies are as follows:(1)Using silica nanoparticles as raw materials,the silica solution modified by n-octyltriethoxysilane(OTS)is sprayed to the pre-cured polyurethane(PU)substrate by spraying method,and the superhydrophobic surface is formed by curing.The drag reduction properties of superhydrophobic surfaces are tested and analyzed using a self-designed rotary viscometer.The results show that the superhydrophobic surface prepared based on OTS@SiO2 nanoparticles shows low surface roughness(Sa=0.298 ?m),strong hydrophobic performance(water contact angle CA=171.4°)and 0° rolling angle;after 50 wear cycles,the superhydrophobic surface still has superhydrophobic performance,and the water contact angle remains above 160°,showing strong mechanical stability.The drag reduction performance of the hydrophobic surface was tested and analyzed by the rotating viscometer,and the rotor sprayed with the superhydrophobic surface could achieve a drag reduction rate of 66.7%at low speed rotation(100 rpm)and 7.4%at a higher speed(500 rpm),showing excellent drag reduction performance.The drag reduction rate of the superhydrophobic rotor can still reach 3.7%at 500 rpm after 2.5 h of continuous impact of sand and gravel,indicating that the surface also shows excellent resistance reduction stability performance.This excellent superhydrophobic performance is attributed to the unique nanoparti cl e-b ased ultrahydrophobic surface low-roughness structure,which has the good performance of converting the solid-liquid interface into a gas-interface to reduce frictional resistance,and the low-roughness surface structure is more stable and less easily destroyed.(2)A superhydrophobic/flexible coupling surface based on silica nanoparticles and natural latex was prepared by electrostatic flocking method,and the physical properties,drag reduction properties and mechanism of the surface were studied.The results show that the superhydrophobic/flexible coupling surface shows excellent hydrophobic performance(water contact angle CA=165°);in addition,the coupling surface also shows excellent self-cleaning,and water droplets can take away dirt on the surface.At the same time,the pressure sensor drag reduction test platform was built independently,with the help of which the coupling surface was tested to achieve the maximum resistance reduction rate of 51.8%and 89.5%in the laminar flow and turbulence states,respectively,showing excellent drag reduction performance.After 15 days of immersion in water,the maximum surface drag reduction rate can still reach 86.7%,showing excellent resistance reduction stability performance.This unique superhydrophobic and flexible surface composite structure achieves a good drag reduction effect through the small friction resistance of the superhydrophobic gas-liquid interface in the laminar flow state,and a higher drag reduction rate is obtained by delaying the transition of the boundary layer by the natural latex flexible surface in the turbulent state.(3)Using silica nanoparticles as raw materials,and using the easy-to-operate spray method,the silica dispersion modified by n-octyltriethoxysilane(OTS)was sprayed to the pre-cured polyurethane(PU)surface to prepare a superhydrophobic surface,and the graphene with good conductivity was applied on the back of the surface to form a continuous film,and then the current was introduced to make the graphene film heat and transfer heat to the superhydrophobic surface,characterizing the properties of the superhydrophobic coating,and exploring the relationship between different wall temperatures and the surface drag reduction effect.The results showed that the prepared coating had good superhydrophobicity,with a water contact angle of 157.9° and a rolling angle of 1°.In addition,the superhydrophobic coating also exhibits a good self-cleaning effect.Similarly,with the help of a rotary viscometer test bench,the resistance reduction rate was tested and analyzed,and the results showed that the superhydrophobic coated rotors that had increased the wall temperature showed a decrease in torque in the speed range,and when the wall temperature was raised to 50?,the drag reduction rate reached 100%.The surface of the coating prepared by this work has micro-nano-level grooves,which can trap air to form an air film,and the barrier effect of the air film converts the solid-liquid contact into a gas-liquid contact with less resistance,and the synergistic effect with the increased wall temperature makes it have good resistance reduction properties.