Study Of Drag Reduction Effect At Solid-liquid Interface With Micro-nano Structures

Reduce the frictional resistance between microfluidic devices and surface vessels and the fluid,which has important significance to improve the device performance and ship speed.Hence,reducing solid-liquid interfacial friction has been the research hotspots.The domestic and foreign scholars have lots of theoretical research on drag reduction problem.However,the study of drag reduction effect of solid-liquid interface with micro/nano structure is not profound enough and the internal mechanism is not clarified.It also lacks of experimental verification.The paper builds up a model of cavitation drag reduction on solid-liquid interface with microstructure and study the influence of surface microstructure to the reduction of frictional resistance theoretically.We suggest the test method for weak frictional resistance of solid-liquid interface and applie this method to measure frictional resistance on the prepared superhydrophobic and superoleophobic surfaces.We also study the influence of surface microstructure to the reduction of frictional resistance by experiment.There are many bubbles on superhydrophobic surface.The bubbles can reuduce contact area between solid and liquid.While the change of bubble angle and microstructure affects the drag reduction effect of solid-liquid interface.The paper builds up a model of cavitation drag reduction at solid-liquid interface with micro structures.We analyze the influence of surface cavitation and microstructure on drag reduction.The theoretical analysis shows that the bubble angle affects the fluid boundary slip length and solid-liquid interfacial friction.When the bubble angle is 0°,the fluid boundary slip length is the largest and the solid-liquid interfacial friction is the smallest;the micro-nano structures affect the drag reduction characteristics and reducing solid-liquid contact can improve the drag reduction effect of superhydrophobic surface.According to the theoretical analysis results,we use one step immersion method and chemical replacement method to prepare the superhydrophobic surface with different micro-nano structures on metal.The experiment result shows that the superhydrophobic surface with conical structure has the best hydrophobicity and drag reduction effect.The prepared surface has water contact angle about 158° and the rolling angle about 4°.When it is in 1.5m/s velocity,the drag reduction rate is about 32%.Meanwhile,the superhydrophobic surface with conical structure has better durability.The surface still keep stable drag reduction effect when it is tested with rushing for 52 hours.So far,the methods of measuring solid-liquid interfacial friction are indirect measuring.These methods analyze the drag reduction at solid-liquid interface by the fluid slip length.The test result is always affected by liquid gravity.The paper suggests the method for direct measurement of weak frictional resistance of solid-liquid interface.We use the resistance sensor to build up the device for testing frictional resistance of solid-liquid interface.The testing device provides an experimental platform for the drag reduction.Based on the suggested test method for frictional resistance of solid-liquid interface,the prepared superhydrophobic surface and water frictional resistance are measured.We prove the model of cavitation drag reduction at solid-liquid interface with microstructure by the experiment.The preparation of superoleophobic surface is still the difficulty and it also restricts the study of drag reduction on superoleophobic surface.The paper suggests the composite method of electro deposition method and oxidation method to prepare the superoleophobic surface on copper.We analyze the influence of micro/nano structures on oleophobic and drag reduction effect.The study finds that compared with the common micro/nano structures,the hanging structure can reduce the actual surface connection of liquid and solid.The structure also can improve the oleophobic property and drag reduction.The prepared superoleophobic surface with hanging structure has oil contact ange about 156° and the rolling angle about 7°;meanwhile,the superoleophobic surface with hanging structure has well drag reduction effect and durability.When it is in 1.5 m/s velocity,drag reduction rate is about 23%.The surface still keep stable drag reduction effect when it is tested with rushing for 52 hours.Based on these,we prepare the superoleophobic surface on the rotor by compound method and study the rotor by the hydraulic floating support prototype.The experiment result shows that the superoleophobic surface can reduce the frictional resistance between rotor and oil and improve the speed of the rotor.