Study On The Drag Reduction Of Superhydrophobic Coating Synthesized By Atmospheric Plasma Jet

The unique wettability property of superhydrophobic surface make it a prospected material that can be applied in the field of self-cleaning products, marine corrosion resistance and drag reduction, biomedicine aspect and so on. Generally, there are mainly two preparation methods to produce that superhydrophobic surface. One is to introduce the low surface energy surface with rough structure. The other is to lead in low surface energy materials on the rough structured solid surface. However, most of the preparation methods are limited by many straitened circumstances, such as highly cost, long process time, complicated preparation procedure and special equipment demanded, et al. The promising atmospheric pressure plasma jet can works in atmospheric environment directly, which remarkably simplifies procedure, increases the deposition rate, improves the safe operationand then reduces the whole cost. The atmospheric pressure plasma jet makes it possible to apply on a large area plate treatment.The superhydrophobic coatings were synthesized by atmospheric pressure plasma jet on aluminum plate and its epoxy painted surfaces in discharging environment, in which hexamethydisiloxane(HMDSO) was used as monomer, and Ar was carrier gas. By means of contact angle tester, the contact angle of the prepared coatings on the surface reached up to 162o and the rolling angle decreased to nearly 1o. Whereas the contact angle of untreated sample was 78o, and the rolling angle was approximately 90o. With the high contact angle and low rolling angle, the deposited coatings show excellent superhydrophobicity, which indicates that the method of atmospheric plasma jet treatment can significantly change the wettability property of specimen's surface, no matter it is hydrophilic. To study the wettability mechanism of superhydrophobic surface, SEM, EDX, FTIR and other testing methods were applied to analyze the surface structure and composition. The result shows that the superhydrophobic coating is composed of accumulated micro-nano particles and large amount of pores and cavities that formed in accumulation process. What's more, the main elements of the coating are Si and O, which constitute Si-O-Si network structure with low surface energy. Both of these two beneficial terms contribute to decrease the water wettability of coated surface. Water droplets on deposited surface present spherical distribution and can fall off the surface while slightly tilted platform, which means it has excellent superhydrophobic behavior. The deposited coatings by plasma kept superhydrophobic properties for over 10 months in the atmosphere. After the coated samples had been soaked in acid and alkali corrosion solution for a period, the surface still maintained high contact angle and low rolling angle. The experimental results of the stability of superhydrophobic coatings in water flow surroundings showed that, with increasing flow velocity and flow time, the attached superhydrophobic coating on the surface of the metal plate would subject more serious damage, which leads to weak hydrophobic effect.The unticorrosion test in seawater was measured for superhydrophobic surfaces deposited on carbon steel plate. Compared to bare steel substrate, the plasma treated plate showed less number of corrosion pots, and the area of corrosion on superhydrophobic surface were much smaller. The results of electrochemical behavior of coated sample showed that the self corrosion potential had improved and the corrosion current density was reduced. What's more, the impedance value increased obviously. All of this implies a fact that the superhydrophobic coatings possess prefect unticorrosion effect to the base metal materiel.To investigate the drag reduction performance of superhydrophobic surface in water flow, the differential pressure and flow resistance testing device was used for atmospheric pressure plasma jet treatment sample. The results showed that, with the flow velocity increasing gradually, the drag reduction effect of the superhydrophobic coated surface increase fast at the beginning and then decrease after a maximum value at about 2.5m/s, that is 37%. The contact model of a water drop on hydrophobic surface was mainly appeared in the form of gas-liquid contact,which could induced water flow evolve to slip flow, causing the boundary shear stress decreased to zero and the friction resistance between water flow and the solid surface was greatly reduced. which results in the drag reduction effect. This potential characteristics of superhydrophobic surface make it extensive application value in the field of ship, warship and various underwater vehicles.