Study On Wetting Behavior Of Droplets On Single Crystal And Polycrystalline Solid Surfaces

With the development of society,the requirements for the practical application of biomedical material compatibility,optical properties of transparent ceramics,anti-oil and anti-fouling of fiber fabrics,surface modification of alloys,pigment synthesis,pesticide spraying and so on have been gradually raised,and the study of wetting behavior has important research value.However,the adsorption of surface water mist such as Al2O3,the staining of polymer fabrics,and the lack of industrial lubricating oil and the pollution limit their use.In this paper,the wetting behavior of water,glycerin and castor oil in PMMA,PTFE,PVC,glass and stainless steel substrates was studied in a normal temperature and semi-closed environment by modified seat drop method.The difference in wetting behavior on α-Al2O3 single crystals(<0001>,<11-20>,<10-10>,<1-102>)and polycrystalline Al2O3 substrates was further investigated from the point of crystal orientation.The contact angle was measured by ImageJ software,and the solid surface energy and interface energy were calculated by theoretical equations such as ZDY method.The main results obtained are as follows:(1)The contact angles of water,glycerin and castor oil on low surface energy polymer substrates(PMMA,PTFE and PVC)are respectively between 108.6～80.1°,96.5～75.5° and 63.9～55.0°.The contact angle decreases with the extension of free evaporation time,and the reduction in surface tension helps to reduce the contact angle.However,the difference in substrate surface groups also has an effect on the wetting behavior.The contact angle is the result of the combination of the liquid type and the surface characteristics of the substrate.Free evaporation causes the transition of droplets from Cassie to Wenzel wetting mode which is an unstable process and is disturbed by multiple factors.It has multiple equilibrium states in the transition process and is not always in the expected minimum energy state.(2)The contact angles of water,glycerin and castor oil on high surface energy substrates(glass and stainless steel)are in the range of 78.3～59.2,70.7～52.8,44.8～39.3°,respectively,and the high surface energy of the substrate helps to improve the wettability,but is subject to a "big" contact angle due to the proportion of polar components in the three-phase pinning and surface energy.(3)The crystal orientation of α-Al2O3 single crystals affects the wetting behavior.The contact angles of water,glycerin and castor oil on the<0001>,<11-20>,<10-10>and<1-102>orientations were 68.3～33.4,87.9～44.4,84.2～34.1,87.9～42.6°,respectively.There are also differences in interface bonding strength,spreading equilibrium time,and solid/liquid interface energy change rate at different orientations.Polycrystalline Al2O3 facilitates liquid spreading.It was confirmed that the surface of the<0001>oriented substrate was Al atoms,and the remaining orientation was O atoms,and the wetting behavior was related to the interaction between solid/liquid molecules.