Study of water repellency and crystal precipitation on multi-scale functionalized filtration membranes

Abstract

In this thesis, we consider the water repellency and salt precipitation on multi-scale functionalized filtration membrane. In particular, the fabrication of superhydrophobic filtration membrane with various industrial applications, experimental investigation of salt crystal precipitation that varies on the surface hydrophobicity and water penetration between superhydrophobic cylinders for the optimization of membrane distillation module are composed of this thesis. We start with the fabrication of superhydrophobic filtration membrane. Using the chemical vapor deposition, nano-textured polymer membrane was fabricated. The static wetting behavior and condensation behavior was investigated. In addition, robustness of superhydrophobicity after condensation and oil wetting was observed. Subsequently, it is investigated that salt crystal precipitation can be varied with the hydrophobicity of filtration membrane surfaces. On moderately hydrophobic surface and superhydrophobic surface, specific shapes of aggregation of salt crystal are observed due to surface temperature, which were pizza, pot and pebble structure. From the evaporation theory of the sessile drop that is governed by diffusion and scaling, we compare the evaporation of pure water drop and saline water drop. Using mass conservation and water potential in the vicinity of liquid-air interface, we predict the thickness of precipitated salt crystal. The static contact angle, evaporation modes and particle image tracking were used to find the reason of forming of pizza, pot and pebble structure. Finally, we consider water drop penetration between superhydrophobic cylinders. To enhance the performance of membrane distillation, it is needed to increase the contact area of hollow fiber that is contact with water. However, since the superhydrophobicity of hollow fiber can disturb the penetration of water, we need to fine optimized distance between adjacent fibers. Using dimensionless analysis and scaling, we predict the distance between two superhydrophobic cylinders, at which a water drop starts to penetrate. As an academic interest, we also investigate water drop impact between two superhydrophobic cylinders. The relation of Weber number and drop radius is studied. Our work provides the fabrication method of superhydrophobic filtration membrane using a plasma, salt precipitation mechanism on hydrophobic surface and theoretical insight for drop penetration.