Water Droplet Impact on Functional Surfaces

Abstract

The impact and spreading of picolitre-sized water droplets on a substrate is of importance in many applications such as rapid cooling, delayed freezing, crop spraying, and inkjet printing. In this thesis, the effects of substrate chemistry, roughness, hardness, charge, and porosity on such droplet impact are studied.
The effect of roughness was investigated through the use of superhydrophobic CF4 plasma fluorinated polybutadiene. Comparison of the maximum spreading ratio and droplet oscillation frequencies with literature models shows that both are found to be lower than theoretically predicted. Further study of the effect of multiple types of surface topography was carried out via the CF4 plasma texturing of honeycomb surfaces, leading to hierarchical surfaces with roughness on two length scales. This led to the discovery that surfaces with similar static contact angles can give rise to different droplet impact dynamics, governed by the underlying surface topography.
The effect of the mechanical properties of the substrate upon picolitre droplets can be important in microfluidics. The oscillatory dynamics of picolitre droplets following impact were found to depend upon the thickness and elasticity of the substrate. Higher oscillation frequencies are measured for softer and thicker films, which are correlated to larger surface deformations around the contact line.
Static buildup during inkjet printing is known to affect print quality. The role of surface charge on picolitre droplet impact onto polymer substrates is found to give rise to increased droplet impact velocities. Higher surface potentials can result in unexpected behaviour such as droplet bouncing or increased contact area diameters leading to a decrease in print resolution.
Printing on porous materials is important as porosity can aid ink adhesion and durability. CF4 plasma fluorination of porous membranes can inhibit droplet spreading laterally over a surface, with little change in the imbibition behaviour in the material, leading to printing that is more highly defined. These hydrophobic membranes remain oleophilic and could also find use in oil–water separation. Similarly, a hydrophilic–oleophobic switching surface can be beneficial in a range of applications such as anti-fogging, self-cleaning, and oil– water separation. Polelectroyle–fluorosurfactant complexes were found to exhibit excellent switching, resulting in a surface that quickly becomes hydrophilic whilst remaining oleophobic.