Wetting and Liquid Flow on Lubricant Infused Nano- and Micro-Structured Surfaces

Abstract

Surfaces with special wettability have the potential to drastically change the way we use technology and our impact on the environment. Inspired by the lotus leaf, superhydrophobic surfaces are artificial self-cleaning materials, which trap air in a micro- and nano-scale hydrophobic topography. Although they have potential use in anti-fouling, and anti-drag applications, the trapped air that enables the superhydrophobic Cassie-wetting state has been shown to be thermodynamically unstable under pressure and shear. Based on the trumpet pitcher plant, Slippery Liquid-Infused Porous Surfaces (SLIPS) utilise similar surface roughness to trap a chemically compatible lubricant via capillary wetting, creating a customisable liquid interface, and are stronger candidates for self-cleaning and anti-fouling surfaces, with the potential for drag reduction. Understanding the nano-scale forces that stabilise the thin lubricant film, and the ability of these films to reduce drag is crucial for their practical use under shear. In this Thesis, the following is reported: Firstly, the fabrication and characterisation of single-scale and hierarchical superhydrophobic surfaces, via spontaneous wrinkling of a rigid Teflon AF film on shrinkable plastic substrates was performed, alongside mechanical robustness testing. Secondly, the dynamics of depletion of a silicone oil layer on square micro-pillar surfaces upon the placement of a water droplet was studied via laser scanning confocal microscopy, visualising the curvature of the lubricant interface between pillars. Finally, using colloidal probe atomic force microscopy, the flow of simple viscous liquids was studied over smooth lubricated surfaces, with the aim to quantify effective interfacial slip. The minimum lubricant film thickness that has slippery properties was quantified and related to macroscopic measurements of roll-off angles of water droplets.