Nature-inspired low friction lubricated surfaces

Abstract

In this thesis I investigatethe wetting and lubrication properties of new types of surface coatings known as Slippery Liquid-Infused Porous Surfaces (SLIPSs). Such lubricated surfaces represent a promising technology which allows the control of the liquid friction and wetting properties of surfaces through the presence of specific oils, theoretically removing any form of pinning. However, unlike for solid surface coatings, there is little current knowledge of how to spatially pattern lubricated surfaces with different wettability in different regions and the properties of the resulting surfaces. This research explores the creation of innovative Multi- Lubricant Lubricated surfaces, referred to as Composite SLIPS, their properties and their potential for droplet self-propulsion. The research investigates the design and manufacture of resilient and long-lasting Composite SLIPSs, focusing on comprehending their underlying physics of wetting, including the impact of surface chemistry and lubricant liquid properties.

Two different techniques for creating spatially patterned lubricated surfaces are investigated. The first approach uses a mechanical method based on 3-D printing whilst the second approach uses a physical-chemical technique based on preferential wetting by lubricants to different substrate materials patterned using photolithography. It is found that the mechanical method can achieve macroscopic patterns on the mm scale and shows that patterned biphilic lubricated surfaces can be achieved, but it is difficult to extend to smaller scales. For the second approach, a layer of Teflon AF1600 is patterned on 50 micron and larger scale using a lift-off process and is used to stabilize a thin film of a perfluorinated oil, Krytox. A superhydrophobic coating (GLACO) is used as the complementary material to stabilize a range of other oils with a focus on olive oil to create a Composite SLIPS surface. To demonstrate the applications of the new techniques, wedge-shaped regions of olive oil are created within a Krytox oil background, which exhibits lower water wettability. This setup drives linear self-propelled motion of water droplets from the wedge tip toward the olive oil’s higher wettability region at the base. A theoretical model is developed to accurately describe this motion in relation to design and material parameters. These findings advance the development of materials with novel wetting and lubrication properties, offering insights for optimizing Composite SLIPS in applications like droplet microfluidics and water harvesting.