Investigating Lubricant Behavior on Lubricant Infused Surfaces

Abstract

Surfaces incorporating stable lubricant overlayers, known as slippery liquid-infused porous surfaces (SLIPS), have been shown to possess impressive anti-fouling properties in a wide array of applications. The extreme repellency of SLIPS mainly arises from the ability of the surface to maintain a lubricant overlayer that masks the solid substrate from impinging droplets, bacteria, ice, and other foulants. Thus, understanding the lubricant behavior in a variety of conditions is critically important for creating high-performing lubricant infused surfaces. We begin by visualizing the lubricant dynamics during water condensation and freezing on SLIPS, an area with important relevance to anti-ice applications and phase-change heat transfer. We then develop and validate new mechanistic models which explain our observations and are also highly relevant to many other applications. Investigating the geometry of a droplet on SLIPS, we find that the lubricant wetting ridge is a dynamic, low-pressure region that can grow quite large as it is exposed to excess lubricant; however, due to the long timescales associated with thin film flow, SLIPS are often far from equilibrium. During droplet motion, lubricant is entrained around a moving droplet by the interplay of capillary pressure in the wetting ridge and viscous dissipation in lubricant thin-films. We develop a mechanistic model for this process which we verify experimentally and identify the growth of the wetting ridge as the main source of lubricant depletion due to a droplet moving on SLIPS. This is a critical development as many potential applications for SLIPS are limited by longevity, and these findings suggest non-intuitive strategies for mitigating lubricant loss over time. Finally, we consider the thin film limit, when lubricant stability is dictated by complex interfacial and long-range interactions. We find that the small size of the wetting ridge, which arises as a result of depletion, plays a key role in destabilizing the lubricant overlayer under and around a droplet. Ultimately, we hope this work will inform future research and lead to improved SLIPS for a variety of applications.