Wetting Behaviour of Droplets and Capillary Liquid Bridges Involving Liquid Infused Surfaces

Abstract

Liquid-infused surfaces (LIS), formed of a nano- or micro-structured porous material that is impregnated with a lubricating fluid, are biomimics of the pitcher plant. Their slippery surface makes them ideal candidates for a wide range of applications, from anti-corrosive, anti-fouling and anti-icing coatings to self-healing surfaces and controlled wetting. Here, LIS are studied to determine the fundamental interactions of the lubricant layer with the porous medium, and with droplets and capillary liquid bridges under different environmental conditions. When LIS are exposed to realistic environmental conditions, they rapidly lose their desirable slippery properties and age, which may compromise their original application. This may occur through a process of oil loss, smoothing of the porous layer and porous substrate degradation. Oil loss is attributed to the formation of micro-emulsion in the lubricant layer which displaces oil away from the surface.

LIS interactions with stretching capillary liquid bridges (CLB) are also considered. Measuring capillary forces on LIS is challenging as they present a surface with little friction or pinning. A novel set-up is created to study the comparatively small forces exhibited by LIS, and extract the bridge geometry. CLBs on non-infused surfaces are strongly affected by contact line pinning, and hysteretic behaviours are commonly observed for the measured contact radii, contact angles, and capillary forces. In contrast, CLBs on LIS experience no pinning and hysteretic behaviours are largely absent. Furthermore, cloaking and the formation of oil ridges change the effective CLB surface tension and contact angle, and gravity has a more dominant effect due to small changes in capillary forces on LIS. In all cases, including for asymmetric bridges (with one LIS and one other surface), it is shown that the measured capillary forces can be well-described by a simple theoretical model that uses the CLB contact radius, contact angle and curvature as input parameters.