How can droplets move easily on a horizontal surface when a droplet of rain doesn’t even slide down a window pane? The answer is that the motion of a droplet on a surface is generally hindered by contact line pinning. This is the liquid equivalent of static friction.
If a water droplet is placed on a SLIPS surface, it’s contact line is not pinned but becomes completely mobile. Although drops readily roll off when the SLIPS is tilted (even by an angle of less than 1°), a droplet placed on a horizontal surface won’t roll off. Instead, it will evaporate over time. However, the profile of the evaporating droplet is rather different to that of a droplet evaporating on a non-SLIPS. The apparent contact angle of the droplet remains constant throughout the evaporation time. The droplet profiles show a spherical cap shape modified by a wetting ridge close to the contact line between the droplet and the surface.
- Evaporation of sessile droplets on Slippery Liquid-Infused Porous Surfaces (SLIPS) J.H. Guan, G.G. Wells, B. Xu, G. McHale, D. Wood, J. Martin and S. Stuart-Cole, Langmuir 31 (2015) 11781-11789.
- Apparent contact angle and contact angle hysteresis on liquid infused surfaces C. Semprebon, G. McHale and H. Kusumaatmaja, Soft Matter 13 (2017) 101-110.