How can droplets move easily on a horizontal surface when a droplet of rain doesn’t even slide down a window pane? 

Illustration of contact line pinning

The motion of a droplet on a surface is hindered by contact line pinning – the liquid equivalent of static friction. If a surface is impregnated with a lubricating oil which is not displaced by water, a droplet of water becomes completely mobile on the surface.  Lubricant impregnated surfaces are examples of Slippery Liquid Infused Porous Surfaces (SLIPS) inspired by the Nepenthes Pitcher plant.

Our experiments have shown that a small droplet of water evaporating on a lubricant impregnated surface/SLIPS has a completely mobile contact line. The apparent contact angle of the droplet remains constant throughout the evaporation time. We have also shown that a droplet on a SLIP surface is easily transported by impacting it with a Surface Acoustic Wave (SAW).

Droplet on a Slippery Liquid Infused Porous Surface

At a fundamental level, a droplet on a SLIP surface rests on a layer of lubricating liquid rather than directly on the underlying solid. The shape of the droplet profile near the solid surface is different to a droplet resting directly on the solid. This is because the droplet “pulls up” some of the lubricant and the balance of forces is described by a Neumann triangle.