Droplet Retention and Shedding on Slippery Substrates

A significant limitation for droplet mobility on solid surfaces is to overcome the inherent pinning of the droplet's contact line that occurs because of chemical/physical heterogeneities. A recent innovation is to use surface texture or porosity to create a stabilized lubricant surface. Droplets on such slippery liquid-infused porous surfaces (SLIPS)/lubricant-impregnated surfaces (LIS) are highly mobile because of the lubricant layer. Low pinning of the contact line reduces the energy required to move a droplet; however, it makes it difficult to accurately position the droplet or to stop its motion altogether. In this paper, a simple structure (step), as small as a few microns in height, is used to introduce controlled droplet pinning on a slippery substrate. The key effect is identified as the capillary force, arising from the interaction between the lubricant menisci created by the step and droplet. The effect of changing step height, lubricant thickness, and initial position on step-droplet interactions has been investigated, showing that droplets can both be repelled from and attracted to the step. To measure the adhesion strength, we report droplet detachment angle measurements under gravity and scaling of force with the lubricant thickness/step height ratio. Under certain conditions, the interaction strength is sufficient to ensure droplet-step attachment even when the surface is rotated to an upside-down orientation. These findings can motivate the design of SLIPS structures, capable of shedding or retaining droplets preferentially, for example, according to the size or wettability, relevant to applications from microfluidics to fog harvesting.