Microdroplet Mobility Mediated by Dynamic Oil Menisci on Lubricant-Infused Surfaces

During dropwise condensation on lubricant-infused surfaces (LISs), we show that oil menisci that form surrounding microdroplets lead to a dynamically changing formation of oil-rich and oil-poor regions which in turn affect the mobility of the microdroplets. Microdroplets robustly self-propel long distances (3 - 6 times their diameters) towards larger droplets in the oil-rich regions, independent of gravity. The sliding velocity depends on droplet sizes, the distance between droplets, and the oil viscosity. The overlapping oil menisci between droplets create an anisotropic oil profile around the moving droplets, leading to unbalanced lateral components of surface tension force along its (apparent) three-phase contact line. We also reveal strong outward flow at the oil-vapor interface of the menisci, effectively transporting nucleated droplets to oil-poor regions at high subcooling degrees. We attribute the internal dynamics to temperature-driven Marangoni flow. Mathematical models based on force balance and scale analysis show good agreement with the experimental data. This work will help inform the selection of lubricant for LISs to enhance droplet mobility and heat transfer performance, and improve our understanding of microscale interfacial thermal-fluid dynamics.