Nucleation Dynamics for Water Condensation on Lubricant-Infused Surfaces

In recent decades, various non-wettable surfaces have been proposed to promote dropwise condensation, featuring much higher heat transfer rates than traditional filmwise condensation. Lubricant-infused surfaces (LIS or SPLIPS) can promote stable dropwise condensation and improve heat transfer rates due to a low nucleation energy barrier and high droplet mobility, leading to high sweeping and re-nucleation rates. However, the nucleation dynamics on lubricant-infused surfaces are considerably less studied and the interplay between the lubricant film and nucleation has been a matter of intense debate. In this work, we conduct water vapor condensation experiments on LIS infused with Krytox oil of varying viscosity within a custom-design chamber with controllable vapor temperature at atmospheric pressure. Using high-speed optical and infrared imaging and optical microscopy, we show that nucleation predominantly occurs in the oil-poor regions where the oil-vapor interfacial temperature is lower than in oil-rich regions. This small temperature difference causes an order magnitude lower free-energy nucleation barrier, leading to enhanced nucleation. The relative area ratio of oil-poor regions to total surface area, i.e., preferred area for nucleation, can be tailored between 11% and 19% by changing the lubricant viscosity from 1627 to 73 cP. We statistically analyze the influence of lubricant viscosity on the new emerging droplets on LIS at a broad range of vapor supersaturation ratios. The results show that the nucleation rate density dramatically increases with vapor supersaturation. More importantly, a strong dependence of nucleation rate density on lubricant viscosity exists in the whole range of supersaturation ratios, which means that higher heat transfer rates can potentially be achieved by lowering lubricant viscosity.