Microdroplet self-propulsion during dropwise condensation on lubricant-infused surfaces

Abstract

Water vapor condensation is common in nature and widely used in industrial applications, including water harvesting, power generation, and desalination. As compared to traditional filmwise condensation, dropwise condensation on lubricant-infused surfaces (LIS) can lead to an order-of-magnitude increase in heat transfer rates. Small droplets (D ≤ 100 μm) account for nearly 85 % of the total heat transfer and droplet sweeping plays a crucial role in clearing nucleation sites, allowing for frequent re-nucleation. Here, we focus on the dynamic interplay of microdroplets with the thin lubricant film during water vapor condensation on LIS. Coupling high-speed imaging, optical microscopy, and interferometry, we show that the initially uniform lubricant film re-distributes during condensation. Governed by lubricant height gradients, microdroplets as small as 2 μm in diameter undergo rigorous and gravity-independent selfpropulsion, travelling distances multiples of their diameters at velocities up to 1100 µm/s. Although macroscopically the movement appears to be random, we show that on a microscopic level capillary attraction due to asymmetrical lubricant menisci causes this gravity-independent droplet motion. Based on a lateral force balance analysis, we quantitatively find that the sliding velocity initially increases during movement, but decreases sharply at shorter inter-droplet spacing. The maximum sliding velocity is inversely proportional to the oil viscosity and is strongly dependent of the droplet size, which is in excellent agreement with the experimental observations. This novel and non-traditional droplet movement is expected to significantly enhance the sweeping efficiency during dropwise condensation, leading to higher nucleation and heat transfer rates.

Cloaking Dynamics on Lubricant‐Infused Surfaces

“…Furthermore, the presence of the lubricant alters interfacial dynamics, affecting the three‐phase contact line which governs phase‐change. [ 20 ] Hence, evaporation and condensation dynamics on SLIPSs/LISs can differ from those observed on solid surfaces.…”

Super liquid repellent surfaces for anti-foaming and froth management

“…The attractive force depends on the size of both bubbles (Supplementary Fig. 5), the height of the wetting ridge, the interfacial tension, and the viscosity of the oil 28 . The bubbles merge upon contact with one another, increasing buoyancy (Fig.…”

Gradient Quasi‐Liquid Surface Enabled Self‐Propulsion of Highly Wetting Liquids

“…Surface‐mediated self‐propulsion of droplets provides a possible approach to passively remove highly wetting condensates during condensation. Specifically, droplets can move without any external energy with the help of asymmetric surface topographies, [ 10,15–23 ] gradient surface wettability, [ 24–29 ] liquid meniscus on SLIPS, [ 30,31 ] and gradient electrostatic charge on superomniphobic surfaces. [ 32 ] In condensation, the surface topographies are flooded by highly wetting liquids and the poor durability of liquid meniscus persists.…”