cleate and grow on liquids and liquid impregnated surfaces†

Title: How droplets nucleate and grow on liquids and liquid impregnated surfaces While nucleation is most likely to occur at the liquid–air interface, the liquid viscosity plays a key role in determining the overall growth of droplets. a Condensation on liquids has been studied extensively in context of breath figure templating, materials synthesis and enhancing heat transfer using liquid impregnated surfaces. However, the mechanics of nucleation and growth on liquids remains unclear, especially on liquids that spread on the condensate. By examining the energy barriers of nucleation, we provide a framework to choose liquids that can lead to enhanced nucleation. We show that due to limits of vapor sorption within a liquid, nucleation is most favoured at the liquid–air interface and demonstrate that on spreading liquids, droplet submergence within the liquid occurs thereafter. We provide a direct visualization of the thin liquid profile that cloaks the condensed droplet on a liquid impregnated surface and elucidate the vapour transport mechanism in the liquid films. Finally, we show that although the viscosity of the liquid does not affect droplet nucleation, it plays a crucial role in droplet growth. Condensation of vapor on a surface can occur either in lmwise or dropwise mode. For many applications including power generation, water harvesting, desalination, and thermal management, dropwise condensation is preferred as it expedites the removal of the condensate from the surface resulting in a signicant increase in condensation heat transfer. 1–6 The magnitude of this increase is related to the three stages of a droplet " life " on a surface: nucleation, growth, and departure. Various routes of optimizing these aspects of dropwise condensation via surface engineering have been proposed. Functionalizing the surface to obtain a hydrophobic surface chemistry and combining it with nano/micro-texturing to achieve superhydrophobicity are some of the ways explored to reduce droplet adhesion as measured through contact angle hysteresis. 5,7–9 In some cases, the reduced adhesion on super-hydrophobic surfaces can also lead to micro-droplet ejection upon coalescence with neighbouring droplets. 10–12 However, under high subcooling conditions, nucleation of a large density of nanoscale droplets within the texture of superhydrophobic surfaces leads to the formation of a liquid lm on the surface. 13–15 In order to overcome such challenges, an alternative surface design with composite solid–liquid materials has gained signicant attention recently. 16–22 Such composite materials consist of nano/micro textured low surface energy surfaces impregnated with a liquid (henceforth also referred …