Condensation Frosting: From Ice Bridges to Dry Zones
The most ubiquitous mode of frost formation on substrates is condensation frosting, where dew drops condense on a supercooled surface and subsequently freeze, and has been known since the time of Aristotle. The physics of frost incipience at a microscopic scale has, nevertheless, eluded researchers because of an unjustified ansatz regarding the primary mechanism of condensation frosting. It was widely assumed that during condensation frosting each supercooled droplet in the condensate population freezes in isolation by heterogeneous nucleation at the solid-liquid interface, quite analogous to the mechanism of icing. This assumption has very recently been invalidated with strong experimental evidence which shows that only a single droplet has to freeze by heterogeneous nucleation (typically by edge effects) in order to initiate condensation frosting in a supercooled condensate population. Once a droplet has frozen, it subsequently grows an ice bridge towards its nearest neighboring liquid droplet, freezing it in the process. Thus ensues a chain reaction of ice bridging where the newly frozen droplets grow ice bridges toward their nearest neighbor liquid droplets forming a percolating network of interconnected frozen droplets. Not always are these ice bridges successful in connecting to their adjacent liquid droplets. Sometimes the liquid droplet can completely evaporate before the ice bridges can connect, thus forming a local dry region in the vicinity of the ice bridge. In this work, we first formulate a thermodynamic framework in order to understand the localized vapor pressure gradients that emerge in mixed-mode phase-change systems and govern condensation and frost phenomena. Following this, we study droplet pair interactions between a frozen droplet and a liquid droplet to understand the physics behind the local ice bridge connections. We discuss the emergent scaling laws in ice bridging dynamics, their relative size dependencies, and growth rates. Thereafter, we show how with spatial control of interdroplet distances in a supercooled condensate and temporal control of the first freezing event, we can tune global frost propagation on a substrate and even cause a global failure of all ice bridges to create a dry zone. Subsequently, we perform a systematic study of dry zones and derive a scaling law for dry zones that collapses all of our experimental data spanning a wide parameter space. We then show that almost always the underlying mechanism behind the formation of dry zones around any hygroscopic droplet is inhibition of growth and not inhibition of nucleation. We end with a discussion and preliminary results of our proposed anti-frosting surface that uses ice itself to prevent frost.