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. / Master of Science / In the movie Iron Man, during the very final battle sequence between our eponymous hero and Iron Monger, there is a moment when Tony Stark realizes that Iron Monger can fly. Immediately Iron Man shoots up into the sky. He tries to reach as high as he can. Iron Monger chases after him. Eventually, high up in the sky amidst the clouds, Iron Monger catches up. He grabs Iron Man in his enormous grip and punches him. It seems like there is no escape for Iron Man. But right then, writhing in Iron Monger’s grip, Tony Stark asks Iron Monger, ‘How’d you solve the icing problem?’ It is then revealed that Iron Monger’s suit has completely frozen over, whereas Iron Man’s suit has no ice whatsoever. Iron Monger shuts down and starts falling from the sky.
The icing problem, which Tony Stark mentions, occurs when supercooled liquid water droplets impact on a chilled substrate and subsequently freeze. Another way of accretion of ice constitutes the frosting problem. The most common mode of frost formation on a surface is called condensation frosting, where the ambient water vapor first condenses on the chilled surface as dew drops, and these liquid droplets subsequently freeze. Until very recently, it was widely assumed that during condensation frosting all the dew droplets freeze in isolation at the solid-liquid interface, without interacting with each other. This assumption, however, is not true. It has recently been shown that in order to initiate condensate frosting, only a single droplet has to freeze by itself, at the solid-liquid interface. Thereafter, frost propagates by the formation of an inter-droplet ice bridge network, where the frozen droplets grow ice bridges toward their nearest neighbor liquid droplets. Interestingly, these ice bridges are not always successful in connecting to their adjacent liquid droplets. If the inter-droplet distance is too large, 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 do extensive experiments to investigate the underlying physics of frost incipience on a microscopic scale. We then derive scaling laws for successful ice connections, their growth rates and for dry zone formations, and end by discussing possible anti-frosting strategies.
It appears that Tony Stark in his universe has solved the icing problem, and most probably also the frosting problem. In reality, however, on earth, we have not. Though anti-icing has received a lot of attention, the same cannot be said about the frosting problem. This work tries to take the first steps towards that. Quite ironically, it looks like ice itself might be the solution to the frosting problem, because of its ability to create dry zones.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/79129 |
| Date | 18 September 2017 |
| Creators | Nath, Saurabh |
| Contributors | Engineering Science and Mechanics, Boreyko, Jonathan B., Yue, Pengtao, Paul, Mark R., Hanna, James, Jung, Sunghwan |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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