# How does water freeze and crystallize on a cold superhydrophobic surface?

Superhydrophobic surfaces are nano-structured to "repel" liquid water and there has been a lot of work done to understand the microscopics of wetting of these nanostructures.

What I currently am interested in understanding is wetting and freezing of water on a cold, below-freezing superhydrophobic surface.

I would expect that the ice would still have mechanical adhesion to the surface, but I don't know to what extent.

So my question is the following: How does water freeze and crystallize on a superhydrophobic surface?

I know this is a general question that would depend on the nature of how superhydrophobicity is achieved, so feel free to specifically treat a certain branch of superhydrophobic surfaces.

As acknowledged in your question, there are a large number of factors at play here. The most basic of these is that there are three fundamental mechanisms by which freezing of a droplet can initiate when in contact with a superhydrophobic surface: 1) heterogeneous nucleation where an ice freezing nucleus (IFN) is present in the water droplet, 2) heterogeneous nucleation where the superhydrophobic surface serves as the IFN, and 3) homogenous freezing of the water droplet.

An excellent demonstration of the first or third scenario above (it is not clear from the article which is actually the case) is given in video format from an article published in Nature$^1$. In this experiment, the water droplet is evaporatively cooled until freezing initiates at the water/air interface, and continues without adhering to the aluminum-based superhydrophobic surface.

Whether or not freezing will initiate at a superhydrophobic surface depends upon the degree of supercooling, the presence or absence of potential IFN within the water droplet, and the potential for the specific material (the superhydrophobic surface) to serve as IFN. It would seem that the latter of these would be unlikely, except that a surface being hydrophobic doesn't mean that it is equally icephobic. Basically, the more the surface resembles the crystalline structure of ice, the more likely it is to act as an IFN.

1) Thomas M. Schutzius, Stefan Jung, Tanmoy Maitra, Gustav Graeber, Moritz Köhme & Dimos Poulikakos, Spontaneous droplet trampolining on rigid superhydrophobic surfaces, Nature 527, 82–85 (05 November 2015) doi:10.1038/nature15738

• Thanks for the answer airhuff. If you don't mind, I'd like to ask a few more questions: – user157879 Jun 3 '17 at 17:33
• 1. Are there any general features of how well ice adheres to hydrophobic surfaces after forming? Naively I would expect them to adhere quite weakly, but it would depend case-by-case. 2. Take the case of standard room humidity and introduce a cold superhydrophobic surface, how would ice form in this case? This seems to be different than if water was pre-existing on the surface – user157879 Jun 3 '17 at 17:45
• I agree with your expectations that ice would tend to adhere pretty weakly to a superhydrophobic surface. Your example of forming ice from the vapor is interesting. I once did a set of experiments where I cooled a glass rod in ambient air so that it would grow ice crystals. I had a terrible time with supercooled water depositing instead of ice because ice likes to form on crystaline-like materials, which glass is not. If I had been using a superhydrophobic material instead of glass, I suspect 2 things: 1) I would have had to cool to much lower temperatures to induce condensation (continued...) – airhuff Jun 4 '17 at 1:41
• (meaning higher levels of supersaturation of water vapor would be required to form condensation) and 2) since the focus of superhydrophobic materials is to be hydrophobic, not necessarily ice-phobic, I suspect that at low enough temperatures ice would have formed rather than supercooled water, and that the ice would have adhered to the surface more readily than if it had formed from the liquid phase. This is admittedly somewhat speculative, as I have little experience with superhydrophobic materials, though I do have a lot of experience growing ice ;) – airhuff Jun 4 '17 at 1:51