We are all familiar with the silica gel packets that are included in our packages and medicine bottles.  Silica gel is hygroscopic, which means that it preferentially collects moisture from the air to keep the surrounding material dry.  But did you know that ice is also hygroscopic?  In this NSF sponsored research, we revealed how the hygroscopic properties of ice govern the growth of frost and can even be exploited to promote anti-frosting surface technology.  Our three primary outcomes can be summarized as follows: (1) Characterized incipient frost formation on surfaces, (2) Revealed when dry zones can form about hygroscopic ice, and (3) Developed a passive anti-frosting surface technology.  We will now explain each of these outcomes in more detail.  

(1) Using top-down microscopy, a cold stage, and a humidity chamber, we captured how frost first forms on a surface under a wide variety of conditions.  We developed a comprehensive model that explains a process we term "inter-droplet ice bridging" that is responsible for the initial layer of frost.  The mechanism for ice bridging is the hygroscopic nature of ice, which causes a frozen dew droplet to harvest water molecules from a neighboring liquid dew droplet.  This collected water forms as an ice bridge, which grows toward the liquid droplet to eventually touch it and freeze it.  Now this newly frozen droplet grows an ice bridge toward the next liquid droplet, and so on, until the entire population of dew droplets has frozen into frost.

(2) When the nearest liquid droplets are sufficiently far away from the ice, the liquid completely evaporates before the ice bridges can connect.  We call this a "dry zone."  Validated by exhaustive experimentation, we developed a model that predicts when dry zones versus ice bridges occur, and also predicts the size of a dry zone for any weather condition.  

(3) By using either chemical or physical patterns, we templated a series of microscopic ice stripes on chilled surfaces.  This array of fine ice stripes created overlapping dry zones, where no frost was able to form on the intermediate surface areas.  We showed that this concept can be exploited to keep at least 90% of a surface frost-free even under extremely chilled and humid conditions.  This concept was demonstrated for both planar surfaces and cylindrical cables.  

The growth of frost on various infrastructure and vehicles results in billions of dollars in economic losses every year in the US alone.  Therefore, our development of a passive anti-frosting surface technology, where the vast majority of the surface stays dry, has a very broad practical impact.  Especially when considering that active methods of frost removal, such as using antifreeze or salts, are both expensive and environmentally harmful.  The impact of our work was broadly disseminated by media outlets such as Scientific American and R&D Magazine.  We also made a local impact to our community by hosting summer camps for middle school and high school students, where we attached magnifying lenses to their smart phones to show them inter-droplet frost growth at work.  

In terms of intellectual merit, surprisingly, the phenomenon of inter-droplet ice bridging was almost completely ignored in the past despite its primary importance in frost growth.  Our models that capture when frost can, and cannot, grow on a surface are therefore an important breakthrough with regards to having a fundamental understanding of frost and the importance of inter-droplet interactions.    

Last Modified: 12/02/2019
Modified by: Jonathan B Boreyko