Graeber Gustav, Dolder Valentin, Schutzius Thomas M, Poulikakos Dimos
Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering , ETH Zurich , Sonneggstrasse 3 , CH-8092 Zurich , Switzerland.
ACS Nano. 2018 Nov 27;12(11):11274-11281. doi: 10.1021/acsnano.8b05921. Epub 2018 Oct 24.
Surface icing affects the safety and performance of numerous processes in technology. Previous studies mostly investigated freezing of individual droplets. The interaction among multiple droplets during freezing is investigated less, especially on nanotextured icephobic surfaces, despite its practical importance as water droplets never appear in isolation, but in groups. Here we show that freezing of a supercooled droplet leads to spontaneous self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We put forth the explanation that, as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble, resulting in ice coverage of the nanotextured surface. Although cascade freezing is observed in a low-pressure environment, it introduces an unexpected pathway of freezing propagation that can be crucial for the performance of rationally designed icephobic surfaces.
表面结冰会影响众多技术过程的安全性和性能。以往的研究大多集中在单个液滴的冻结上。尽管多个液滴在冻结过程中的相互作用具有实际重要性，因为水滴从不孤立出现，而是成群出现，但对其的研究较少，尤其是在纳米纹理憎冰表面上。在此，我们表明过冷液滴的冻结会导致自发自热并引发强烈汽化。由此产生的快速传播的蒸汽前沿在到达相邻过冷液滴时会立即导致它们级联冻结。我们提出的解释是，当蒸汽接近寒冷的相邻液滴时，它会导致局部过饱和并形成空气中的微观冰晶，这些冰晶充当冻结成核位点。这种机制的顺序触发和传播导致整个液滴群迅速冻结，从而使纳米纹理表面被冰覆盖。尽管在低压环境中观察到了级联冻结，但它引入了一种意想不到的冻结传播途径，这对于合理设计的憎冰表面的性能可能至关重要。
