Influence mechanism of a single microbubble on frost crystal growth characteristics on a sessile frozen water droplet

Frosting is a prevalent phenomenon in nature and industry, but often brings negative effects. As common impurities inside water droplets, air bubbles can change the freezing morphology and affect frost crystal growth. To reveal the underlying mechanism, freezing and frosting experiments of a sessile water droplet containing an air bubble on horizontal and inclined cold plates were carried out. On a horizontal cold plate, the bubble shows a negligible effect on the freezing front migration, with the relative deviation of freezing rates among different positions within 8%. Meanwhile, the freezing tip deflection generally increases with bubble size and offset ratio. On an inclined cold plate, the freezing tip naturally inclines downward under gravity but turns counterclockwise upwards after injecting an air bubble on the upper side. Based on the above freezing characteristics, the frosting behavior is further analyzed. On a horizontal cold plate, as bubble size increases from 0.1 μL to 0.3 μL, the maximum frost crystal heights are 2.037, 2.004, 1.811, 1.813, and 1.801 mm, respectively. The growth rate of the maximum frost crystal decreases monotonically with increasing air bubble size. Besides, the bubble also leads to asymmetric frost growth between the bubble-containing side and the bubble-free side. This study reveals the regulatory mechanism of air bubbles on frosting, providing theoretical guidance for improving anti-frosting/defrosting surfaces and designing ice-suppressing surfaces.