Bubble formation in freezing droplets

Water droplet icing on solid surfaces has been intensively studied in recent years but still with many mechanisms unrevealed. Here, we report a bubble formation phenomenon that always occurs in freezing droplets, from millimeter-sized sessile droplets to microscale condensed droplets, during icing and condensation frosting. In the second stage of icing (the first stage is nucleation and recalescence), air dissolved in liquid water is separated out in the ice front, forming many isolated bubbles. Coupled with the droplet icing physics, a theoretical model was proposed to predict the bubble formation and elucidate its influencing factors. Due to the bubble formation, the final ice droplet is actually a porous medium, rather than a complete solid ice. These results bring new insight into icing physics, helping modify icing models and improve anti-icing and deicing techniques.