Solidification characteristics of two-dimensional water droplets

Frost and ice are ubiquitous in nature and industry, and sometimes cause problems. The solidification of water droplets is in the early stage of frosting and icing and has attracted extensive research interest. However, due to physical occlusion, solidification characteristics inside a droplet are always unclear. In this study, Hele‒Shaw cells were used to produce cross-sectional slices of water droplets deposited on hydrophilic and hydrophobic surfaces, referred to as two-dimensional droplets. The solidification characteristics of these droplets were investigated at micrometer spatial and millisecond temporal scales. Results show that the maximum dendrite growth velocity reached 0.45 m/s during the recalescence stage. Using the side‑view freezing front height from a three‑dimensional droplet as a proxy for the true front height introduces errors ranging from ‒35% to +45%. For a ‒30°C substrate, the maximum longitudinal temperature difference within the droplet reached 7.3°C. Additionally, micro-scale trapped air bubbles with equivalent diameters ranging from 18 to 78 µm switch their growth mode from spheroidal to longitudinal approximately 250 ms into the freezing stage, corresponding to about 17% of the total growing time. These findings provide new insight into frosting and icing physics and may inform enhanced defrosting and de‑icing strategies.