A mechanism for the bubble formation during evaporative crystallization of saline droplets below boiling point

Abstract

This study explores the peculiar phenomenon of bubble formation during evaporative crystallization at temperatures below boiling point, focusing on high latent heat solid-forming salts (Na2CO3, K2CO3, and Na2SO4) compared to lower latent heat solid-forming salts (NaCl, KNO3, and NH4Cl). Droplet evaporative crystallization experiments are conducted at temperatures ranging from 58 °C to 110 °C. The findings consistently show that salts with high latent heat of solid formation exhibit bubble nucleation and growth during crystallization. A plausible mechanism for this bubble formation is proposed, involving the interplay of thermal gradients and crystallization rates. Crystallization releases a significant amount of latent heat, leading to localized temperature increases near the crystal-liquid interface, often exceeding the boiling point. This can trigger bubble formation even if the substrate is below boiling. Energy balance at the interface during crystallization is used to interpret the local temperature rise at the crystal-liquid interface. The study also examines the influence of various parameters (initial concentration, substrate temperature, surface wettability, and different salts) on bubble nucleation and growth, emphasizing the critical role of latent heat release in these processes. This research offers valuable insights into the mechanisms of bubble formation in evaporative crystallization for the first time in the author's best knowledge.