Development of a Novel Freezing-Levitation System for Controlled Contact-Free Generation of Supercooled Droplets

Abstract

Supercooled droplets are liquid droplets existing below their equilibrium freezing point without solidification, which occur widely in natural and industrial environments and play critical roles in atmospheric science, cryopreservation, and aviation safety. Supercooled droplets exhibit unique characteristics, such as a rapid freezing process triggered by impingement or other disturbances. To better reveal the dynamic and thermal processes experienced by supercooled droplets, controlled laboratory generation of supercooled droplets is essential for systematic investigations. In this study, we developed a novel freezing-levitation system to produce supercooled droplets without direct contact or support. Both high-speed imaging and infrared thermography were applied to characterize the supercooling process and the subsequent droplet impinging dynamics. Different types of water droplets, i.e., deionized-water and tap-water, were used in the experiments and compared in terms their supercooling and freezing behaviors. The observations revealed that, while a clear recalescence event occurred for the tap water droplet during supercooling, the deionized water droplet remained in liquid state for an extended period. Droplet impact experiments showed that both supercooled and non-supercooled droplets spread similarly during the inertial stage. However, their retraction varied, i.e., the supercooled droplet showed suppressed recoil, while the non-supercooled droplet underwent full capillary retraction before freezing. These findings demonstrate that the developed freezing-levitation system can consistently and precisely produce supercooled droplets that can offer reliable experimental conditions in examining the thermal and hydrodynamic features of supercooled droplet in both static and dynamic environments.