Freezing of a spreading droplet

Abstract

Hypothesis: The wettability of a droplet on pre-cooled surface can be predicted by the Overall Energy Balance (OEB) approach incorporating factors such as inertial, viscous, surface energy, gravitational energy, and heat transfer. For a critical Stefan number, it is also hypothesized that nucleation or recalescence occurs more rapidly, while total droplet freezing is delayed, a behavior attributed to the cessation of three-phase contact line (TPCL) movement.

Experiments: In this study, we have employed a jet-based deposition technique to ensure that the deposited droplet is free from any unwarranted external body forces. The substrates examined include copper, aluminum, brass, and stainless steel selected for their varying thermal conductivities. Substrate temperatures ranged from -20C∘-0C∘. High-speed and infrared thermal cameras were utilized to capture the physical phenomena during the droplet deposition and freezing.

Findings: The theoretical model, developed using the OEB approach, closely matched the experimental observations, validating the hypothesis. The study demonstrated that dimensionless numbers, including Weber, Reynolds, Bond, Stefan, and Peclet numbers, govern droplet spreading on pre-cooled substrates. It was confirmed that supercooling effects are negligible when droplets are deposited using a jet-based technique. The spreading rate of a freezing droplet was found to be proportional to the substrate temperature, with slower rates observed at lower temperatures. Additionally, the total freezing time of the droplet was the highest at -20C∘, despite nucleation occurring fastest at this temperature.

Keywords: Droplet freezing; Droplet spreading; Heat transfer; Overall energy balance.