How different freezing morphologies of impacting droplets form

Area of Science:

• Fluid dynamics
• Materials science
• Phase transitions

Background:

• Droplet freezing is governed by spreading and solidification dynamics.
• Typically, droplet spreading is much faster than solidification, leading to spherical cap shapes.
• Understanding coupled spreading and solidification is crucial for controlling freezing outcomes.

Purpose of the Study:

• To investigate droplet freezing under conditions of enhanced spreading and rapid solidification.
• To explore the impact of droplet impact velocity and substrate temperature on freezing morphologies.
• To elucidate the relationship between hydrodynamics, heat transfer, and ice formation.

Main Methods:

• Experiments involved room-temperature water droplets impacting subcooled superhydrophilic surfaces in a nitrogen-filled chamber.
• High-speed imaging was used to capture both droplet impact dynamics and solidification processes.
• Varying impact velocities and substrate temperatures allowed for systematic study.

Main Results:

• Five distinct freezing morphologies were observed.
• Morphology variation was attributed to the competition between solidification and impact hydrodynamics timescales.
• A phase diagram was developed to map freezing morphologies based on relevant timescales.

Conclusions:

• The study demonstrates control over droplet freezing morphologies by manipulating impact and freezing parameters.
• The findings provide insights into the complex interplay of fluid dynamics and phase change during impact freezing.
• The developed phase diagram serves as a predictive tool for freezing outcomes.