Contact Line Ice Nucleation Is the Dominant Freezing Mechanism for Water on Macro- and Microscopic Polypropylene...

Area of Science:

• Materials Science
• Physical Chemistry
• Surface Science

Background:

• Ice nucleation on hydrophobic surfaces is critical for applications like aircraft icing and cryopreservation.
• Predicting freezing on plastics is challenging due to uncertainty in nucleation site (interface vs. contact line).

Purpose of the Study:

• To investigate ice nucleation onset locations on macroscopic and microscopic polypropylene plastics.
• To differentiate between interface and contact line freezing mechanisms.

Main Methods:

• High-speed videography (≥2100 fps) to observe freezing initiation.
• Contact angle measurements during cooling cycles under nitrogen flow.
• Analysis of freezing on polypropylene sheets and fibers with varying droplet sizes (10 μL, 1 μL, 5 nL).

Main Results:

• Contact line nucleation dominated, observed in 90% of cases for 10 μL droplets on flat polypropylene.
• Droplet contact angles decreased with cooling, indicating a pinned contact line.
• Contact line nucleation was 3.5 times more frequent than interface nucleation on polypropylene microfibers.

Conclusions:

• Polypropylene sheets and fibers exhibit a strong preference for ice nucleation at the three-phase contact line.
• Understanding contact line thermodynamics is key to predicting freezing behavior on hydrophobic surfaces.