Contact Nucleation Caused by Pressure Perturbation?

The reason why contact nucleation (an ice nucleating particle makes contact with a supercooled water drop and initiates freezing) is more efficient than immersion nucleation (same ice nucleating particle is immersed inside a supercooled drop and initiates freezing) has been elusive for over half a century. Our recent lab experiments show that a supercooled water drop in motion can freeze at a higher temperature compared with motionless drop. We observe that agitation of a droplet, either by electrowetting or by mechanical vibration, induces freezing of supercooled water at distorted contact lines. We propose that pressure perturbations due to the distorted contact line might be the cause of ice nucleation at high temperatures. Freezing efficiencies obtained from previous contact-nucleation experiments are used to estimate the effective pressure perturbations responsible for the freezing. We also provide a physically based and experimentally constrained parameterization of contact nucleation, which might be useful for cloud models. Verifying our hypothesis requires directly measuring or simulating the pressure perturbation at small temporal and spatial scales relevant to ice nucleation. But on the way to unveil the mystery, our new results imply that, in addition to temperature and aerosol material properties, pressure perturbations and contact-line geometry may also contribute to ice nucleation.