Ice Formed By Contact Freezing: Pressure Matters, Not Just Temperature

Ice nucleation is the crucial step for ice formation in atmospheric clouds, and therefore underlies climatologically-relevant precipitation and radiative properties. Progress has been made in understanding the roles of temperature, supersaturation and material properties, but an explanation for the efficient ice nucleation occurring when a particle contacts a supercooled water drop (contact nucleation) has been elusive for over half a century. We have explored ice nucleation initiated at constant temperature, and observe that mechanical agitation of a droplet induces freezing of supercooled water at distorted contact lines. Results show that symmetric motion of supercooled water on a vertically oscillating substrate does not freeze, no matter how we agitate it. However when the moving contact line is distorted with the help of trace amounts of oil or inhomogeneous pinning on the substrate, freezing can occur at temperatures much higher than in a static droplet, equivalent to ~10¹⁰ increase in nucleation rate. We propose that pressure perturbations due to the distorted contact line might be the cause of ice nucleation at high temperatures. Indeed the observed freezing-temperature increase scales with contact line speed in a manner consistent with the pressure hypothesis. Our experiments demonstrate a strong preference for ice nucleation at three-phase contact lines compared to the two-phase interface, and they also show that movement and distortion of the contact line are necessary contributions to stimulating the nucleation process. These new results imply that in addition to temperature and aerosol material properties, it may be important to know about dynamic properties that influence the water surface.