What is special about contact nucleation of ice? The role of a moving or distorted three-phase contact line

Description

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 ~10^10 increase in nucleation rate. Several possible mechanisms are proposed to explain the observations. One plausible explanation among them, decreased pressure due to interface curvature, is explored theoretically and compared with the observational results quasi-quantitatively. Indeed the observed freezing-temperature increase scales with contact line speed in a manner consistent with the pressure hypothesis. Whatever the mechanism, the 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.