Nucleation at contact lines where fluid-fluid interfaces meet solid surfaces

Water droplets that only contact solid particles freeze at higher temperatures than those in which the particle is completely immersed. The difference between the two situations is that only when the particle contacts the droplet is there a contact line where the air-water interface meets the surface of the particle. If the particle is immersed in the droplet the contact line disappears. To try to better understand this we study nucleation in a simple model: the three-state Potts model. This model has the three phases that are required to study nucleation at a contact line. Using computer simulation we calculate exactly the nucleation rates at the contact line, and at interfaces and in the bulk. We find that the nucleation rate at the contact line is orders of magnitude higher than it is anywhere else. Classical nucleation theory calculations suggest that this finding should be generic and so may also be true for much more complex systems, such as ice nucleating in a water droplet. Thus we may have found the nucleation mechanism that underlies the experimental observations.