A model for a spreading and melting droplet on a heated substrate

We develop a model to describe the dynamics of a spreading and melting drop let on a heated substrate. The model, developed in the capillary-dominated limit, is geometrical in nature and couples the contact line, trijunction, and phase-change dynamics. The competition between spreading and melting is characterized by a single parameter K-T that represents the ratio of the c haracteristic contact line velocity to the characteristic melting ( or phas e-change) velocity. A key component of the model is an equation of motion f or the solid. This equation of motion, which accounts for global effects th rough a balance of forces over the entire solid liquid interface, including capillary effects at the trijunction, acts in a natural way as the trijunc tion condition. This is in contrast to models of trijunction dynamics durin g solidi cation, where it is common to specify a trijunction condition base d on local physics alone. The trijunction dynamics, as well as the contact angle, contact line position, and other dynamic quantities for the spreadin g and melting droplet, are predicted by the model and are compared to an is othermally spreading liquid droplet whose dynamics are controlled exclusive ly by the contact line. We nd that in general the differences between the d ynamics of a spreading and melting droplet and that of an isothermally spre ading droplet increase as K-T increases. We observe that the presence of th e solid phase in the spreading and melting configuration tends to inhibit s preading relative to an isothermally spreading droplet of the same initial geometry. Finally, we nd that increasing the effect of spreading promotes m elting.