Capillary Mediated Melting of Ellipsoidal Needle Crystals

Abstract

Measurements of video data on melting dendritic crystal fragments in reduced gravity show that a fragment’s ellipsoidal axial ratio, C/A, rises initially until it melts down to a pole-to-pole length of C ≈ 5 mm. At that point we observe a sudden fall in the C/A ratio with time, as the polar regions melt toward each other more rapidly than C/A times the melting speed, dA/dt, of the equatorial region. This accelerated melting allows the C/A ratio to fall from values around 10–20 (needle-like) towards values approaching unity (spheres) just before total extinction occurs. Analytical and numerical modeling will be presented that suggest that the cause of these sudden changes in kinetics and morphology during melting at small length scales is due to a crystallite’s extreme shape anisotropy. Shape anisotropy leads to steep gradients in the mean curvature of the solid-melt interface near the ellipsoid’s poles. These curvature gradients act through the Gibbs-Thomson effect to induce unusual thermo-capillary heat fluxes within the crystallite that account for the observed enhanced polar melting rates. Numerical evaluation of the thermo-capillary heat fluxes shows that they increase rapidly with the C/A ratio, and with decreasing length scale, as melting progresses toward total extinction.