Capillarity Effects In Mushy Zones Under Microgravity Conditions

Abstract

The Isothermal Dendritic Growth Experiment (IDGE) was a component microgravity experiment included as a portion of the United States Microgravity Payload Mission (USMP4) launched by NASA late in 1997. Video data obtained from the IDGE show that PVA dendrites may be melted into detached fragments, or crystallites, without suffering any detectable relative motion with respect to the surrounding quiescent melt phase, despite the mass density difference between crystal and melt. The observed dendritic melting and fragmentation occur by heat conduction devoid of convective effects. Agreement between conduction melting theory and these microgravity experiments was found earlier if melting occurs under shape-preserving conditions, where needle-like ellipsoid-shaped fragments shrink by melting at a constant C/A ratio, where C is the semi-major axis length, and A is the semi-minor axis length. We report new observations of melting kinetics affected by capillarity, where the C/A ratio of individual crystallites does not remain constant. Capillary effects that accompany the melting of slender crystallites lie outside conventional heat conduction theory. The data presented here show that melting kinetics remain dominated by heat conduction from the surrounding melt, which is modified by heat fluxes internal to the crystallites. The capillary-induced heat flux results from steep interfacial curvature gradients on the crystallites just prior to their extinction.