Measurements of the evolution of porosity in a mushy layer

Abstract

We suggest improvements to an experimental technique designed to make local measurements of liquid volume fraction, or porosity, in a mushy layer that forms during dynamic solidification of a multicomponent melt. In a set of control experiments we measured the electrical resistance of a medium of known porosity in which the solid particles were Fontainebleau sand, and the electrolytes were aqueous solutions of ammonium chloride. The results confirm that an appropriate description can be obtained using Archie's law with a cementation exponent of 1.5. In a second set of experiments we solidified aqueous solutions of ammonium chloride by cooling from below under conductive conditions. The solid phase formed was ice with a plate-like morphology, and we compared values of porosity obtained from the electrical measurements with those deduced from temperature measurements assuming local thermodynamic equilibrium. The results indicate that this progressively solidifying porous medium can also be described by Archie's law, and good agreement between electrical and thermal porosities is obtained for a cementation exponent of 1.72. We interpret this higher value as implying a higher tortuosity for this porous medium when compared with that made of Fontainebleau sand.