J. Fluid Mech. (2001), vol. 432, pp. 201–217. Printed in the United Kingdom c  2001 Cambridge University Press 201 Diffusion-controlled solidification of a ternary melt from a cooled boundary By ANNELI AITTA, HERBERT E. HUPPERT AND M. GRAE WORSTER Institute of Theoretical Geophysics, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 9EW, UK (Received 6 April 2000 and in revised form 25 September 2000) We present details of an experimental study of crystallization adjacent to a cooled boundary from an aqueous solution of potassium nitrate and sodium nitrate. This transparent system is typical of many ternary melts that do not form solid solutions, including examples in igneous petrology and metallurgy. We have measured the rates of advance of the front of crystallization and the eutectic front, behind which the system is completely solid. From careful measurements of the concentration and temperature fields, we have been able to infer the location of an internal phase boundary: the cotectic front separating a region in which only one component of the ternary system forms crystals from a region in which two components form crystals. Our experiments were conducted under conditions in which fluid flow is minimal, so that rates of crystallization are determined principally by the diffusive transport of heat. We have confirmed that the thicknesses of the various regions all grow in proportion to the square root of time, as is expected of diffusion-limited growth, and have determined the constants of proportionality for a range of different initial concentrations and boundary temperatures. We have found evidence to suggest that there may be a significant nucleation delay in the secondary and tertiary crystallization. Our measurements of concentration provide much more information about the ternary phase diagram than has hitherto been available. 1. Introduction The formation of solids by the cooling of liquid melts is a fundamental process in a wide range of industrial and natural settings. Although there are some examples of the solidification of (almost) pure liquids, such as the making of ice cubes in a freezer, most situations involve the solidification of liquids made up of many constituents. Examples include the fabrication of modern super alloys and the solidification of molten lava. Many fundamental aspects of solidification of multi-component melts can be determined from studies of the solidification of binary, or two-component, melts. For example, they explain how a component of the melt can be preferentially incorporated into the solid phase, causing the melt to become enriched in its other components. Such enrichment results in local constitutional supercooling (Tiller et al. 1953); the melt adjacent to the front of solidification finds itself at a temperature below its equilibrium freezing temperature, owing to the variation of freezing temperature with the composition of the melt. This can result in an instability of the phase boundary (Mullins & Sekerka 1964) and the formation of a mushy layer in which the solid