INFLUENCE OF MIXING IN LIQUID ON UNIDIRECTIONAL SOLIDIFICATION RATE IN TRANSPARENT ORGANIC ALLOY

In the transparent organic crystal known as succinonitrile-acetone bin ary alloy, transient behavior of unidirectional solidification is dire ctly observed within the range where the planar interface is stable, b y means of a microscopic interferometer. Interface morphology and soli dification rates are obtained by bright-field observations. Interferen ce fringes are used to determine the gradient of the temperature and o f the solute concentration in the liquid ahead of the solid-liquid int erface. Although the solidification direction is taken such that the t hermal convection is suppressed, experimental data on solidification r ates and concentration gradients agree well with numerical values base d not on the diffusion-controlled model, but on the boundary layer mod el which assumes fluid mixing beyond the boundary layer. One of the re asons why fluid mixing occurs is thought to be the thermosolutal conve ction induced by the concentration gradient built up ahead of the soli d-liquid interface. Thickness of the boundary layer estimated from exp erimental data of solidification rates agrees quantitatively with thos e obtained from interference fringes and from tracer analysis.