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Materials and Methods

We elected to maintain the same anion and cation in all our experiments, and concentrate on the effects of changes in the other variables. We chose cobalt(II) chloride hexahydrate, CoCl$_2\cdot$6H$_2$O, as the metal salt we would use throughout, and sodium silicate as the reacting solution. Aqueous sodium silicate solutions were prepared from a concentrated solution, 6.25 M with respect to silica, composed of ca. 27% SiO$_2$ and 14% NaOH, by diluting it with bidistilled water. We prepared aliquots of dilutions from 1:1 to 1:100 v/v, giving us concentrations between 6.25 M and 0.0625 M with respect to silica.

For ease of data collection, we wished to have an approximately two-dimensional system. For this reason we performed the experiments in a 1 mm thick Hele-Shaw cell [31,32] of dimensions 1.5 cm $\times$ 2 cm formed by two parallel glass plates sealed at their edges. Changes in solution refractive index observed by a Mach-Zehnder interferometer were used to follow the experiments. Solution refractive index is linked to composition; it varies approximately linearly with the concentration of a dissolved solute. We measured the refractive indices of sodium silicate and cobalt chloride solutions: the refractive index of 1.56 M sodium silicate solution is 1.35, and that of 0.781 M solution is 1.34, while a saturated 4.10 M solution of cobalt chloride has refractive index 1.41, and a 2.05 M solution, 1.37. A concentration gradient in a solution is then visible with the interferometer as a curving or bowing of the interference fringes.

The experimental procedure was to insert a seed crystal into the growth cell, and, after placing the cell oriented vertically in the test arm of the interferometer, to fill the cell with sodium silicate solution using a syringe to begin the experiment (Fig. 2a).

Experiments were performed in a temperature-controlled laboratory maintained at 20$\pm$1$^\circ$C.


next up previous
Next: Mechanism of growth Up: Formation of Chemical Gardens Previous: Introduction
Julyan Cartwright 2002-12-13