Fabrication of centimeter-sized colloidal crystals using pH gradient and gelled crystal template

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Aqueous dispersions of charged colloidal silica particles showed a novel one-directional crystal growth, when pyridine, a weak base, was diffused from one end of the dispersions. The colloidal crystal consisted of pillar-shaped crystal grains whose height and width were in the order of cm and sub-cm, respectively. By numerical analysis of the growth curves based on the reaction-diffusion equation, the growth process was explainable in terms of i)the diffusion of pyridine with neutralization reactions between weakly acidic silica surfaces, ii)charging up of the silica particles and iii)the charge-induced crystallization of the dispersions. The presence of freely diffusing species, which extended the highly charged region forward, was essentially important for the one-directional growth. A similar one-directional crystal growth was observed when NaHCO_3, a salt of a weak acid and strong base, was used, but the growth was hardly observed by using a strong base NaOH.