We have observed a crossover between a spatially confined precipitation regime and periodic pattern formation regime. This unusual behavior was observed when electrolyte solutions of Na 2 HPO 4 and CaCl 2 were allowed to diffuse into an agarose gel from opposite ends. The formation of the confined precipitate occurs when the electrolyte flux J is the same at both sides of the gel. The time of formation and the width of the precipitate are a function of J and both follow the scaling relation v ϳ ͑J͞D͒ 2b with b 0.40 6 0.2 and D the diffusion coefficient. The growth of periodic bands of precipitate was observed when J was different at both gel ends. [S0031-9007(96)01256-2]

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We have used small-angle light-scattering (SALS), microscopy, and measurements to study structural changes produced in unbuffered agarose gels as ions migrate under applied electric fields (3-20 V/cm). Anisotropic, bowtielike, light-scattering patterns were observed, whose development occurred more quickly at higher fields. The horizontal lobes were more pronounced at higher polymer concentration. Analysis of the SALS data with a simple model of scattering from anisotropic rods in an electric field is consistent with anisotropic rodlike domains on the order of 10-15 microm in length, which align perpendicular to the electric field. The anisotropic domains in the gel reach almost the same level of orientation, regardless of the field strength. Microscope imaging revealed anisotropic domains on the same length scale, also aligned perpendicular to the field. Profiles of pH variation across the gel, measured by video photography, indicate that the anisotropic patterns appear when the H+ and OH- ions, migrating in opposite directions, meet. Calculations of pH profiles using a model based on electrodiffusion reproduce several features of measured pH profiles, including the power-law dependence on the electric field of the time at which the oppositely charged fronts meet. Ions migrating from both ends of the gel produce pH changes that are correlated with macroscopic shrinking and orientation of the gel.

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