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Propagating fronts in thin tubes: concentration, electric, and pH effects in a two-dimensional precipitation pulse system.

In this paper, we studied the dynamics of a CaCO3 precipitate deposition pulse in a thin, long tube connecting two reservoir sinks of coprecipitates. The pulse profile, as well as the time t(c) and distance x(c) of the first appearance of precipitate, is studied as a function of the initial concentration of CO(3)(2-) in the right reservoir, [CO(3)(2-)](0), and later as a function of an applied external electric field at different voltages. The time variations of the pulse location and the pH at the center of the tube are determined. The distance from the calcium chloride sink (x) at any fixed time decreases as [CO(3)(2-)](0) increases. The time evolution of the front location exhibits a crossover between an early time regime and a late time regime. The pH-time curve shows a marked resemblance with a sigmoid shape. At any time, the pH consistently increases with [CO(3)(2-)](0). In the presence of a constant electric field applied across the tube (fixed voltage), t(c) decreased with the field strength, whereas x(c) exhibited a correlated increase. Irregularities in the variation of distance with the applied voltage (at a fixed time) were noted. The pH experiences a slight increase with the applied voltage. The pulse width exhibits a nonlinear time dependence, of the form w = a + bt(1/6). The shape of the deposition pulse deviates from a Gaussian distribution. This study is of special interest in the experimental simulation and modeling of precipitate deposition and potential clogging in microcapillary channels.