Dissipation Structure of Electrochemical Hydrodynamic Convection

Colloidal electroconvection in a thin horizontal cell. II. Bulk electroconvection of water during parallel-plate electrolysis

We recently have reported [J. Chem. Phys. 122, 164701 (2005)] a family of electroconvective patterns that arise when charge-stabilized colloidal dispersions are driven by constant (dc) vertical electric fields. Competition between gravity and electrokinetic forces acting on the individual spheres in this system leads to the formation of highly organized convective instabilities involving hundreds of spheres. Here, we report a distinct class of electroconvective patterns that emerge in confined aqueous dispersions at higher biases. These qualitatively resemble the honeycomb and labyrinthine patterns formed during thermally driven Rayleigh-Benard convection, but arise from a distinct mechanism. Unlike the localized colloidal electroconvective patterns observed at lower biases, moreover, these system-spanning patterns form even without dispersed colloidal particles. Rather, they appear to result from an underlying electroconvective instability during electrolysis in the parallel plate geometry. This contrasts with recent theoretical results suggesting that simple electrolytes are linearly stable against electroconvection.

Article

A theoretical treatment of the double-diffusive convection instability is presented, with a focus on the isothermal case of two solutes in a common solvent, initially forming a distinct interface. Linear stability analysis is used to determine the wave number of the marginal mode and symmetry properties of the corresponding fluid flow for realistic values of two control parameters: (i) the ratio of the diffusion coefficient of the solute in the lower solution to that in the upper solution (τd), and (ii) the ratio of the excess densities (relative to the solvent) of the upper to the lower solutions (τp). It is shown that the instability has two regimes: (i) τd-3/2 < τρ < τd-1 in which diffusion precedes convection, and (ii) τd-1 < τρ < 1 in which convection is immediate. The two regimes are separated by a point (τρ = τd-1) through which the wavelength diverges and the fluid velocity undergoes a continuous symmetry change. Above this point, columns of alternating up-and-down flow fade continuously into the solutions above and below the original interface. Below it, the flow within a column undergoes at least one reversal. The horizontal nodal plane associated with such a reversal should be observable.Key words: double-diffusive convection, fluid instability, pattern formation, solutions.

Magnetic effect for electrochemically driven cellular convection

Hydrodynamic instability analogous to Rayleigh-Bénard convection is observed in an electrolytic solution between two parallel copper wire electrodes. The laser interferometric technique can reveal the dissipation structure created by the motion of the fluid, which is controlled electrochemically. It is shown that under the presence of horizontal magnetic field the roll cells move horizontally along the electrodes. The electrochemically driven convection is simply controlled and monitored by setting and measuring the electrochemical parameters and forms many kinds of spatiotemporal patterns, especially under the magnetic field. The phenomenon is modeled by considering a Boussinesq fluid under a concentration gradient. The stability of the resulting equations is studied by linear stability analysis. The time dependent nonlinear system is investigated numerically and the main features of the experimental response are reproduced.