Several processes occur when parallel plate electrodes are used to apply uniform electric fields to aqueous electrolytes. Below the threshold for electrochemical reactions, dissolved ions redistribute around the electrodes and screen the field in the bulk. At higher biases, water dissociates into hydroxyl and hydronium ions that flow to the oppositely charged electrodes, where they recombine into molecular gases that dissolve into the electrolyte. The anionic and cationic fluxes ordinarily intermingle to maintain local electroneutrality. This not only minimizes the system's electrostatic energy, but also helps to maintain mechanical stability because the counterpropagating fluxes exert no net force on the water.
The mechanical stability of steady hydrolysis can be disrupted by adding macroscopic charged objects such as dispersed colloidal spheres. The spheres do not engage in electrochemical reactions, and their fixed charges favor a departure from local electroneutrality in the surrounding electrolyte (1). Once the ionic fluxes are spatially separated, they entrain counterpropagating flows of water around the spheres (2,1,3). These flows exert hydrodynamic forces on the spheres, and their motions, in turn, redirect the flows (4,2,5,3). We previously reported (6,7) that the interplay of microscopic electrohydrodynamic flows around individual spheres can give rise to large-scale convective instabilities characterized by highly organized flow patterns involving thousands of spheres. These colloidal electroconvective patterns form at biases just above the threshold for steady electrolysis, and their structure depends on the properties and concentration of the colloidal spheres. Consequently, they appear not to reflect underlying electroconvective instabilities in the electrolyte itself (6,7).
The present article describes a distinct category of bulk electroconvective patterns that form at higher biases even without dispersed colloidal spheres. Their appearance demonstrates that bulk electroconvective instabilities can occur in simple electrolytes under steady driving by uniform fields. These observations therefore provide an experimental resolution to a long-standing debate regarding the mechanical stability of simple electrolytes in the parallel-plate geometry (9,10,8).
Qualitatively similar instabilities have been observed during the electrodeposition of copper from aqueous electrolytes in the parallel plate geometry (11). In that case, convection was attributed to density gradients resulting from current-driven concentration gradients in the dissolved copper ions. Electrohydrodynamically driven convection also has been reported during the electrolysis of model non-aqueous electrolytes (12,13) in vertical slit pores. In this case, fluid flow appears to be nucleated by a charge injection mechanism, and does not involve gravity.
Until recently, the onset of electroconvection without gravity in simple electrolytes was believed to resemble the intrinsic convective instability in plasmas (8). Unlike plasma convection, however, the electrohydrodynamic instability in electrolytes is entirely suppressed by homogeneous boundary conditions (9). Bulk gravity-free electroconvection in simple electrolytes therefore should occur only in the presence of inhomogeneous boundary conditions (10), and the resulting patterns should be pinned to the underlying inhomogeneities. This is consistent with observation of bulk electroconvection during the electrodeposition of branched metallic structures from aqueous electrolytes (15,14). We find, by contrast, that electrolysis of pure water in a horizontal planar slit pore yields drifting cellular electroconvection patterns not obviously associated with electrode structures under conditions not conducive to plasma-like convective instabilities (9,8).
Because the high-bias electrohydrodynamic patterns described below emerge from and supplant the low-bias colloidal electroconvective instabilities we have previously described (6,7), we organize our present observations as extensions of that work. The patterns' phenomenology then allows us to distinguish the pattern forming mechanism. Section 2 describes the experimental apparatus, whose ability to create large-scale electroconvective patterns is discussed in Sec. 3. Observations described in this section also demonstrate that the electroconvective patterns are largely insensitive to the dispersed colloids' properties or concentration. Section 4 discusses the patterns' dependence on other experimental conditions such as pH and electrode uniformity. The full set of observations is summarized in Sec. 5 to provide an overview of electroconvective pattern formation in water.