A comprehensive investigation of electroconvection in canonical electrochemical environments [electronic resource]
Keywords: Mechanical Engineering.

Electrochemical interfaces, such as the interfaces of liquid electrolytes with ion-selective membranes and electrodes, play key roles in many important technologies including electrodialysis for water purification, lab-on-a-chip systems and electrodeposition. The coupling of ion transport, electrostatics, and fluid flow at these interfaces generates a rich variety of phenomena some of which, like electric double layers and electroosmosis, have been studied for more than a century. However, only recently was it found that, beyond a critical applied voltage, a hydrodynamic instability occurs at these interfaces. In aqueous systems at room temperature, voltages of approximately one volt and larger generate chaotic fluid motion with many of the properties of classical turbulence, but at near-zero Reynolds number. The resulting phenomena exhibit structures with scales ranging from tens of nanometers to sub-millimeter. This fluid flow, known as electroconvection, enhances mixing thereby providing transport well-beyond what would exist in otherwise diffusion-limited processes. Interactions of electroconvective flows with the electrochemical environment govern system-level behavior. This dissertation explores three model problems which provide insight into fundamental interactions between electroconvection and mean shear, surface patterning, and nominally non-selective surfaces. A novel numerical method, based on physical asymptotic insights, for the conservative transport of charged species was developed to enable these investigations. This method was implemented in a new parallel direct numerical simulation code for solution of the Poisson--Nernst--Planck and Navier--Stokes equations. The first model problem explored consists of an electrolyte between an ion-selective membrane and reservoir subject to applied shear at voltages within the chaotic regime. We find that shear reduces mean current density by ~10% in the regimes investigated by decorrelating concentration and velocity fluctuations. In addition, we quantify the effect of shear on various statistical quantities including mean profiles, fluctuating profiles, ensemble-averaged transport terms, and spectra. The second model problem explores opportunities for enhancing overall transport by controlling electroconvection via patterning of electrochemical interfaces. We find that electroosmotic flows generated by the patterns interact with electroconvection caused by the instability to increase overall transport by up to ~80%. We also analyze homogeneous membranes finding a novel regime of electroconvection where coexistence of multiple convective states can lead to a multivalued current. The third model problem is an ideally-polarizable cylinder in an infinite electrolyte subject to an applied electric field, the canonical model problem for induced-charge electroosmosis. Induced-charge electroosmosis flows had previously been assumed to be stable. However, we find that they are subject to the same instability leading to electroconvection. This result opens possibilities for design of microdevices utilizing chaos-enhanced mixing.