Overlimiting Current in Nonuniform Arrays of Microchannels: Recirculating Flow and Anticrystallization.

Overlimiting current (OLC) through electrolytes interfaced with perm-selective membranes has been extensively researched for understanding fundamental nano-electrokinetics and developing efficient engineering applications. This work studies how a network of microchannels in a nonuniform array, which mimics a natural pore configuration, can contribute to OLC. Here, micro/nanofluidic devices are fabricated with arrays of parallel microchannels with nonuniform size distributions, which are faced with a perm-selective membrane. All cases maintain the same surface and bulk conduction to allow probing of the sensitivity only by the nonuniformity. Rigorous experimental and theoretical investigation demonstrates that overlimiting conductance has a maximum value depending on the nonuniformity. Furthermore, in operando visualization reveals that the nonuniform arrays induce flow loops across the microchannel network enhancing advective transport. This recirculating flow eliminates local salt accumulations so that it can effectively suppress undesirable salt crystallization. Therefore, these results can significantly advance not only the fundamental understanding of the driving mechanism of the OLC but also the design rule of electrochemical membrane applications.

Surface Conduction in a Microchannel.

Ionic current through a microchannel has drawn significant attention not only for fundamental electrokinetic research but also for the development of novel micro/nanofluidic applications. Among various ion transport mechanisms, surface conduction, which is a predominant mechanism in micro/nanofluidic devices, has been theoretically characterized based on two-dimensional analysis. However, its infinite axis assumption has become a barrier for direct application in practical micro/nanochannel networks. In this work, we conducted rigorous experiments to include all of the three-dimensional length scales. There, L/ A, the perimeter to area ratio of the microchannel cross-section, came up as a single parameter to quantitatively interpret the surface conductive ion transportation. Overlimiting conductance of microchannel devices increased with larger perimeter, which is equivalent to specific surface area, even with the same cross sectional area. Finally, a micro/nanofluidic diode with a different L/ A value on its forward and reverse channel was demonstrated as a simple application. The analysis presented could provide a practical guideline to design a micro/nanofluidic application.