Figure 13.

Schematic representations of common mechanisms to sustain overlimiting current in a microchannel between a reservoir on the left and a CEM on the right. The volume average conductivity in (a) exhibits classical linear diffusion (continuous line) up to a region of depletion (dashed line), where charge carriers are transported by (b) surface conduction, (c) electroosmosis, or (d) electroconvection (visualized experimentally by Gu et al.⁵⁶⁴). The relative contributions of these effects depend on the width of the channel, such that surface conduction dominates in narrow channels and electroconvection in wide ones. Reproduced with permission from ref (527). Copyright 2011 American Physical Society.