Evidence shows concentration polarization and its propagation can be key factors determining electroosmotic pump performance

Porous structures with submicron pore diameters and low ionic strength electrolytes yield more efficient electroosmotic (EO) pumps. For these conditions, however, electric double layers may carry a substantial portion of ionic current, creating an imbalance between current carried by anions versus cations. This leads to the formation of net neutral regions of ion depletion and enrichment on opposite sides of the pump. We visualize ionic concentration enrichment and depletion using a custom visualization setup with an embedded porous glass EO pump. Visualizations and conductance measurements indicate that concentration polarization (CP) zones are formed and propagate in EO pump systems with order 100–1000 μM ionic strength. To date, no EO pump model has taken CP into account and yet CP has a significant effect on pumping rate and power consumption. We propose pore-volume-to-surface-area ratio as a new Duhkin number length scale for predicting CP regimes in EO pumps. CP and its propagation can have profound and long-range effects on ionic conductivity and electric fields in EO pumps.