Electro-osmotic flow enhancement in carbon nanotube membranes

In this work, experimental evidence of the presence of electro-osmotic flow (EOF) in carbon nanotube membranes with diameters close to or in the region of electrical double layer overlap is presented for two different electrolytes for the first time. No EOF in this region should be present according to the simplified theoretical framework commonly used for EOF in micrometre-sized channels. The simplifying assumptions concern primarily the electrolyte charge density structure, based on the Poisson-Boltzmann (P-B) equation. Here, a numerical analysis of the solutions for the simplified case and for the nonlinear and the linearized P-B equations is compared with experimental data. Results show that the simplified solution produces a significant deviation from experimental data, whereas the linearized solution of the P-B equation can be adopted with little error compared with the full P-B case. This work opens the way to using electro-osmotic pumping in a wide range of applications, from membrane-based ultrafiltration and nanofiltration (as a more efficient alternative to mechanical pumping at the nanoscale) to further miniaturization of lab-on-a-chip devices at the nanoscale for in vivo implantation.