Reversing flow in charged membranes

Image of ions in blue, green and red.

Solvent with water and free-moving ions.

New computations by Maurizio Porfiri, Institute Professor at NYU Tandon, and Alain Boldini, Ph.D. candidate at NYU Tandon, suggest that under certain circumstances osmotic flow can change direction across a charged membrane, leading to an increase in concentration differences. The researchers expect the effect to be observable in microfluidic systems, where fluid volumes are small and highly controllable.

To model such a system, Boldini and Porfiri imagined a porous material with fixed charges. The results showed that if the ions are large enough in size they can inflate the membrane, creating a pressure that causes the solvent to flow in the direction opposite to that observed in osmosis.

The researchers believe that their predicted flow reversal could be demonstrated in microfluidic devices, where the phenomenon could be used to concentrate compounds for nanofabrication and other applications.