Rising tension

Contrary to classical theoretical predictions, electric fields increase the solid–liquid interfacial tension of ionic solutions moving in zeolite nanochannels

Credit: © 2009 AIP

Electric fields are used to manipulate liquid droplets on hydrophilic surfaces — known as electrowetting — for applications such as fabricating 'lab-on-a-chip' devices and liquid lenses with adjustable focal lengths. However, despite the abundance of information on the physical mechanisms governing the movement of ionic solutions through micrometre channels, the effect of electric fields on ions moving through nanochannels is still unclear.

The major difficulty in analysing ionic transport in nanochannels is the lack of appropriate experimental methods for accurately determining the solid–liquid interfacial tension. Now, Yu Qiao and colleagues¹ from the University of California, San Diego, the University of Texas-Pan American and Columbia University have used the pressure-induced infiltration method to study the effect of electric field on ionic transport through nanopores in zeolite Y samples.

Discs of electrically conducting hydrophobic zeolite were placed in a steel cylinder containing lithium chloride solution, which was forced through the nanochannels in the zeolite sample by a piston. Surprisingly, the solid–liquid interfacial tension increased when an electric field was applied, which is the opposite effect to that predicted by macroscopic theory, and was attributed to charge separation in the nanochannels. The results could have important implications for research in fields such as biomedicine and catalysis.