Ions drive macroscopic flows in water

Moving miniscule volumes of fluid through narrow channels is a process with many applications, from inkjet printing to modern biological lab techniques. A recent approach for generating and controlling such fluid flows involves injecting ions into a liquid and applying a voltage, coaxing the charges into dragging the fluid along. Many of these approaches require large electric potentials to inject the ions, however, limiting their usefulness for aqueous solutions.

In work reported in The Journal of Chemical Physics, a team of researchers from Osaka University has proposed a new method for generating electrically induced flows that requires much lower voltages than prior methods. By using an exchange membrane to introduce ions into a reservoir of water, researchers were able to generate a flow through a 1-square-millimeter channel using a potential of just 2 volts.

The team measured the movement of microscopic tracer particles, observing their flow through the channel in response to an applied voltage. An electrically neutral solution showed no flow, while a concentrated solution, created by the exchange membrane after 18 hours, showed a flow in the same direction as positively charged ions.

The researchers also derived a detailed theoretical model for the complex coupling between the ion currents and fluid flow, putting the calculations to the test in their device. The authors say that improvements to the exchange membrane could reduce the lead time needed to inject a sufficient number of ions into the water reservoir, opening up potential applications for the transport of drugs and specimens for lab-on-a-chip devices.

Source: “Cation-induced electrohydrodynamic flow in aqueous solutions,” by Kentaro Doi, Fumika Nito, and Satoyuki Kawano, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5006309 .