Researchers at MIT and Ben-Gurion University in Israel have discovered a way to exploit the mild electric charge that covers most surfaces to remove ions from fluids. The discovery, detailed in a theoretical paper published in September 2011 in the journal Physical Review Letters, points to an alternative way to purify water.
The method, on which experimental work is underway, involves inserting fluids into microchannels, which are small-diameter tubes about the width of a strand of hair. Normally when an electric current crosses fluid in a tube, the transfer of ions through the fluid follows a single mechanism, called diffusion. The team of researchers, who include Vicki Dydek, a graduate student in the Department of Chemical Engineering at MIT, has discovered a secondary mechanism for ions to move.
Because electrons can’t travel through liquid, when an electric current is hooked up to a container of liquid, a chemical reaction converts the electrons to ions, which can conduct a current because they carry positive and negative charges. Scientific convention states that if the charges in an ionic system were averaged out, the positive and negative charges would even out to neutral. But this accounting overlooks the surface charge of the container that holds the fluid.
The team showed that while this surface charge is usually miniscule compared with the charge of the liquid, which may be more than a thousand times greater, when the volume of the liquid is shrunk to the size of a microchannel, the two charges can be comparable. This enables the ions to conduct along the surface of the charged channel, moving in some places faster than in the bulk of the channel. “It’s like an HOV lane,” Dydek said.
This isn’t the first time scientists have suggested that surface charge can play a significant role in ion transfer. But this paper is the first to show how this surface charge can enable the current flowing through a liquid to surpass its normal limit. Perhaps more importantly, the paper also demonstrates that this process creates ion-depleted regions in the fluid. These depleted regions could potentially be exploited to produce de-ionized water for drinking, or to remove heavy metals from water at polluted sites.
Dydek, who will begin work as a post-doctoral researcher at Harvard Medical School starting this February, plans to continue her research in microfluidics, this time turning her attention to modeling blood flow. “I really feel that math is universal,” she said. “I honestly think that in our time, our generation, the frontier of knowledge is the human body… Mathematical modeling offers us a window into what’s happening, on a very small scale, without running a lot of expensive experiments.”