Inside a narrow water-filled channel, the wall naturally carries a small electric charge. That charge attracts ions in the water, creating a thin layer of oppositely charged water close to the wall. When a voltage is applied along the length of the channel, this thin layer starts to move and drags the rest of the water along with it. As a result, the water flows in the direction you choose.
Water flows through microchannels
Rob Lammertink studies how molecules and ions behave near membranes and interfaces. In his lab, he demonstrates electro-osmosis in a microchannel no wider than a human hair. “You place positive and negative electrodes inside the liquid channel and create a large voltage difference,” he explains. “That forces the liquid to flow.”
Water moves by electro-osmosis at 500, 250 and 750 volts
Does electro-osmosis always work?
Electro-osmosis does come with limitations. The first is voltage. For a microchannel just a few centimetres long, Rob already needs around 700 volts to make the water flow at about one millimetre per second. That is roughly three times the mains voltage in a typical home. “If you wanted to do this over distances of metres, you would need tens of thousands of volts,” says Rob. “That simply isn’t practical. You’d be dealing with voltages comparable to high-voltage power lines.”
The second limitation is not about scale, but about contact. Electro-osmosis relies on a direct interaction between electric charge and liquid, which means the electrodes must physically touch the fluid. “You cannot use electromagnetic radiation to create electro-osmosis,” Rob explains. “The electrodes have to be in contact with the liquid.”
Small scale, big impact
Within those limits, electro-osmosis is an extremely useful tool. Lab-on-a-chip systems use it to move tiny amounts of liquid through microscopic channels without any moving parts. This is essential for portable medical diagnostics and on-site chemical analysis. The technology can, for example, be used to analyse DNA and proteins.
Rob's research group at University of Twente investigates related phenomena using microfluidic platforms to understand how electric charge and fluid transport interact in and around membranes. That knowledge can be applied directly to water purification, for example, removing pharmaceutical residues and other micropollutants from water but also to the production of green hydrogen through water electrolysis.
