As the march of miniaturization continues, there is an ever-increasing demand for devices that can control the small-scale motion of liquids in technologies such as liquid crystal displays, printing and chemical synthesis. Recent innovations in electrofluidic control have achieved fluid flow speeds of several millimetres per second. But a new method developed by Menno Prins and colleagues at Philips Research Eindhoven in the Netherlands exploits the intrinsically small-scale effects of capillarity - ideally suited to miniature devices - to reach flow speeds a hundred times faster (M W J Prins et al 2001 Science 291 277).
When two fluids that don’t mix – or are ‘immiscible’ – are placed in a vessel, an interface develops between them. Prins and co-workers based their device on the behaviour of the interface – or meniscus – between a conducting fluid and an insulating fluid inside a cylindrical channel with a diameter of just 0.35 millimetres. They found that the tension in the meniscus and the tension in the interface between the conducting fluid and the channel wall – which has a water-repellent coating – compete with each other. The tensions can be adjusted electrically relative to one another to modify the behaviour of the fluids.
Electrodes embedded in the walls of the channel create a potential difference between the wall and the conducting fluid. The charges in the channel wall attract the conducting fluid, reducing the tension of the interface between them. The conducting fluid therefore flows along the inner wall of the channel, reaching speeds of several centimetres per second. The channel’s effective diameter falls and this ‘electrocapillary’ effect causes the insulating fluid to flow along the centre of the tube.
The new device is made up of thousands of these tiny cylindrical channels, which can be selectively operated. It is unaffected by gravity and meets many other criteria for a commercially viable device – it is electrically controlled, reversible, quick to respond, and uses little power.
Existing fluid control techniques have limited the miniaturization of inkjet printers. “Electrocapillary technology will allow more nozzles to be integrated into a single printer head, increasing resolution and printing speeds”, Prins told PhysicsWeb. The device can also act as an optical filter because it is easy to selectively fill or empty the channels. “It could switch optical signals in telecommunications systems or spatially filter x-rays for improved image quality and a lower radiation dose in medical x-ray imaging”, says Prins.
