A tangled surface of Teflon-coated silicon nanowires can be used to electronically tune the shape of water droplets, research shows. The trick could lead to sophisticated new chemistry techniques, in which liquid droplets are moved around with great precision.
The novel surface was created by French researchers investigating a technique known as 鈥渆lectrowetting鈥�, where a droplet is placed on a water-repellent surface and its shape is changed by applying a voltage. Using this method with very hydrophobic surfaces 鈥� on which water drops roll like marbles 鈥� could give chemists fine control over chemical reactions.
One promising way to make a 鈥渟uper-hydrophobic鈥� surface requires a nanostructure that props droplets like a human on a bed of nails, combined with a water-repellent coating like Teflon. Until now, however, it has not been possible to manipulate droplets in quite the right way using this set-up 鈥� they can be squashed but will not spring back into shape. Some of the deformed droplet creeps into the gaps between the 鈥渘ails鈥�, or pillars, preventing the whole drop reforming.
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Rabah Boukherroub and colleagues from the Institute of Electronics, Microelectronics and Nanotechnology in Lille, France, took a different approach. Instead of using upright pillars they grew a tangled mass of silicon nanotubes.
鈥淚t鈥檚 like spaghetti,鈥� Boukherroub told New 杏吧原创. 鈥淚t seems the water doesn鈥檛 penetrate that arrangement as much.鈥�
Reversible manipulation
Increasing the voltage across a water droplet caused it to squash down. When the voltage was removed, it sprang back into shape. Although the French team only managed to distort their drops a little, this should be enough to push them around by controlling the voltage between many different surface electrodes.
鈥淲e are working on designing another system we hope will allow greater reversible manipulation,鈥� Boukherroub says. Their idea is to combine the nano-scale structure with one tens or hundreds of times bigger to mimic the lotus leaf which strongly repels water using many tiny bumps (also see Anti-fog glass-coating has clear applications)
鈥淓lectrowetting should make completely electronic control of water possible,鈥� Boukherroub says. 鈥淚t could be better than messy pumps and valves.鈥�
Glen McHale at Nottingham Trent University in the UK is also investigating ways to reversibly distort droplets. 鈥淚t is interesting to see that they have a surface that doesn鈥檛 impale the drop,鈥� he says. McHale and colleagues have already achieved this feat using a different method. They make 鈥渓iquid marbles鈥� by coating droplets with a powder of hydrophobic material.
鈥淭he powder sticks to the air-water interface all over the droplet,鈥� he explains. 鈥淭he result is as if it鈥檚 suspended on tiny stilts all over.鈥� Applying voltage to the 鈥渓iquid marbles鈥� makes them change shape due to electrowetting 鈥� but they readily spring back. Using a line of twenty electrodes the Uk team have managed to push their liquid marbles along.
