Researchers in the US have discovered a way of modifying metal surfaces so that they “pump” liquids uphill. The method, which involves exposing the surfaces to pulses of intense laser light, could be exploited in the future for analysing fluids “on-chip” or for biological sensing.

Earlier this week, physicsworld.com reported on research by Chunlei Guo and Anatoliy Vorobyev at the University of Rochester, New York, in which they claimed laser pulses could “blacken” a lightbulb’s tungsten filament and thereby boost its efficiency towards 100 %. Now, Guo and Vorobyev have used a similar technique to affect the surface “wettability” of platinum and gold plates.

“In a gravity-defying way, the treated metal surfaces make liquids sprint vertically uphill at an unprecedented speed of 1 cm/s,” they write.

Pits, globules and grooves

The researchers have used a horizontally-polarized laser, which sends pulses of light 65 femtoseconds (65 x 10-15 s) long at a wavelength of 800 nm onto the metal surfaces. They scanned the laser horizontally and vertically until they had treated a circular area 24 mm in diameter. Images from a scanning electron microscope showed a resultant structure of fine pits and globules superimposed on larger, periodic grooves.

Examining the surfaces with a video camera, Guo and Vorobyev found these grooves could suck up methanol when the surface was horizontal, vertical or inclined at 45°. The researchers believe the phenomenon is due to the so-called “Marangoni effect”, in which fluid flow results from a gradient in surface tension. The distance between the grooves, at just 100 µm, means that molecules in the methanol can find themselves more attracted to the metal than to neighbouring methanol molecules, and therefore tend to creep forwards.

The horizontal orientation produced the fastest speed at 1.6 cm/s, while the vertical orientation produced the slowest speed at 1 cm/s. “To our knowledge these are the highest liquid moving speeds one has observed on a metal surface,” they report in an upcoming issue of Applied Physics Letters.

Could work like a ‘microprocessor’

Applications of the treated metals could include microfluidics wherein fluids could be manipulated on sub-millimetre scales, say researchers. They also highlight a potential medical application because blood could be directed precisely along a defined path to a sensor for disease diagnosis.

“Imagine a huge waterway system shrunk down onto a tiny chip, like the electronic circuit printed on a microprocessor, so we can perform chemical or biological work with a tiny bit of liquid,” said Guo in a press statement.