Light pushes liquid uphill

Physicists in China have devised a new way of sending tiny amounts of liquid uphill and around bends. They do so using very narrow, artery-like structures made from a liquid crystal polymer that changes shape when exposed to light. The technology could be used in so-called laboratories-on-a-chip or in tiny mechanical systems activated by light, they say.

The work is the latest development in a field known as microfluidics, which involves studying the behaviour of fluids with volumes as small as a trillionth of a litre that are channelled along tubes a few micrometres in diameter. Labs-on-a-chip integrate many such channels on a single tiny substrate in order to carry out on a far smaller scale – and potentially much more quickly – the kinds of experiments usually performed using test tubes, beakers and other traditional lab equipment. Applications include wide-ranging medical diagnoses and the monitoring of atmospheric trace gases.

Microfluidic operations are often implemented using external transducers – such as pumps, valves or electrodes – that are in direct contact with the fluid. However, according to Damien Baigl of the École Normale Supérieure in Paris, the use of such transducers tends to make devices complex, expensive and fragile, and can also limit potential applications. Light, on the other hand, can impart energy to fluids without direct contact, he points out. It can also be tuned over a wide range of wavelengths and power levels, provides excellent spatial and temporal resolution and is biocompatible.

Capillary forces

Exploiting light’s attractive properties in a practical device is a challenge, however. One option is to use the force of light directly – either through radiation pressure or using what are known as optical tweezers to trap tiny solid particles in a liquid – but this tends to require very powerful laser beams. An alternative approach is to make use of the capillary forces caused by liquids’ adhesion to other molecules. These are commonly seen when a liquid rises up the inside of a narrow tube in opposition to gravity.

This latter approach has been taken by Yanlei Yu of Fudan University in Shanghai, and colleagues. Yu’s team exploits the fact that a droplet of liquid confined in a cone-shaped tube will be pushed towards the narrower end of the tube as a result of the lower pressure there. The trick was to find a substance to make structures that could be changed from cylinders to cones when exposed to light. Specifically, the researchers needed to identify a liquid-crystal structure that was mechanically and also chemically robust when undergoing such transformations.

They succeeded. To do so they took inspiration from natural arteries, vessels that carry blood away from the heart using alternate layers of muscle and elastic material. The team built tubes about 0.5 mm across made up of multiple layers of liquid crystal polymer, and exposed the tubes to blue light from an LED. The light shifted the orientation of the liquid crystal molecules so as to slightly squeeze the tubes – changing their cross section from a circle to an ellipse and thereby increasing their cross-sectional area. By progressively increasing the intensity of the light along the length of the tubes the researchers were able to transform the tubes into cones.

Slug sandwich

By sandwiching a “slug” of one fluid between portions of another inside a tube and illuminating the tube, the researchers found, as expected, that they were able to move the slug in the direction of decreasing light intensity (see video, above). They did so with various combinations of fluids, including silicone oil interspersed with either air or rapeseed oil, as well as with more complex fluids used in biomedical and chemical engineering. They were able to repeat the trick for tubes inclined at up to 17°, for snakelike tubes, helical tubes and also tubes shaped in a Y that allowed two silicone oil slugs to fuse. They could even push slugs around when placing a 1 mm thick piece of lean pork between light source and tube. “These experiments demonstrate that our [tubes] have promise for application in microfluidic systems embedded in biological tissues,” they write.

Miles Padgett of the University of Glasgow, who works on optical tweezers, is also confident that the latest work could lead to important applications. “So often labs-on-a-chip look more like chips in a much larger lab,” he says. “So replacing external pumps with light sources that could themselves be integrated and provide sufficient drive to mix liquids together is a significant step forward.”

Baigl describes the work as “a totally new concept” in light-driven microfluidics, arguing that it “greatly expands the available optical toolbox” in this field. The challenge now, he says, will be to “demonstrate that real-world microfluidic applications can be achieved by this method, using light instead of pumps”.

The research is reported in Nature.

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