Slick new surface wicks oil in one direction

Imagine nozzles that never clog or machinery that cleans and oils itself – these may be around the corner.

A team of engineers in Finland led by Juan Li designed and created a flat surface that moves oil droplets without any energy input. Microscopic raised tracks coated with an oil-repelling polymer forced oil drops in one direction.

The technique, published in Science Advances, could be used in a range of industrial applications – not only to move oils where they’re needed but also to “transport unwanted stray droplets away from critical areas of devices”, says study co-author and Aalto University engineer Robin Ras.

Forcing liquids to run in one direction, seemingly against the laws of physics, is a surprisingly common occurrence in nature. The Texas horned lizard, for instance, uses microchannels in its skin to “wick” condensed fog and dew to its mouth where it slurps it up. Last year, engineers copied those microchannels and designed a water-wicking surface.

Water’s strength lies in its immense surface tension – that is, molecules comprising a water drop’s surface are very strongly attracted to each other. This allows some insects to walk on a puddle’s surface. 

But oil is a different kettle of fish. With low surface tension, its surface molecules allow a drop to spread out rather stay in a cohesive blob. 

In 2000, a Japanese trio reported a surface that could move oil along, but it needed energy in the form of light to keep the drops rolling. So Li and colleagues decided to make a patterned surface that spontaneously, and without power, moved oil along in one direction without it spreading out and leaving a residue.

The best pattern, they found, was a radial one. Tiny “microstripes”, which look a bit like railway tracks, were arranged like spokes on a wheel.

When coated with an oil-repelling polymer, a drop of oil placed anywhere on the pattern zoomed towards the centre, guided by the microstripes. By tweaking the microstripes, they were able to steer the drops inwards, outwards and pinned to one spot. 

There was no visible oily residue on the surface. When examined under a microscope, only small smudges of oil were spotted near the centre of the pattern, but they didn’t affect the material’s oil-transporting abilities.

And after 10 months in storage, the surfaces still worked as well as they did on day one.

Their work, the researchers write, is not limited to oil – forcing ink in inkjet printers in one direction could lead to fewer blockages.