Thin tubes coated with superhydrophobic materials reverse the normal rules of flowing liquids, making the slow ones fast and the fast ones slow, scientists have discovered. The finding could prove useful in helping run certain tests more rapidly.

Thick syrupy materials such as honey are called viscous, a word whose dictionary definition is literally “having or characterized by a high resistance to flow.” That might need an adjustment after scientists at Aalto University, Finland, discovered circumstances where such liquids flow faster than non-viscous ones like water.

Hydrophobic surfaces repel water and other polar liquids. “A superhydrophobic surface consists of tiny bumps that trap air within the coating, so that a liquid droplet that rests on the surface sits as if on a cushion of air,” said Professor Robin Ras in a statement. 

On a tilted superhydrophobic surface, water will slide off easily since there is minimal friction to resist gravity’s pull. Viscous liquids will do the same but move more slowly in keeping with their usual behavior.

That was assumed to be the end of the matter until Ras and colleagues coated the insides of thin capillary tubes with a superhydrophobic material. To their astonishment, the viscous liquids actually moved faster, at least when the ends of the tubes were closed. A second material produced the same result.

“When a droplet is confined to a sealed superhydrophobic capillary, the air gap around the droplet is larger for more viscous liquids,” Dr Maja Vuckovac, first author of the paper in Science Advances, said. Less viscous liquids narrow the space between themselves and the surrounding walls.

In a sealed tube, a falling droplet compresses the air below until its pressure resists the fall. Viscous droplets, however, have so much air around them they can act as a pump, their downward pressure causing the gas below to pass through the surrounding gap to the space above the droplet.

Within this distinct environment, the consequences are dramatic. Glycerol is a thousand times more viscous than water, normally moving at a relative snail’s pace, yet it flows 10 times faster in Ras and Vuckovac’s tubes. 

By adding fluorescent tracer particles to their liquids and illuminating them with UV light, the authors were able to study the droplets’ interior movements. They found low-viscosity droplets rotate internally, making up for their slow progression by spinning in place. Meanwhile, stickier drops barely spin as they fall.

The findings don’t matter for the bulk movement of liquids like oil through pipelines, at least initially. However, diagnostic tests that rely on transporting tiny amounts of liquids such as blood are common and becoming ever more important.

Traditionally, the response to liquids moving too slowly has been to apply more pressure, but this risks damaging the equipment above a certain level. Superhydrophobic coatings on tubes could be the answer, at least for very small quantities, but this work shows the effectiveness of this will depend on the liquid.  

Maybe the times really are a-changin'.