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In test tubes with a special coating, the more viscous the liquid, the faster it flows



Physicists were surprised to find that in specially coated tubes, the larger the viscous liquid, the faster it flows

Drops of honey on a hydrophobic surface. Credit: Aalto University

It is well known that thick viscous liquids ̵

1; like honey – flow more slowly than liquids with low viscosity, like water. The researchers were surprised to find that this behavior turned upside down when liquids flowed through chemically coated capillaries. In fact, through these specially coated tubes, liquids a thousand times more viscous flow ten times faster.

The speed at which various fluids flow through pipes is important for a wide range of applications, from industrial processes such as refineries to biological systems such as the human heart. Traditionally, if you need to speed up the flow of fluid through a pipe, you increase the pressure on it. However, this technique has its limits; there is only so much pressure you can put on the pipe before you risk breaking it. This is especially true for thin and narrow tubes, such as those used in microfluids to produce drugs and other complex chemicals, so researchers are investigating whether they can increase the rate at which fluids flow through narrow tubes without increasing pressure.

In an article published October 16 in the journal Scientific achievements, researchers have found that by coating the inside of pipes with liquid repellent compounds, they can cause viscous liquids to flow faster than those with low viscosity.

“The superhydrophobic surface consists of tiny bumps that trap air inside the coating, so that a drop of liquid that rests on the surface sits as if on an air cushion,” explains Professor Robin Ras, a research group at the Department of Applied Physics at Aalto University. the industry of extremely water-repellent coatings, including recent publications in Science and Nature.







Quick explanation of the work, including sample data from the article. Credit: Aalto University

The hydrophobic coatings themselves do not accelerate the flow of more viscous liquids. If you put a drop of honey and a drop of water on a surface with a superhydrophobic coating, and then tilt the surface so that gravity makes the drops move, water with low viscosity will flow faster.

But when the droplet is confined to one of the very narrow tubes used in microfluids, things change dramatically. In this system, the superhydrophobic coating on the walls of the tube creates a small air gap between the inner wall of the tube and the outer part of the drop. “We found that when a drop is confined to a sealed superhydrophobic capillary, the air gap around the drop becomes larger for more viscous liquids. This larger air gap allows viscous liquids to move through the tube faster than less viscous ones under gravity,” he says. Dr. Maya Vuchkots, the first author of the article.

The size of the effect is quite significant. Drops of glycerin, a thousand times more viscous than water, flow through the tube more than ten times faster than drops of water. The researchers filmed the drops as they moved along the tube, tracking not only how fast the liquid moved through the tube, but also how the liquid flowed inside the drop. For viscous liquids, the liquid inside the drop hardly moved at all, whereas in the drops with lower viscosity, a rapid stirring motion was detected.

Physicists were surprised to find that in specially coated test tubes, the more viscous the liquid, the faster it flows

A drop of honey in a superhydrophobic covered test tube. Credit: Aalto University

“The most important discovery is that less viscous liquids also managed to penetrate a little into the air cushion surrounding the droplets, making a thinner air gap around them. This means that the air under the low viscosity droplet in the tube could not move. as fast as for a more viscous drop with a thick air gap. With less air to seep through the low-viscosity droplets, they were forced to move through the tube at a lower speed than their more viscous counterparts, ” explains Dr. Matilda Beckholm, one of the project’s researchers.

The team has developed a fluid dynamics model that can predict how droplets will move in test tubes coated with various superhydrophobic coatings. They hope that further work on these systems can have significant applications for microfluids, a variety of chemical engineering techniques used to accurately control liquids in small quantities and to produce complex chemicals such as drugs. Having the ability to predict how coatings can be used to modify fluid flow, coatings can be useful to engineers developing new microfluidic systems.


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More information:
The movement of droplets that improve viscosity in sealed superhydrophobic capillaries Scientific achievements (2020). DOI: 10.1126 / sciadv.aba5197

Provided by Aalto University



Citation: In test tubes with a special coating, the greater the viscous liquid, the faster it flows (2020, October 16). Retrieved October 16, 2020. From https://phys.org/news/2020-10- specially-coated-tubes-viscous- liquid.html

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