In specifically coated tubes, the extra viscous a liquid is, the quicker it flows

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In specially coated tubes, the more viscous a liquid is, the faster it flows


Physicists surprised to find that in specially coated tubes, the more viscous a liquid is, the faster it flows
Drops of honey on a hydrophobic floor. Credit score: Aalto College

It is broadly recognized that thick, viscous liquids—like honey—circulation extra slowly than low-viscosity liquids, like water. Researchers have been stunned to search out this habits flipped on its head when the liquids circulation by way of chemically coated capillaries. Actually, by way of these specifically coated tubes, liquids a thousand occasions extra viscous circulation ten occasions quicker.


The velocity at which completely different fluids circulation by way of pipes is essential for a wide variety of purposes: from industrial processes resembling oil refineries to organic programs just like the human coronary heart. Historically, if you’ll want to make a fluid circulation quicker by way of a pipe, you improve the strain on it. This method, nevertheless, has its limits; there’s solely a lot strain you may put right into a pipe earlier than you run the chance of bursting it. That is very true for skinny and slim pipes, like those utilized in microfluidics for producing medication and different advanced chemical substances, so researchers are investigating whether or not they can improve the velocity at which liquids circulation by way of slim tubes with out having to extend the strain.

Within the paper revealed on 16 October within the journal Science Advances, researchers discovered that by coating the within of the pipes with compounds that repel liquids, they may make viscous liquids circulation quicker than these with low viscosity.

“A superhydrophobic floor consists of tiny bumps that traps air inside the coating, so {that a} liquid droplet that rests on the floor sits as if on a cushion of air,” explains Professor Robin Ras, whose analysis workforce at Aalto College’s Division of Utilized Physics has made a spread of attention-grabbing discoveries within the space of extraordinarily water repellent coatings, together with latest papers in Science and Nature.

A fast explainer of the work, together with pattern information from the paper Credit score: Aalto College

Superhydrophobic coatings themselves do not velocity up the circulation of the extra viscous liquids. Should you place a drop of honey and a drop of water on a superhydrophobic coated floor after which tilt the floor so gravity makes the droplets transfer, the low-viscosity water will circulation down quicker.

However when a droplet is confined to one of many very slim tubes utilized in microfluidics, issues change drastically. On this system, the superhydrophobic coating on the partitions of the tube creates a small air hole between the within wall of the tube and the surface of the droplet. “What we discovered was that when a droplet is confined to a sealed superhydrophobic capillary, the air hole across the droplet is bigger for extra viscous liquids. This bigger air hole is what allowed for the viscous fluids to maneuver by way of the tube quicker than the much less viscous ones when flowing attributable to gravity,” says Dr. Maja Vuckovac, the primary creator of the paper.

The dimensions of the impact is sort of substantial. Droplets of glycerol, a thousand occasions extra viscous than water, circulation by way of the tube greater than ten occasions quicker than water droplets. The researchers filmed the droplets as they moved by way of the tube, monitoring not solely how briskly the liquid moved by way of the tube, but in addition how the liquid flowed contained in the droplet. For viscous liquids, the liquid contained in the droplet hardly moved round in any respect, whereas a quick mixing movement was detected within the decrease viscosity droplets.

Physicists surprised to find that in specially coated tubes, the more viscous a liquid is, the faster it flows
A droplet of honey in a superhydrophobic coated tube. Credit score: Aalto College

“The essential discovery is that the less-viscous liquids additionally managed to penetrate a bit into the air cushion surrounding the droplets, rendering a thinner air hole round these. Which means the air beneath a low-viscosity droplet within the tube could not transfer out of the best way as quick as for a extra viscous droplet with a thicker air hole. With much less air managing to squeeze previous the low-viscosity droplets, these have been compelled to maneuver down the tube with a slower velocity than their extra viscous counterparts,” explains Dr. Matilda Backholm, one of many researchers on the venture.

The workforce developed a fluid dynamics mannequin that can be utilized to foretell how droplets would transfer in tubes coated with completely different superhydrophobic coatings. They hope that additional work on these programs might have vital purposes for microfluidics, a kind of chemical engineering approach that’s used to exactly management liquids in small portions and in manufacturing advanced chemical substances like medicines. By with the ability to predict how the coatings can be utilized to switch fluid circulation, the coatings could also be useful for engineers growing new microfluidics programs.


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Extra data:
Viscosity-enhanced droplet movement in sealed superhydrophobic capillaries. Science Advances (2020). DOI: 10.1126/sciadv.aba5197

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Aalto College

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In specifically coated tubes, the extra viscous a liquid is, the quicker it flows (2020, October 16)
retrieved 18 October 2020
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