It is common knowledge that thick, viscous liquids – like honey – flow more slowly than low-viscosity liquids like water. The researchers were surprised that this behavior was turned on its head when the fluids flow through chemically coated capillaries. Liquids flow through these specially coated pipes ten times faster than a thousand times more viscous.
In the article published in the journal on October 16 Advances in scienceThe researchers found that by coating the inside of the pipes with liquid-repellent compounds, viscous liquids can flow faster than those with low viscosity.
“A superhydrophobic surface consists of tiny bumps that trap air in the coating, so that a drop of liquid that rests on the surface sits like an air cushion,” explains Professor Robin Ras, whose research team at the Institute for Applied Physics at Aalto University has one Made a number of interesting discoveries in extremely water-repellent coatings, including recent publications in science and nature.
Superhydrophobic coatings themselves do not accelerate the flow of the more viscous liquids. If you put a drop of honey and a drop of water on a superhydrophobic coated surface and then tilt the surface so that the droplets move under gravity, the low viscosity water will flow down faster.
However, when a droplet is confined to one of the very narrow tubes used in microfluidics, things change drastically. In this system, the superhydrophobic coating on the walls of the tube creates a small air gap between the inside wall of the tube and the outside of the droplet. We have found that the air gap around the droplet is larger for more viscous liquids when a droplet is confined to a sealed superhydrophobic capillary. This larger air gap allowed the viscous liquids to move through the tube faster than the less viscous ones when they were flowing due to gravity, ”says Dr. Maja Vuckovac, the first author of the paper.
The size of the effect is quite substantial. Glycerin droplets, which are a thousand times more viscous than water, flow through the pipe more than ten times faster than water droplets. The researchers filmed the droplets as they moved through the tube and tracked not only how fast the liquid moved through the tube, but also how the liquid flowed within the droplet. In the case of viscous liquids, the liquid in the droplet hardly moved, while a rapid mixing movement was observed in the droplets with lower viscosity.
‘The key discovery is that the less viscous liquids were also able to penetrate a little into the air cushion surrounding the droplets, creating a thinner air gap around them. This means that the air under a low viscosity droplet in the pipe cannot move out of the way as quickly as it would with a more viscous droplet with a thicker air gap. Since less air was able to pass the low-viscosity droplets, they had to move more slowly through the tube than their more viscous colleagues, ”explains Dr. Matilda Backholm, one of the researchers on the project.
The team developed a fluid dynamics model that can be used to predict how droplets would move in tubes coated with various superhydrophobic coatings. They hope that further work on these systems could have significant applications for microfluidics, a type of chemical engineering technique that can precisely control liquids in small quantities and manufacture complex chemicals such as drugs. By predicting how the coatings can be used to change fluid flow, the coatings can be useful to engineers developing new microfluidic systems.
Materials provided by Aalto University. Note: The content can be edited by style and length.
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