Water usually flows faster than honey. But when liquids are put in narrow tubes coated with liquid-repelling compounds, liquids with higher viscosity, flow faster.
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And that’s more than just a physics fun fact. The speed at which fluids flow through pipes is important for a large range of applications, from industrial processes to biological systems.
Scientists were surprised to find this out when they experimented with superhydrophobic coatings. Their findings are reported in the journal Science Advances.
“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,” says research leader Robin Ras.
The coatings themselves don’t speed up the flow of the more viscous liquids, Ras explains. If you place a drop of honey and a drop of water on a coated surface then tilt it to let gravity do its work, the low-viscosity water will flow more quickly.
However, when a droplet is confined to one of the very narrow tubes used in microfluidics, things change drastically. The superhydrophobic coating creates a small air gap between the inside wall of the tube and the outside of the droplet.
More about this over at Cosmos Magazine.
(Image Credit: Aalto University)