“This means that the air beneath a low-viscosity droplet in the tube couldn’t move out of the way as fast as for a more viscous droplet with a thicker air gap,” she says. The crucial discovery, says co-author Matilda Backholm, was that less-viscous liquids also managed to penetrate a bit into the air cushion surrounding the droplets, rendering a thinner air gap around these. “This larger air gap is what allowed for the viscous fluids to move through the tube faster than the less viscous ones when flowing due to gravity.” “What we found was that when a droplet is confined to a sealed superhydrophobic capillary, the air gap around the droplet is larger for more viscous liquids,” says first author Maja Vuckovac.
The superhydrophobic coating creates a small air gap between the inside wall of the tube and the outside of the droplet. However, when a droplet is confined to one of the very narrow tubes used in microfluidics, things change drastically. 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. The coatings themselves don’t speed up the flow of the more viscous liquids, Ras explains. “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. A team at Aalto University decided to experiment with superhydrophobic coatings. You can make liquids flow faster by increasing the pressure, but that has obvious risks, particularly in thin or narrow pipes.
The speed at which fluids flow through pipes is important for a large range of applications, from industrial processes to biological systems. In fact, they report in a paper in the journal Science Advances, glycerol, which is a thousand times more viscous than water, was seen to flow 10 times faster.Īnd that’s more than just a physics fun fact. Physicists in Finland were surprised to find that in narrow tubes coated with compounds that repel liquids, the more viscous that liquid is, the faster it flows. Water flows more quickly than honey – except, perhaps, when it doesn’t.
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