Scientists Make a Sticky Liquid Flow Faster Than Water

The surprise finding was observed when the fluids moved through specially treated tiny tubes.
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Peter Gwynne, Contributor

(Inside Science) -- Intuition and observation agree on what appears to be a solid scientific fact: The more viscous -- that is, sticky -- a liquid is, the more slowly it will flow. Molasses, for example, moves much more sluggishly than water. 

But researchers at Finland’s Aalto University have shown that this behavior reverses in certain conditions. They have found that when tiny tubes called capillaries are specially treated, droplets of viscous liquids can flow inside them much faster than droplets with less viscosity.

The discovery, reported in the journal Science Advances, has potential applications in fields as diverse as chemical engineering and medicine.

The Aalto team, headed by physicist Robin Ras, relied on a property called superhydrophobicity -- literally, “extreme aversion to water.” 

“When a hydrophobic surface like wax or Teflon has roughness, it becomes superhydrophobic,” Ras explained. In a capillary tube about the width of a human hair, such a surface can sustain a thin air layer that shields any liquid in the tube from the solid surface.

“We noticed that for tubes that have a superhydrophobic coating on the inside, the air layer is thicker for viscous liquid droplets than for low-viscous water droplets,” Ras said.

When the ends of the tube are closed and the tube is tilted, each droplet acts as a piston that compresses the air below it and creates low pressure above it, he added. The pressure difference drives the air upward through the gap between the liquid and the tube wall. As the air moves upward, the pressure below is reduced, allowing the droplet to flow down. Since air has virtually no viscosity, it flows more easily through the thick gap of viscous droplets than through the thin one of water droplets.

Videos of droplets traveling through the tubes revealed the extent of the effect. Droplets of glycerol, a thousand times more viscous than water, flowed through the tube 10 times faster than water droplets did.

The researchers have since developed a model to predict how droplets will move in capillaries coated with different superhydrophobic coatings. Further study, they say, could lead to applications in microfluidic processes that control the manufacture of small quantities of chemicals for medical and other uses.

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Peter Gwynne is a freelance writer and editor based in Hyannis, Massachusetts, who covers science, technology and medicine.