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Honey and other highly viscous liquids flow faster than water in specially coated capillaries.The surprising finding was made by Maja Vuckovac and colleagues at Aalto University in Finland, who also showed that this counterintuitive effect stems from the suppression of internal flow within more viscous droplets.Their results directly contradict current theoretical models of how liquids flow in superhydrophobic capillaries.
The field of microfluidics involves controlling the flow of liquids through tightly confined regions of capillaries—usually for the manufacture of devices for medical applications.Low viscosity fluids are best for microfluidics because they flow quickly and effortlessly.More viscous fluids can be used by driving them at higher pressures, but this increases mechanical stress in the delicate capillary structures — which can lead to failure.
Alternatively, the flow can be accelerated using a superhydrophobic coating that contains micro- and nanostructures that trap air cushions.These cushions significantly reduce the contact area between the liquid and the surface, which in turn reduces friction – increasing flow by 65%.However, according to current theory, these flow rates continue to decrease with increasing viscosity.
Vuckovac’s team tested this theory by looking at droplets of varying viscosities as gravity pulled them from vertical capillaries with superhydrophobic inner coatings.As they travel at constant speed, the droplets compress the air below them, creating a pressure gradient comparable to that in the piston.
While droplets showed the expected inverse relationship between viscosity and flow rate in open tubes, when one or both ends were sealed, the rules were completely reversed.The effect was most pronounced with glycerol droplets—even though 3 orders of magnitude more viscous than water, it flowed more than 10 times faster than water.
To uncover the physics behind this effect, Vuckovac’s team introduced tracer particles into the droplets.The motion of the particles over time revealed a fast internal flow within the less viscous droplet.These flows cause the fluid to penetrate into the micro- and nano-scale structures in the coating.This reduces the thickness of the air cushion, preventing the pressurized air beneath the droplet from squeezing through to balance the pressure gradient.In contrast, glycerin has almost no perceptible internal flow, inhibiting its penetration into the coating.This results in a thicker air cushion, making it easier for the air beneath the drop to move to one side.
Using their observations, the team developed an updated hydrodynamic model that better predicts how droplets move through capillaries with different superhydrophobic coatings.With further work, their findings could lead to new ways to create microfluidic devices capable of handling complex chemicals and drugs.
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