But 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. ‘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. 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,’ says Dr Maja Vuckovac, the first author of the paper.
The size of the effect is quite substantial. Droplets of glycerol a thousand times more viscous than water flow through the tube more than ten times faster than water droplets. The researchers filmed the droplets as they moved through the tube, tracking not only how fast the liquid moved through the tube, but also how the liquid flowed inside the droplet. For viscous liquids, the liquid inside the droplet hardly moved around at all, whereas a fast mixing motion was detected in the lower viscosity droplets.
‘The crucial discovery is that the less-viscous liquids also managed to penetrate a bit into the air cushion surrounding the droplets, rendering a thinner air gap around these. 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. With less air managing to squeeze past the low-viscosity droplets, these were forced to move down the tube with a slower speed than their more viscous counterparts,’ 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 different superhydrophobic coatings. They hope that further work on these systems could have significant applications for microfluidics, a type of chemical engineering technique that is used to precisely control liquids in small quantities and in manufacturing complex chemicals like medicines. By being able to predict how the coatings can be used to modify fluid flow, the coatings may be helpful for engineers developing new microfluidics systems.
The work was carried out using the OtaNano research infrastructure. OtaNano provides state-of-the-art working environment and equipment for nanoscience and -technology, and quantum technologies research in Finland. OtaNano is operated by Aalto University and VTT, and is available for academic and commercial users internationally. To find out more, visit their website.
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Viscosity-Enhanced Droplet Motion in Sealed Superhydrophobic Capillaries
Maja Vuckovac, Matilda Backholm, Jaakko V. I. Timonen, Robin H. A. Ras, Science Advances, DOI: https://dx.doi.org/10.1126/sciadv.aba5197
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