Viscous droplets glide on air

Experience tells us that high-viscosity liquids flow more slowly than thinner ones. Although extra pressure or a hydrophobic surface can help a viscous fluid flow faster, those interventions work equally well on the fluid’s less-viscous counterparts and preserve their advantage. But new research shows there’s at least one exception to the rule: Maja Vuckovac and coworkers at Aalto University in Espoo, Finland, have uncovered a situation in which viscosity enhances, rather than impedes, fluid flow through a tube.

To induce the counterintuitive result, the researchers squeezed droplets of fluids with different viscosities into glass capillary tubes treated with Hydrobead, a commercially available superhydrophobic coating. Then they oriented the tubes vertically, as shown in the first image, so gravity pulled the droplets down. When both ends of the capillary were open, the less viscous droplets moved faster. But if one or both ends were closed, the trend reversed. For example, despite being 1000 times more viscous, a glycerol droplet slid through the tube 10 times as fast as a water droplet.

Each falling droplet acts like a piston, building up a pressure difference between the air in front of and behind it. Because of the closed capillary end (or ends), air is forced to flow around the droplet. Conveniently, each droplet is already surrounded by a thin air layer, known as a plastron. The thicker the plastron is, the faster air can flow through it—and the speedier the droplet will be.

Vuckovac and coworkers found that viscosity affects the plastron’s average thickness: Droplets of glycerol and other high-viscosity liquids maintain a nearly flat air–liquid interface, as illustrated in the second figure, whereas thinner liquids deform in response to the hydrophobic coating’s micron-scale roughness. Increasing the viscosity from 1 mPa∙s to 1000 mPa∙s thickened the plastron by about a factor of two, from 3.2 ± 1.3 μm to 7.2 ± 1.5 μm. Vuckovac and coworkers expect that the new knob they’ve discovered will be useful for tuning liquid transport in micro- and nanofluidic devices. (M. Vuckovac et al., Sci. Adv. 6, eaba5197, 2020 .)