Sometimes the thicker the liquid, the faster it flows.

"We found that the more viscous a droplet is when confined in a sealed superhydrophobic capillary, the larger the air gap around the droplet. The larger air gap makes the natural flow rate of the viscous fluid faster - by gravity." Dr. Maia Wolkowiak, lead author of the paper, said.

The effect is quite impressive. Glycerin is 1,000 times more viscous than water, and flows 10 times faster than water.

"The key finding is that the lower viscosity fluid permeates the surrounding air cushion, thereby thinning the air gap. This meant that the air beneath the low-viscosity droplets in the tube could not support the faster flow." Dr. Matilda Backholm, one of the researchers on the project, explains.

The team has developed a fluid dynamics model that can be used to predict how droplets will move in tubes with different superhydrophobic coatings. They hope that these studies will have important applications in microfluidics.

Microfluidics is a chemical engineering technique that can be used to precisely control small amounts of liquid and used to make complex chemicals, such as pharmaceuticals.