PNAS First Look Blog

Science journalists discuss a selection of new papers from PNAS

Controlling drag with tunable bubble mattresses

Scanning electron microscopy image of a representative microfluidic device, showing two main microchannels for gas (Pg) and liquid (Qw) streams connected by gas-filled side channels.

Scanning electron microscopy image of a representative microfluidic device, showing two main microchannels for gas (Pg) and liquid (Qw) streams connected by gas-filled side channels.


Bubbles can reduce the drag that slows ships and submarines in the water. As such, researchers want to exploit cushions of air to help create faster torpedoes and attack vessels as well as drastically cut the enormous volumes of fuel needed to ship freight around worldwide. Novel devices that emit mattresses of bubbles reported this week in the Proceedings of the National Academy of Sciences now could help researchers find ways to make hulls even more slippery than ever.

Chemical engineer Rob Lammertink at the University of Twente in the Netherlands and his colleagues designed and fabricated microfluidic chips that could influence the flow of fluids much as microelectronic chips steer the flow of electricity. These devices consisted of two parallel microscopic channels, one for liquid, the other for gas, that were connected by side channels in between. By controlling the gas pressure, the researchers could easily and precisely control how much bubbles jutted into the channel filled with liquid. Particles scattered into the liquid could reveal how the liquid was flowing over this cushion of air.

The scientists experimented with how much the bubbles protruded into the liquid — the more the bubbles protruded, the more the walls of the bubbles were angled against the flowing liquid. They found the most slippery protrusion angle was about 10 degrees, corresponding to a 23 percent reduction in drag. These findings roughly matched computer simulations that suggested a 21 percent reduction in drag when the protrusion angle was in the range of -2 to 12 degrees.

“It is the first experimental proof that the geometry of a bubble has a strong influence on the hydrodynamics of the liquid flowing along it,” Lammertink said.

In the future, Lammertink and his colleagues want to see how bubble geometry might influence how liquids absorb gases. Such research can improve transport of gases through membranes into liquids — “think of artificial blood oxygenation, or soft drink carbonation,” Lammertink said.

Categories: Applied Physical Sciences
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