Bubbles burst the same way in any orientation

Thin sheets wrinkle when squeezed because they need less energy to buckle than to compress. That simple idea explains a range of systems, from the formation of fingerprints to the shape of Earth’s crust where two tectonic plates meet. Viscous liquids, such as spray paint and magma, buckle too. For example, when a rupture releases the gas from a bubble on a liquid’s surface, the unsupported hemispherical thin film forms radial wrinkles around the bubble’s edge.

Conventional wisdom is that the collapse and wrinkling result from gravitational forces. But Boston University’s James Bird and his colleagues have now shown that upside-down bubbles have the same popping behavior as right-side-up and sideways ones. Their observation demonstrates that gravity doesn’t drive bubbles’ collapse.

When the researchers punctured 1-cm-radius silicone oil bubbles, the capillary and gravitational forces were unbalanced. But if gravity were the dominant force, an upside-down bubble (see the video) wouldn’t retract upward.

Bird and his colleagues estimated that the capillary force is 80 times as large as the gravitational force. They also investigated the collapse speed for different viscosities of silicone oil and bubble thicknesses. The more viscous the oil was, the slower the bubble shrank. Bubble collapse, they concluded, is a balance between capillary and viscous forces.

Bubbles’ wrinkling behavior, shown in the top image, complicated the group’s calculations; surface tension smooths out wrinkles but also seems to initiate them. In models in which gravity and viscosity are responsible for bubble collapse, the number of wrinkles depends on the size of the hole. But when the researchers introduced a sealed valve to a bubble, releasing pressure was all it took to introduce wrinkles, no hole needed. The wrinkles result from a hoop compression that overcomes the smoothing influence of surface tension and buckles the film. The compression scales with the thickness of the film, which is thicker toward the base of a bubble, so wrinkles appear only away from the center.

The number of wrinkles hangs on a balance of inertia, bending, and compression. Bird and his group found that silicone oil and blown-glass bubbles that were 0.8–2 cm in radius formed anywhere from 8 to 96 wrinkles. They predicted the number of wrinkles from a force balance equation analogous to the one that describes wrinkles on elastic disks. That correspondence suggests a future extension to viscoelastic films, such as the fluid that lines the respiratory tract. When we breathe and cough, thin bubble films in the fluid break up and produce potentially pathogen-bearing aerosols. (A. T. Oratis et al., Science 369, 685, 2020 .)