News Release 

Surface tension, not gravity, drives viscous bubble collapse

American Association for the Advancement of Science

By demonstrating that surface tension - not gravity - drives the collapse of surface bubbles in viscous liquids, a new study flips the previous understanding of how viscous bubbles pop on its head. When bubbles rise to the surface of a liquid, they form a thin-film dome supported by the trapped gas inside, which eventually ruptures. In low-viscosity liquids, bubbles rupture rapidly - over a period of milliseconds - and the process is largely dominated by surface tension and inertia. Bubbles can also form in viscous liquids, a phenomenon prevalent in both natural and industrial settings. However, unlike in thin liquids, bubbles in viscous liquids bust far more slowly, and collapse in on themselves, resulting in structural instability characterized by radial wrinkles that appear around the bubble periphery. Previous studies have concluded that the weight of the thin liquid film at the top of the bubble is responsible for both the bubble's collapse and the observed surrounding wrinkling instability, suggesting that gravity is the primary driver of viscous bubble rupture. Here, Alexandros Oratis and colleagues flip the expirements and come to different conclusions. In addition to repeating the original experiments that led to the gravity-dominated paradigm, Oratis et al. also examined viscous bubbles positioned both sideways and upside down. They show that gravity plays a negligible role in bubble collapse. Instead, the surface tension and dynamic stress of the liquid forming the bubble are the main factors guiding the behavior of viscous bubbles, and the observed wrinkling instability. A prediction of the model they develop is that bubble wrinkling will not occur for all conditions of bubbles in viscous liquids; the authors further describe conditions where wrinkling would not be expected.

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