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Air bubbles sink and steel spheres rise in an unusual fluid

PORTLAND, OREGON—In a liquid, bubbles are supposed to rise and steel is supposed to sink. But engineers have reported exactly the opposite in a water-based mixture that contains a gelling agent known as carbopol, commonly used as a thickener in cosmetics. These “complex fluids” behave like a solid when at rest but can flow like a liquid when they’re pushed on. (Whipped cream and mashed potatoes are a couple of examples.) The team filled a container with roughly half a liter of the transparent fluid and inserted a steel sphere a few millimeters in diameter. They then banged the container upward on the underside of a table, producing a downward force. For the nearly massless air bubbles, this downward force was stronger than the upward-pointing buoyant force, causing the bubbles to sink, the team reported here at the 69th Annual Meeting of the American Physical Society Division of Fluid Dynamics last week. But the steel sphere, with its significantly larger inertia, tended to keep going in the direction of the initial upward movement and therefore rose (as seen in video). The finding may pave the way for stronger building materials by helping remove large air voids from wet concrete, the authors say.