Physics: Turning buoyancy on its head

Tiny boats that float upside down underneath a levitating layer of liquid, displaying unusual buoyancy forces, are reported in Nature this week. This observation challenges our intuitive understanding of liquid–air interfaces and could prompt future investigations of the behaviour of liquid boundaries.

Under the action of gravity, liquids in a container, such as a laboratory flask, will typically fall to the bottom of the vessel. However, under certain circumstances, vibrating the liquid vertically can make it levitate above a less dense layer (such as a cushion of air). Emmanuel Fort and colleagues reveal that this vertical shaking also causes buoyancy to flip at the lower surface of the levitated liquid — as if the gravity there has been inverted. They perform experiments with liquids (silicon oil or glycerol) in containers, shaking the vessels vertically and injecting air into the base of the system until the liquid starts levitating. Objects, such as a model boat, are shown to float upside down on the lower interface of the levitating liquid. The authors explain that the vertical vibrations are causing this apparent anti-gravity effect. Additionally, air bubbles in the lower regions of the liquid are also seen to sink rather than rise, a related anti-gravity effect that has been observed in previous experiments.

The observations made by Fort and colleagues defy Archimedes’ principle, in which an upward buoyant force, equal to the weight of displaced fluid, is exerted on an immersed body. They show that this buoyancy force is mirrored on the lower interface of the levitating liquid layer, and they demonstrate this using boats that float on both the upper and lower layers of the liquid simultaneously. However, this is more than just a nautical curiosity: such phenomena might have practical applications in the transport of gas or materials trapped in fluids, suggest Vladislav Sorokin and Iliya Blekhman in a News & Views commentary