The Weird Physics of Levitating Liquids and Upside-Down Buoyancy
c0lo writes:
Nature video for the doi:10.1038/s41586-020-2643-8.
Vibration overcomes gravity on a levitating fluid
Counter-intuitive phenomena that arise in fluids under the action of vibration have attracted considerable research interest since the 1950s. For example, in a vibrating volume of fluid, gas bubbles can sink and heavy particles can rise. Moreover, a layer of fluid can be levitated above a layer of air by shaking the system vertically at a relatively high frequency (of the order of 100 hertz or more). Writing in Nature, Apffel et al. report another remarkable phenomenon associated with a vibrating, levitated layer of fluid: objects can float upside down on the lower interface of the fluid, as if gravity were inverted (Fig. 1). These phenomena have strong potential for practical use, for example in systems that involve gas bubbles suspended in fluids (such as bubble column reactors used for gas-liquid reactions), and for the segregation and transport of material inclusions in fluids (as used in mineral processing and waste-water treatment).
The extraordinary behaviours of vibrating fluids are just a small fraction of the surprising phenomena that arise as a result of high-frequency vibrations more generally. Probably the most well-documented examples are the Stephenson-Kapitza pendulum, in which a rigid pendulum balances upside down from a vibrating point of suspension, and the Chelomei pendulum, in which a washer that can slide along a rod seems to 'float' when the rod is vibrated vertically.
Kapitza pendulum - the downwards hanging equilibrium position becomes unstable
Chelomei's pendulum explained - a rod with a sliding disc, the ensemble being vibrated vertically.
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