Scientific Seminars

What buoyancy really is: Updating Archimedes' Principle

Alberto Parola
Uni-Insubria, Dip Sci & Tech.

2012-12-05    14:00    Brera - Cupola Fiore

The fate of an object in a homogeneous fluid is simple, as Archimede noticed back in 200 B.C.: if the object weights less (more) than the fluid it floats (it sinks). Often in nature as well as in industrial settings the suspending fluid is complex, filled of a variety of species that differ in size and densities. Sedimentation phenomena involving nano-particles or macromolecules have been the subject of extensive investigation as they find important application in geophysics, chemical engineering, biology, astrophysics, and medicine. As an example, scientists add heavy salts or colloidal nano-particles to macromolecular suspensions to create a density gradient that separates the different components. In such 'crowded fluids', there are surprising effects on sedimentation: Archimedes' principle - arguably, the oldest law in physics -- is at stake. The key question we address is the following: what is the buoyancy felt by a particle settling in a dispersion of other particles with different size and/or material density? By performing targeted equilibrium measurements on binary particle mixtures, we show that the standard Archimedes' expression is just a limiting approximation, valid only for mesoscopic objects settling in a molecular fluid, and we provide a fully general expression for the actual buoyancy force. This ``Generalized Archimedes Principle'' allows to explain existing puzzling results and to demonstrate the occurrence of unexpected effects, such as denser particles floating on top of a lighter fluid, which we actually observe and quantify.