Monday, April 23, 2012

1204.4548 (Roberto Piazza et al.)

What buoyancy really is. A Generalized Archimedes Principle for
sedimentation and ultracentrifugation
   [PDF]

Roberto Piazza, Stefano Buzzaccaro, Eleonora Secchi, Alberto Parola
Particle settling is a pervasive process in nature, and centrifugation is a much versatile separation technique. Yet, the results of settling and ultracentrifugation experiments often appear to contradict the very law on which they are based: Archimedes Principle - arguably, the oldest Physical Law. The purpose of this paper is delving at the very roots of the concept of buoyancy by means of a combined experimental-theoretical study on sedimentation profiles in colloidal mixtures. Our analysis shows that the standard Archimedes' principle is only a limiting approximation, valid for mesoscopic particles settling in a molecular fluid, and we provide a general expression for the actual buoyancy force. This "Generalized Archimedes Principle" accounts for unexpected effects, such as denser particles floating on top of a lighter fluid, which in fact we observe in our experiments.
View original: http://arxiv.org/abs/1204.4548

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