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Issue 27, 2012
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What buoyancy really is. A generalized Archimedes' principle for sedimentation and ultracentrifugation

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Abstract

Particle settling is a pervasive process in nature, and centrifugation is a 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 into 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.

Graphical abstract: What buoyancy really is. A generalized Archimedes' principle for sedimentation and ultracentrifugation

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Article information


Submitted
15 May 2012
Accepted
26 May 2012
First published
14 Jun 2012

Soft Matter, 2012,8, 7112-7115
Article type
Communication

What buoyancy really is. A generalized Archimedes' principle for sedimentation and ultracentrifugation

R. Piazza, S. Buzzaccaro, E. Secchi and A. Parola, Soft Matter, 2012, 8, 7112
DOI: 10.1039/C2SM26120K

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