Sommaire:

Particle settling is a pervasive process in nature, and centrifugation of insoluble solids is a much versatile industrial separation technique. Sedimentation studies also originated powerful analytical methods such as ultracentrifugation, widely used in the separation of cellular organelles, carbon nanotubes, or graphene. 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 my talk is delving at the very roots of the concept of buoyancy by means of a combined experimental and 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. These findings should be highly relevant for the interpretation of many experiments in geophysics, chemical engineering, biology, and medicine, and may provide a tool to devise new ultracentrifuge separation methods sensitive to particle size.

Pour plus d'informations, merci de contacter Stocco A.