What buoyancy really is: Archimedes stepping into a crowded bath

Condensed Matter lunchtime seminar

What buoyancy really is: Archimedes stepping into a crowded bath

  • Event time: 1:00pm
  • Event date: 27th June 2013
  • Speaker: Roberto Piazza (Dipartimento CMIC Giulio Natta Politecnico di Milano)
  • Location: CSEC Seminar Room,

Event details

Gravity or ultracentrifuge settling of particles or macromolecules has been the subject of extensive investigation for many decades, and finds important application in geophysics, chemical engineering, biology, and medicine. Usually, the settling process involves several disperse species, either because natural and industrial dispersions display a wide size distribution, or because additives are put in on purpose to allow for density--based fractionation of the suspension, as in Density Gradient Ultracentrifugation (DGU).

Such a macromolecular crowding, however, may have surprising effects on sedimentation, for it strongly affects the buoyancy force felt by a settling particle. In this talk, we show that, as a matter of fact, the standard Archimedes principle is just a limiting law, valid only for mesoscopic particles settling in a molecular fluid, and we provide a fully general expression for the actual buoyancy force providing a microscopic basis to the general thermodynamic analysis of sedimentation in multi-component mixtures. The effective buoyancy also depends on the particle shape, being much more pronounced for thin rods and disks. Our model is successfully tested on simple colloidal mixtures, and used to predict rather unexpected effects, such as denser particles floating atop of a lighter fluid, which we actually observe in targeted experiments. This 'Generalized Archimedes Principle' may provide a tool to devise novel separation methods sensitive to particle size and shape.}, place = {CSEC Seminar Room

This is a weekly series of informal talks given primarily by members of the soft condensed matter and statistical mechanics groups, but is also open to members of other groups and external visitors. The aim of the series is to promote discussion and learning of various topics at a level suitable to the broad background of the group. Everyone is welcome to attend..

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