Defining entropy classically: Lessons from colloidal experiments

Condensed Matter lunchtime seminar

Defining entropy classically: Lessons from colloidal experiments

  • Event time: 1:00pm
  • Event date: 15th June 2015
  • Speaker: Mike Cates (Formerly School of Physics & Astronomy, University of Edinburgh)
  • Location: Room 2511,

Event details

I will discuss some paradoxes that arise when trying to understand entropy from a purely classical viewpoint, as is appropriate when dealing with colloidal particles in suspension. One concerns the fact that colloids are not indistinguishable; however, because we choose not to distinguish them when making measurements, they behave as though they were. Another is that the relative abundance of colloidal clusters of different shapes looks like it should depend on their moments of inertia. This seems extremely strange in an overdamped system and would lead to some peculiar "isotope effects" in mixtures of colloids of identical chemistry but different masses (such as core-shell particles with different cores). Fortunately, all isotope effects vanish when calculations are done correctly. A more general conclusion is that to explain the behaviour of colloids, it is not merely useful, but obligatory, to adopt an informatic rather than dynamical definition of entropy. This introduces an element of subjectivity which, though unsurprising to followers of Gibbs, can still traumatize some people, such as a referee who wrote: "Do the authors try to convince readers that the amount of heat somebody measures in experiment may depend on somebody's subjective feeling?". So long as the word "feeling" is replaced by "information", the answer is yes: the amount of heat flowing depends on what else you do and do not choose to measure in an experiment.

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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