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Issue 33, 2015
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Celebrating Soft Matter's 10th anniversary: Testing the foundations of classical entropy: colloid experiments

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Abstract

Defining the entropy of classical particles raises a number of paradoxes and ambiguities, some of which have been known for over a century. Several, such as Gibbs' paradox, involve the fact that classical particles are distinguishable, and in textbooks these are often ‘resolved’ by appeal to the quantum-mechanical indistinguishability of atoms or molecules of the same type. However, questions then remain of how to correctly define the entropy of large poly-atomic particles such as colloids in suspension, of which no two are exactly alike. By performing experiments on such colloids, one can establish that certain definitions of the classical entropy fit the data, while others in the literature do not. Specifically, the experimental facts point firmly to an ‘informatic’ interpretation that dates back to Gibbs: entropy is determined by the number of microstates that we as observers choose to treat as equivalent when we identify a macrostate. This approach, unlike some others, can account for the existence of colloidal crystals, and for the observed abundances of colloidal clusters of different shapes. We also address some lesser-known paradoxes whereby the physics of colloidal assemblies, which ought to be purely classical, seems to involve quantum mechanics directly. The experimental symptoms of such involvement are predicted to be ‘isotope effects’ in which colloids with different inertial masses, but otherwise identical sizes and properties, show different aggregation statistics. These paradoxes are caused by focussing one's attention on some classical degrees while neglecting others; when all are treated equally, all isotope effects are found to vanish.

Graphical abstract: Celebrating Soft Matter's 10th anniversary: Testing the foundations of classical entropy: colloid experiments

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Publication details

The article was received on 28 Apr 2015, accepted on 15 Jul 2015 and first published on 15 Jul 2015


Article type: Tutorial Review
DOI: 10.1039/C5SM01014D
Citation: Soft Matter, 2015,11, 6538-6546
  • Open access: Creative Commons BY license
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    Celebrating Soft Matter's 10th anniversary: Testing the foundations of classical entropy: colloid experiments

    M. E. Cates and V. N. Manoharan, Soft Matter, 2015, 11, 6538
    DOI: 10.1039/C5SM01014D

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