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27.05.2025 14:30 s.t. - Seminarroom 25.32 O2.51
Bill van Megen: „Existence of the partition function and thermodynamic equilibrium - A perspective from hard-sphere colloids”
RMIT Melbourne, Australia
Existence of the (canonical) partition function is axiomatic to definition of the thermodynamic equilibrium state and existence of the attendant thermodynamic properties. To date this premise is without unequivocal experimental verification or contradiction. In addressing this rather fundamental issue we determine that the statistics of fluctuations of the particle number density and its flux of suspensions of hard-sphere particles, measured by dynamic light scattering, are equivalent to the statistics derived by random sampling of the microstates of the canonical ensemble. This provides a dynamical test of thermodynamic equilibrium. Complementing this with the conclusion of diffusing wave spectroscopy experiments, that hydrodynamic interactions are established on the speed of sound, we determine that there’s a one-to one correspondence between the frequency of propagating longitudinal phonons; sound; and the dephasing time of fluctuations of the longitudinal particle current density. The implication is that while equilibrium applies on times (ca. 10-10 s.) it takes sound to propagate the interparticle distance, its signature is not revealed until the accumulation of independent microstates of the canonical ensemble is such that by virtue of the Central Limit Theorem the statistics of fluctuations of dynamical variables are Gaussian.
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