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Salt particles that have dissolved in water never spontaneously come back together in solution to reform solid particles. Moreover, a gas that has expanded in a vacuum remains dispersed and never spontaneously reassembles. The unidirectional nature of these phenomena is the result of a thermodynamic state function called entropy (S). Entropy is the measure of the extent to which the energy is dispersed throughout a system, or in other words, it is proportional to the degree of disorder of a...
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A pure, perfectly crystalline solid possessing no kinetic energy (that is, at a temperature of absolute zero, 0 K) may be described by a single microstate, as its purity, perfect crystallinity,and complete lack of motion means there is but one possible location for each identical atom or molecule comprising the crystal (W = 1). According to the Boltzmann equation, the entropy of this system is zero.
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Evolution of Staircase Structures in Diffusive Convection
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Excess entropy and Stokes-Einstein relation in simple fluids.

S A Khrapak1, A G Khrapak1

  • 1Joint Institute for High Temperatures, Russian Academy of Sciences, 125412 Moscow, Russia.

Physical Review. E
|November 16, 2021
PubMed
Summary
This summary is machine-generated.

The Stokes-Einstein relation in dense liquids is linked to excess entropy. Its validity onset is observed around s_ex ≲ -2, with gas-liquid behavior demarcated at s_ex ≃ -1.

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Area of Science:

  • Physical Chemistry
  • Statistical Mechanics
  • Condensed Matter Physics

Background:

  • The Stokes-Einstein relation connects self-diffusion and shear viscosity in dense liquids.
  • Understanding the conditions for its validity is crucial for fluid dynamics and material science.

Purpose of the Study:

  • To identify the precise conditions on phase diagrams where the Stokes-Einstein relation holds true.
  • To investigate the role of excess entropy as an indicator for the relation's validity across different interaction potentials.

Main Methods:

  • Simulations of four model systems with distinct pairwise interaction potentials: Lennard-Jones, Coulomb, Debye-Hückel, and hard spheres.
  • Analysis of the reduced excess entropy (s_ex) as a predictor for Stokes-Einstein relation validity.

Main Results:

  • The Stokes-Einstein relation's validity is strongly correlated with reduced excess entropy (s_ex).
  • Onset of SE relation validity consistently observed at approximately s_ex ≲ -2 across all model systems.
  • The boundary between gas-like and liquid-like fluid behavior is characterized by s_ex ≃ -1.

Conclusions:

  • Reduced excess entropy (s_ex) serves as a reliable indicator for the applicability of the Stokes-Einstein relation in dense liquids.
  • The findings provide a universal criterion for the Stokes-Einstein relation's validity based on thermodynamic properties, applicable across diverse interaction potentials.