Classical fluids

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Classical fluids^[1] are systems of particles which retain a definite volume, and are at sufficiently high temperatures (compared to their Fermi Energy) that quantum effects can be neglected. A system of hard spheres, interacting only by hard collisions (e.g., billiards, marbles), is a model classical fluid. Such a system is well described by the Percus-Yevik equation. Common liquids, e.g., liquid air, gasoline etc., are essentially mixtures of classical fluids. Electrolytes, molten salts, salts dissolved in water, are classical charged fluids. A classical fluid when cooled undergoes a freezing transition. On heating it undergoes an evaporation transition and becomes a classical gas that obeys Boltzmann statistics.

A system of charged classical particles moving in a uniform positive neutralizing background is known as a one-component plasma (OCP). This is well described by the Hyper-netted chain equation (see CHNC). An essentially very accurate way of determining the properties of classical fluids is provided by the method of molecular dynamics. An electron gas confined in a metal is NOT a classical fluid, where as a very high-temperature plasma of electrons could behave as a classical fluid. Such non-classical Fermi systems, i.e., quantum fluids, can be studied using quantum Monte Carlo methods, Feynman path integral equation methods, and approximately via CHNC integral-equation methods.