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Abstract

In contrast to ordinary fluids, whose critical dynamics is well understood nowadays, the isothermal non-ideal fluid has received much less attention so far. An isothermal flluid might serve as a useful approximation for monolayer films, which are known to admit for liquid-vapor-like phase separation below a critical point. A pure fluid at its critical point shows dramatic slowing down in its dynamics, owing to a divergence of the order-parameter susceptibility and the coeffient of heat transport. Under isothermal conditions, however, sound waves provide the only possible relaxation mechanism for order-parameter fluctuations. Here, we study the critical dynamics of an isothermal non-ideal fluid via scaling arguments and computer simulations of the corresponding fluctuating hydrodynamics equations. We show that, below a critical dimension of 4, the order-parameter dynamics of an isothermal fluid effectively reduces to “model A”, characterized by overdamped sound waves and a divergent bulk viscosity. In contrast, the shear viscosity remains finite above two dimensions. Possible applications of the model to monolayer films are discussed.