Nature of Self-Diffusion in Fluids

Abstract

The nature of the self-diffusion in low-molecular fluids is discussed. The particular attention is paid to atomic fluids (such as argon), liquid metals, and associated fluids (such as water). The self-diffusion coefficient in the fluids of all indicated types is considered to be the sum of two components: one of them is associated with the transfer of molecules by hydrodynamic vortex modes, and the other is generated by the circulatory motion of local molecular groups. The both components have a collective nature, they are genetically related to each other and differ only by their scales: the former is mesoscopic, the latter is nanoscopic. Manifestations of the collective vortical transport of molecules as specific features in the time dependence of the root-mean-square displacement of molecules are discussed. Sound arguments are proposed concerning the inadequacy of the activation mechanism of thermal molecular motion in low-molecular liquids. The immanent contradiction of exponential temperature dependences for the viscosity and self-diffusion coefficients is proved. In all cases, the preference is given to qualitative arguments.

Прояви Існування Колективного Переносу В Атомарних Рідинах Та Рідких Металах

“…Moreover, it was assumed that the temperature dependence of this component may have an activation character, similar to what takes place in solids. The falsity of such ideas has been emphasized many times in works [6,[14][15][16][17][18][19]. In works [6,16,18], it was ultimately proved that the so-called "single-particle" component of the self-diffusion coef-…”

“…As was shown in work [16], the "single-particle" component is described by the Stokes-Einstein formula in which the radius of the molecule is determined from the analysis of the shear viscosity. As a result, the self-diffusion coefficient of a molecule is expressed by the sum (see also works [14,17]…”