Issues of Modeling Molecular Flows in Three-Aggregate Systems

Principles of approaches used to describe molecular flows in dispersed phases are discussed. The approaches are based on the current continuum mechanics methods, the physical interpretation of hydrodynamics and non-equilibrium thermodynamics that differently introduce the concept of pressure in the bulk phase. Differences are emphasized in these three mentioned model-free methods of determining the pressure, which equally imply the use of experimental data and do not allow the calculation of the pressure. To implement pressure calculations it is needed to apply the statistical physics methods closing the calculations of the mechanical and thermodynamic properties in wide temperature and density ranges, which also have different definitions of pressure. The calculation of pressure in fluids and three-aggregate systems of limited volumes is considered based on the lattice gas model. The introduction of local and average pressure values in these fluids is discussed along with the question how they depend on indirect interactions between solids through the fluid phase, apart from the direct surface potential effect of solids. Microscopic hydrodynamics enables the calculations of local non-equilibrium corrections in the flow to the equilibrium pair distribution function of molecules, which affect all characteristics of dispersed phases. These corrections are related to local hydrodynamic velocities, and they govern the conditions of local equilibrium used to calculate local dissipative coefficients in any part of the system. It is shown that in all situations, the second law of thermodynamics makes it possible to indicate the correct definitions of the pressure value, which provides the possibility of its modeling.