Viscosity as the product of its ideal low-concentration value and a thermodynamic function

L Marchioni and MA Di Muro and M Hoyuelos, PHYSICAL REVIEW E, 111, 064128 (2025).

DOI: 10.1103/lg89-g16h

The behavior of viscosity, eta, as a function of concentration in dense fluids remains an unsolved problem, as is the case with other transport coefficients. Boltzmann's theory and the Chapman-Enskog method predict the value of the viscosity at low concentrations, eta 0. Here, the hypothesis eta = phi eta 0 is proposed, where phi is a function of the thermodynamic state that represents the effects of interactions as concentration increases. We consider that eta 0 is the viscosity in an ideal hypothetical system, where the condition of small interactions applies for the whole density range (phi -> 1 for low concentration). The method proposed to verify this hypothesis involves coupling the system with a solvent represented by a Langevin thermostat, characterized by a damping time td. Molecular dynamics simulations show that different values of noise intensity modify eta and eta 0, but do not affect phi. This result supports the assumption that phi is a state function, since the thermodynamic state remains unaltered by the presence of damping and noise. Simulations were conducted for particles that interact via a pseudohard sphere or a Lennard-Jones potential.

Return to Publications page