General Nonlinear 2-Fluid Hydrodynamics of Complex Fluids and Soft Matter

TL;DR

This paper develops general nonlinear 2-fluid hydrodynamic equations for complex fluids like liquid crystals and polymers, highlighting the importance of convective velocities and stress distribution, with simplified models for experimental comparison.

Contribution

It introduces a comprehensive framework for 2-fluid hydrodynamics of complex fluids, emphasizing nonlinearities and the role of convective velocities and stress division.

Findings

Proper velocities are material dependent and cannot be arbitrarily chosen.

Stress division depends on convective velocities and material properties.

A simplified set of equations and a linearized model are provided for experimental comparison.

Abstract

We discuss general 2-fluid hydrodynamic equations for complex fluids, where one kind is a simple Newtonian fluid, while the other is either liquid-crystalline or polymeric/elastomeric, thus being applicable to lyotropic liquid crystals, polymer solutions, and swollen elastomers. The procedure can easily be generalized to other complex fluid solutions. Special emphasis is laid on such nonlinearities that originate from the 2-fluid description, like the transport part of the total time derivatives. It is shown that the proper velocities, with which the hydrodynamic quantities are convected, cannot be chosen at will, since there are subtle relations among them. Within allowed combinations the convective velocities are generally material dependent. The so-called stress division problem, i.e. how the nematic or elastic stresses are distributed between the two fluids, is shown to depend partially on the choice of the convected velocities, but is otherwise also material dependent. A set of reasonably simplified equations is given as well as a linearized version of an effective concentration dynamics that may be used for comparison with experiments.