Combined extension and torsion of a swollen cylinder in unsteady conditions for the mechanical characterization of a hydrogel: An application of the continuum theory of mixtures

Due to the growing technological interest on gels, there is a need for more and more accurate mechanical and thermodynamic experimental characterizations. To this aim, we have simulated the stress relaxation behavior of a cylindrical gel sample subjected to a finite combined extension and torsion. To account for the biphasic nature of the gel, we have used the continuum theory of mixtures in unsteady conditions, adopting a Flory–Rehner expression to describe the Helmholtz free energy of the gel. The results of the simulations showed a pronounced relaxation of the axial force along with a shallow relaxation of the torque: while the former effect is a direct consequence of the solvent outflow, the latter one is only an indirect consequence of solvent movement. Both properties can be conveniently used to evaluate the mechanical and diffusive characteristics of a gel with an ordinary rheometer. We have also performed a parametric study of the model to highlight the main influence of the material constants appearing in the constitutive equation.