Large Deformation Rheometry and Structure-Property Relationships in Biopolymer Physical Gels

We examine experimentally the nonlinear rheology of two biopolymer physical gel systems and develop mathematical network models to describe the observed nonlinear viscoelastic behavior. A gluten dough (a protein biopolymer gel), and hagfish slime (a biopolymer/biofiber water-swollen network) each exhibit similar nonlinear rheology in which the average elastic modulus strain-softens at large input strain, yet the local tangent elastic modulus strain-stiffens. Such behavior has been observed for other biopolymer networks including snail pedal mucus gel and a keratin filament network. This juxtaposition of simultaneous softening and stiffening suggests a network structure composed of nonlinear elastic strain-stiffening elements (here modeled as Finite Extensible Nonlinear Elastic (FENE) elements), in which network connections are sufficiently destroyed as elements are stretched (consistent with a physical gel). Rheological tests include Large Amplitude Oscillatory Shear (LAOS) and inertio-elastic ringing, which provide the initial observations to motivate the choice of the models, as well as a test for the success of the structure-property relationship.