Poroviscoelasticity of whey protein hydrogels at different length and time scales

The mechanical and solvent transport properties of model whey protein hydrogels were characterized by mechanical indentation. Using cylindrical indenters with a large size range, 0.2–6 mm, it is confirmed that most of the force relaxation measured during indentation is poroelastic in nature, i.e. due to solvent transport. An additional, smaller, viscoelastic relaxation is observed with a relaxation time of ∼10 s. A new microindentation apparatus was built to test hydrogels using small indenters, resulting in constant solvent diffusivities and permeabilites with indenter sizes 0.2–1 mm, at roughly ∼4·10⁻¹⁰ m²/s and ∼5·10⁻¹⁸ m² respectively, regardless of the swelling conditions considered. Relaxations, however, do not reach constant force values at very long times, increasing the uncertainty of the poroviscoelastic analysis. Despite this, the auxetic behaviour of whey protein hydrogels is highly likely, as inferred from consistent drained Poisson's ratios lower than 0.