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Yoghurt under stress
Biogels formed through the self-association of polysaccharide coils, collagen, actin filaments or attractive globular proteins play a major role in biochemistry and microbiology, biological networks and cell mechanics as well as in food science. Their mechanical properties are the physical basis of biomechanics and morphogenesis. These biomaterials all behave as elastic solids under small deformations but display remarkable non-linear behaviour generally featuring stress- or strain-stiffening and fractures prior to irreversible rupture.
Here we explore the mechanical behaviour of a model protein gel (acid-set sodium caseinate, i.e. "yoghurt") under stress : either constant shear stress that drives it to irreversible rupture ; or swelling-induced compressive stress that causes wrinkling of a thin sheet of gel. The former emphasises the similarity between biogels and brittle solids, in excellent agreement with recent fiber-bundle models that include damage accumulation on elastic fibers. The latter highlights the soft nature of biogels, in particular their porosity and the influence of the solvent.