The cover of the recent Physical Review Letters (issue 120 (20), 2018) features our work on fatigue in soft solids. The corresponding article "Strand Plasticity Governs Fatigue in Colloidal Gels" is published in the same issue. Moreover, a focus story in Physics (issue 11 (50), 2018) highlights the relevance of this scientific research in a broader context.
Soft Solids Break Like Liquids
Repeated deformation of a solid material gradually weakens its structure, ultimately leading to macroscopic fracture. For hard materials like concrete and metal this fatigue process is characterized by the accumulation and growth of small microcracks, which propagate catastrophically once a crack reaches a critical size. However, much less is known about the mechanism of fatigue in soft, disordered solid materials, such as skin, foodstuffs, cosmetics and implants.
By combining experiments and computer simulations we explored weakening in model soft solids, consisting of a heterogeneous network of strongly aggregated colloidal particles. Contrary to the prevailing picture, we find that such colloidal gel networks do not fail by the progressive rupture of structural elements, but rather by a liquid-like flow within individual gel strands. Even slightly pulling on the material leads to particle rearrangements that create slack in the gel strands and soften the material. Repeated loading then leads to the formation of thicker and thinner regions in a gel strand until it breaks up in a fashion that is very similar to the Rayleigh Plateau instability in liquid jets.
Our results could be a first step towards a general description of failure in disordered soft solids, and therefore contribute to a better predictability of the mechanical response and lifetime of soft materials.