The repeated loading of a solid leads to microstructural damage that ultimately results in catastrophic material failure. While posing a major threat to the stability of virtually all materials, the microscopic origins of fatigue, especially for soft solids, remain elusive. Here we explore fatigue in colloidal gels as prototypical inhomogeneous soft solids by combining experiments and computer simulations. Our results reveal how mechanical loading leads to irreversible strand stretching, which builds slack into the network that softens the solid at small strains and causes strain hardening at larger deformations. We thus find that microscopic plasticity governs fatigue at much larger scales. This gives rise to a new picture of fatigue in soft thermal solids and calls for new theoretical descriptions of soft gel mechanics in which local plasticity is taken into account.
In
this paper, we demonstrate the stabilization of polystyrene
microspheres by encapsulating them with dumbbell-shaped colloids with
a sticky and a nonsticky lobe. Upon adding a depletant, an effective
short ranged attraction is induced between the microspheres and the
smaller, smooth lobes of the dumbbells, making those specifically
sticky, whereas the interaction with the larger lobes of the dumbbells
is considerably less attractive due to their rough surface, which
reduces the overlap volume and leaves them nonsticky. The encapsulation
of the microspheres by these rough-smooth patchy dumbbells is investigated
using a combination of experiments and computer simulations, both
resulting in partial coverage of the template particles. For larger
microspheres, the depletion attraction is stronger, resulting in a
larger fraction of dumbbells that are attached with both lobes to
the surface of microspheres. We thus find a template curvature dependent
orientation of the dumbbells. In the Monte Carlo simulations, the
introduction of such a small, curvature dependent attraction between
the rough lobes of the dumbbells resulted in an increased coverage.
However, kinetic constraints imposed by the dumbbell geometry seem
to prevent optimal packing of the dumbbells on the template particles
under all investigated conditions in experiments and simulations.
Despite the incomplete coverage, the encapsulation by dumbbell particles
does prevent aggregation of the microspheres, thus acting as a colloid-sized
steric stabilizer.
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