Soft colloids can overlap and deform, and they may thus be compressed up to volume fractions φ that cannot be explored with hard particles.
In my talk I will first provide a coherent overview of the behavior of colloidal particles interacting via a soft repulsive potential by determining experimentally the φ-dependent structural, dynamical, and rheological properties of soft colloids crossing their glass transition. Our results, supported by numerical simulations, show that the glassy dynamics of soft colloids is markedly different from what has been assumed so far. On the one hand, regardless of their softness, colloids in the so-called supercooled regime exhibit a sharp increase of the equilibrium relaxation time and stretched correlators, signaling heterogenous dynamics akin to those of hard colloids. On the other hand, in contrast to hard spheres, soft colloids at extremely high densities enter a peculiar aging regime due to particle softness and their dynamics is characterized by compressed correlators, signaling the intermittent release of internal stresses that appear in concomitance with the onset of positive yield stresses [1].
I will also show that the transition to this novel regime coincides with the onset of an anomalous decrease of local order with increasing density, reminiscent of the reentrant glass transition predicted theoretically in ultrasoft systems [2-4].
The second part of my talk will be dedicated to the yielding transition of soft glassy systems.
The fluidization of arrested states induced by an external drive, occurs with very similar macroscopic features in a variety of soft systems [5, 6], despite profound differences in their microscopic structural features [6]. This suggests the presence of an underlying general framework, which has been addressed in experimental [7] and numerical [8-10] works, leading to contrasting results: if it is now well established that yielding is associated with a dynamic transition at the microscale [7, 11-13], the nature of such transition is still debated.
In our lab we have recently performed an extensive experimental investigation of the yielding transition under oscillatory strain in a variety of soft glassy samples using a home-made rheo-light scattering setup. We show that the microscopic dynamics do exhibit a well-defined transition at yielding, from ultraslow ballistic dynamics to much faster, diffusive-like dynamics. We rationalize these findings by introducing a simple on-lattice model where the local dynamics result from both spontaneous and shear-induced relaxation processes, and from the dynamical coupling between neighboring sites. A mean field solution of our model yields an equation of state describing yielding as a “cusp catastrophe", a well-known class of problems, akin to a first order liquid-vapor phase transition, involving an abrupt jump of the system properties upon a continuous change of control parameters. Numerical simulations of the model show that disorder in the coupling constants plays a major role in the sharpness of the yielding transition.
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Phys. Rev. E, 97, 040601(R) (2018)
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