Multiple dynamic regimes in concentrated microgel systems

Abstract

We investigate dynamical heterogeneities in the collective relaxation of a concentrated microgel system, for which the packing fraction can be conveniently varied by changing the temperature. The packing fraction-dependent mechanical properties are characterized by a fluid-solid transition, where the system properties switch from a viscous to an elastic low-frequency behaviour. Approaching this transition from below, we find that the range ξ of spatial correlations in the dynamics increases. Beyond this transition, ξ reaches a maximum, extending over the entire observable system size of approximately 5 mm. Increasing the packing fraction even further leads to a second transition, which is characterized by the development of large zones of lower and higher dynamical activity that are well separated from each other; the range of correlation decreases at this point. This striking non-monotonic dependence of ξ on volume fraction is reminiscent of the behaviour recently observed at the jamming/rigidity transition in granular systems. We identify this second transition as the transition to 'squeezed' states, where the constituents of the system start to exert direct contact forces on each other, such that the dynamics becomes increasingly determined by imbalanced stresses. Evidence of this transition is also found in the frequency dependence of the storage and loss moduli, which become increasingly coupled as direct friction between the particles starts to contribute to the dissipative losses within the system. To our knowledge, our data provide the first observation of a qualitative change in dynamical heterogeneity as the dynamics switches from purely thermally driven to stress driven.

Glass-like dynamics of collective cell migration

“…1D. This combination of growing dynamic heterogeneities and slowing migration speed with increasing cell density is strikingly reminiscent of the nature of the relaxations observed in supercooled fluids approaching the glass transition, suggesting the possibility of an analogy between cell motion within a confluent layer and the crowding within a particulate system approaching a glass transition with increasing density (12,13).…”

Increasing the density melts ultrasoft colloidal glasses

“…Such a perturbation not only shifts the glass transition to larger densities [13,14] but also affects qualitatively the dynamics near the transition, in particular the glass fragility [13,15,16]. Experiments have also investigated the physics of glass aging [17], mostly in microgels, with a particular interest on the jamming transition intervening deep into the glass phase [18,19].…”

“…Such a perturbation not only shifts the glass transition to larger densities [13,14] but also affects qualitatively the dynamics near the transition, in particular the glass fragility [13,15,16]. Experiments have also investigated the physics of glass aging [17], mostly in microgels, with a particular interest on the jamming transition intervening deep into the glass phase [18,19].In this work we investigate whether the anomalous equilibrium phase diagram and transport properties of soft repulsive sphere systems also impact their transition to a non-ergodic glassy state. This possibility was evoked for star polymers [11] for a narrow range of star functionalities, but was not confirmed numerically because crystallization intervened before slow dynamics set in.…”

Microfluidic Rheology of Soft Colloids above and below Jamming

“…[22] finds β = 0.45, while Refs. [23][24][25][26] fit to forms that cross between different limiting viscosities at low and high strain rates. The value β = 1/2 is predicted near jamming for viscously-interacting athermal particles [10].…”