  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 characterised 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 of spatial
correlations in the dynamics increases. Beyond this transition, the spatial
correlation range 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 characterised 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 the spatial correlation length on volume fraction
is reminiscent of the behaviour recently observed at the jamming/rigidity
transition in granular systems (Lechenault et al. 2008). 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 switch from
purely thermally-driven to stress-driven.
