  Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) particles of a nearly
constant swelling ratio and with polydispersity indices (PDIs) varying over a
wide range (7.4% - 48.9%) are synthesized to study the effects of
polydispersity on the dynamics of suspensions of soft PNIPAM colloidal
particles. The PNIPAM particles are characterized using dynamic light
scattering (DLS) and scanning electron microscopy (SEM). The zero shear
viscosity ($\eta_{0}$) data of these colloidal suspensions, estimated from
rheometric experiments as a function of the effective volume fraction
$\phi_{eff}$ of the suspensions, increases with increase in $\phi_{eff}$ and
shows a dramatic increase at $\phi_{eff}=\phi_{0}$. The data for $\eta_{0}$ as
a function of $\phi_{eff}$ fits well to the Vogel-Fulcher-Tammann (VFT)
equation. It is observed that increasing PDIs results in increasingly fragile
supercooled liquid-like behavior, with the parameter $\phi_{0}$, extracted from
the fits to the VFT equation, shifting towards higher $\phi_{eff}$. The
observed increase in fragility is attributed to the prevalence of dynamical
heterogeneities (DHs) in these polydisperse suspensions, while the simultaneous
shift in $\phi_{0}$ is ascribed to the decoupling of the dynamics of the
smallest and largest particles. Finally, it is observed that the intrinsic
nonlinearity of these suspensions, estimated at the third harmonic near
$\phi_{0}$ in Fourier transform oscillatory rheological experiments, increases
with increase in PDIs. Our results are in agreement with theoretical
predictions and simulation results for polydisperse hard sphere colloidal
glasses and clearly demonstrate that jammed suspensions of polydisperse
colloidal particles can be effectively fluidized with increasing PDIs.
