Suspensions of small ultra-soft colloids remain liquids in overcrowded conditions

Concentrated suspensions of small ultra-soft colloids (ultra-low crosslinked microgels) are investigated with scattering and steady shear rheology to capture their equilibrium dynamics. The suspensions lack dynamic arrest, although the slow relaxation time $\tau_2$ follows exponential growth with increasing generalized packing fraction, $\zeta$. The zero-shear viscosity grows weakly with $\zeta$, and never diverges in contrast to other soft glass formers, e.g.~star-polymers, microgels, green particles. Their high compressibility allows these ultra-soft spheres to diffuse even in overcrowded environments.