Signatures of Nonergodicity Transition in a Soft Colloidal System

We have investigated the dynamics of an ultrasoft colloidal star suspension over a wide concentration range with the aim at exploring its liquid-to-solid transition. Whereas dynamic light scattering reveals a nonergodic plateau in the intermediate scattering function when the suspension solidifies macroscopically, qualitatively similar to the hard-sphere analogue, the rheological response seems richer; although, in the ergodic regime, the star suspension relaxes via a fast cooperative and a slow self-diffusion process, its corresponding rheological response is characterized by an almost self-similar behavior resembling that of a weak gel, and eventually, at higher volume fractions, it conforms to the glassy rheological response with a strong, frequency-independent storage modulus and a weakly frequency-dependent loss modulus. The implication is that the liquid-to-glass transition in these ultrasoft colloids occurs via an extended intermediate self-similar structural star rearrangement with a frequency-independent loss angle, accompanied by the emergence of a weak slow mode in the intermediate scattering function. The significant polymeric layer of these systems should be responsible for this behavior.