Dynamical Threshold of Diluteness of Soft Colloids

Soft colloids are hybrids between linear polymers and hard colloids. Their solutions exhibit rich phase phenomenon due to their unique microstructure. In scaling theories, a geometrically defined overlap concentration c* is used to identify the concentration regimes of their solutions characterized with distinct conformational properties. Previous experiments showed that the average size of soft colloids remains invariant below c* and varies characteristically above it. This observation reveals the causality between the conformational evolution and the physical overlap between neighboring particles. Using neutron scattering, we demonstrate that the competition between the interparticle translational diffusion and intraparticle internal dynamics leads to significant conformational evolution below c*. Substantial structural dehydration and slowing-down of internal dynamics are both observed before physical overlap develops. Well below c*, a new threshold of diluteness c_D* emerges as the crossover between the characteristic times associated with these two relaxation processes. Below this dynamically defined c_D*, the two relaxation processes are essentially uncoupled, and therefore, the majority of the soft colloids retain their unperturbed conformational dimensions. Our observation demonstrates the importance of incorporating dynamical degrees of freedom in defining the threshold of diluteness for this important class of soft matter.