Structure–dynamics decoupling in soft-colloid suspensions

The accepted paradigm in materials science is that the internal structure of a material determines its macroscopic properties. This connection is reflected in the particle dynamics, which are known to become slower at length scales comparable to the mean interparticle distance. This implies that the q-dependent relaxation time, with q the magnitude of the scattering wave vector, correlates with the structure factor of the material. This is known as de Gennes narrowing in simple atomic liquids, and is a behavior also seen in colloidal suspensions, where the quantities at play are more easily accessible experimentally. We here find that this familiar correlation can breakdown for soft-colloid suspensions. In both experiments and simulations, we find that the q-dependent relaxation time of the suspension does not follow the structure factor, but that instead, it remains unchanged relative to the length scale at which it is measured. We justify this unusual behavior by alluding to single-particle elasticity and how this aspect allows additional relaxation pathways for the characteristic time of the suspension to remain unaffected by the suspension structure. Our findings challenge the prevailing wisdom that the structure of a material unequivocally determines its properties.