Spatially uniform dynamics in equilibrium colloidal gels
Abstract
Gels are low-packing disordered networks of interacting particles that are structurally arrested and able to support weak stresses. Most of colloidal gels are formed by quenching the system into a thermodynamic unstable region. The system then phase-separates into a colloidal-rich and a colloidal-poor phases and remains kinetically trapped into a disordered network which is characterized by aging and complex dynamics. These gels, formed by arrested spinodal decomposition, are thus heterogeneous both in space and time. For particles with limited valence, the coexistence region shifts to lower values of density and temperature. It has been shown that these systems can be cooled down to very low temperatures (much smaller than the attraction energy scale) without phase separation. These so called ‘equilibrium gels’ are characterized by the formation of an empty-liquid state, in which particles form more connections – as the temperature is reduced – until all possible bonds are formed and the system reaches its lowest energy state. Here, we present the first experimental evidence that colloidal gels formed by the reversible aggregation of limited valence particles do not show any appreciable spatial or temporal dynamic heterogeneities, in contrast to gels formed by spinodal decomposition. Taking advantage of the base-pairing specificity and the temperature tunability of the DNA interactions, we investigate a system consisting of DNA nanostars composed of four double stranded arms departing from a common flexible core. Each arm terminates with a single-stranded, self-complementary ‘sticky’ DNA sequence, which provides the interaction between different nanostars. By investigating the system at different temperatures using a combination of dynamic light scattering and Photon Correlation Imaging, a recently introduced technique which allows to measure the sample dynamics with spatial resolution, we show that temporal and spatial heterogeneities on the sample dynamics are basically absent. This evidence is even more striking when compared to the results obtained on the very same system, but where gelation is obtained by a sudden quench in the coexistence region: the gel obtained by phase separation displays strong concentration and dynamic heterogeneities. In summary, we provide experimental proof that the ability of equilibrium gels of tetravalent DNA particles to approach the gel state via a sequence of equilibrium steps results in a spatially and temporally homogeneous system; in contrast, the spinodal coarsening leaves its mark on the system, resulting in heterogeneous structure and dynamics of gels obtained by phase separation.
