Homogeneous dynamics in DNA equilibrium 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 arrested spinodal decomposition: the system is quenched into a thermodynamic unstable region, 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. In contrast, systems composed of particles with limited valence (i.e., limited number of bonds) can be cooled down to very low temperatures (much smaller than the attraction energy scale) without phase separation. Indeed, these so called ‘equilibrium gels’ form an empty-liquid state, in which particles develop more connections until all possible bonds are formed and the system reaches its lowest energy state. Here, we present the first experimental evidence that equilibrium gels do not show any appreciable spatial or temporal dynamic heterogeneities, as opposed 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 dynamics heterogeneities.
