Glassy States in Asymmetric Mixtures of Soft and Hard Colloids

Abstract

By employing rheological experiments, mode coupling theory, and computer simulations based on realistic coarse-grained models, we investigate the effects of small, hard colloids on the glassy states formed by large, soft colloids. Multiarm star polymers mimic hard and soft colloids by appropriately varying the number and size of their arms. The addition of hard colloids leads, depending on their concentration, to either melting of the soft glass or the emergence of two distinct glassy states. We explain our findings by depletion of the colloids adjacent to the stars, which leads to an arrested phase separation when the repulsive glass line meets the demixing binodal. The parameter-free agreement between experiment, theory, and simulations suggests the generic nature of our results and opens the route for designing soft-hard colloidal composites with tunable rheology.

Asymmetric soft-hard colloidal mixtures: Osmotic effects, glassy states and rheology

“…We present a comparative rheological investigation of different states of asymmetric binary softhard colloid mixtures. Using the same system studied before [61], [62], we extend our earlier studies and report on the transient nonlinear rheology (evolution and relaxation of stress under constant shear, response to large amplitude oscillatory shear deformation) of RG, APS and DG states. We discuss the phenomenological link of the rheological properties to morphology and compare simple repulsive glasses from hard spheres, microgels and stars.…”

“…We used the experimental system of references [61] and [62]. It consists of two different multiarm 1,4-polybutadiene (PBD) stars, as soft and hard sphere respectively, by taking advantage of the well-known fact that their pair interaction potential depends on star functionality, f. In this way, we ensured that the mixture is purely entropic.…”

“…The system, initially in a repulsive glassy state in the absence of HS-like star additives, reaches an ergodic liquid-like state for cHS≥0.2% wt. As the concentration of HS-like stars is increased further, the mixture re-solidifies (cHS>3% wt) and transforms into an arrested phase-separated state [61],characterized by a storage modulus similar to that of the initial repulsive glass (cHS=0% wt), hence suggesting that the re-entrant state is not an attractive glass ( Figures 5 and 6). Indeed attractive glasses are characterized by larger values of the storage modulus when compared to repulsive glasses, due to the presence of both caging and attractive forces.…”

“…Consequently, considering mixtures of small hard spheres and large star polymers has emerged as a natural challenge to further enhance these capabilities and explore new properties via experiments, simulations and theory [59], [60], [61]. In this spirit, we have recently reported on the state diagram of such mixtures [61], [45], [62] that is here sketched in Figure 1. We present a comparative rheological investigation of different states of asymmetric binary softhard colloid mixtures.…”

Glass–glass transition during aging of a colloidal clay

“…Interestingly a significant change of the elastic modulus is found: for times upto roughly 2 days (Wigner glass) G 0 is compatible with the long time behaviour reported in previous measurements 28 (G 0 E10 3 Pa); however, for times above 3 days, G 0 is roughly one order of magnitude higher (G 0 E10 4 Pa). Such an increase was found in systems with depletion interactions when passing from a repulsive to an attractive glass 14,32 . This result thus supports the existence of two glasses with significantly different elastic properties and suggests a crucial role of attraction for the occurrence of the spontaneous glassglass transition observed in Laponite suspensions.…”