Depletion, melting and reentrant solidification in mixtures of soft and hard colloids

Abstract

We present extensive experimental and theoretical investigations on the structure, phase behavior, dynamics and rheology of model soft-hard colloidal mixtures realized with large, multiarm star polymers as the soft component and smaller, compact stars as the hard one. The number and length of the arms in star polymers control their softness, whereas the size ratio, the overall density and the composition are additional parameters varied for the mixtures. A coarse-grained theoretical strategy is employed to predict the structure of the systems as well as their ergodicity properties on the basis of mode coupling theory, for comparison with rheological measurements on the samples. We discovered that dynamically arrested star-polymer solutions recover their ergodicity upon addition of colloidal additives. At the same time the system displays demixing instability, and the binodal of the latter meets the glass line in a way that leads, upon addition of a sufficient amount of colloidal particles, to an arrested phase separation and reentrant solidification. We present evidence for a subsequent solid-to-solid transition well within the region of arrested phase separation, attributed to a hard-sphere-mixture type of glass, due to osmotic shrinkage of the stars at high colloidal particle concentrations. We systematically investigated the interplay of star functionality and size ratio with glass melting and demixing, and rationalized our findings by the depletion of the big stars due to the smaller colloids. This new depletion potential in which, contrary to the classic colloid-polymer case, the hard component depletes the soft one, has unique and novel characteristics and allows the calculation of phase diagrams for such mixtures. This work covers a broad range of soft-hard colloidal mixture compositions in which the soft component exceeds the hard one in size and provides general guidelines for controlling the properties of such complex mixtures.

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.…”

“…In general, the increase of   with particle concentration is attributed to the reduced free volume due to crowding [68]. Furthermore, in the DG state both depletants and depleted particles freeze, the elastic modulus increases sharply ( Figure 6) while the soft stars shrink due to the osmotic pressure exerted by the HS-like stars [62]. Indeed, taking into account the mean-field pressure exerted by the HS-like stars [62] for cHS>40 % wt, soft stars become dramatically squeezed and assume a collapsed configuration akin to that observed under poor solvency conditions.…”

“…On the other hand, the hard star is softer than experimental hard sphere systems (PMMA latex or silica particles), however this does not affect the message of the work, as discussed below. Details about the synthesis of the stars can be found elsewhere [63], [62]. Their volume.…”

“…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.…”

“…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.…”