Depletion gels from dense soft colloids: Rheology and thermoreversible melting

Abstract

SynopsisUpon addition of small nonadsorbing linear polymers, colloidal glasses composed of large hard spheres melt and eventually revitrify into the so-called attractive glass regime. We show that, when replacing the hard spheres by star polymers representing model soft particles, a reentrant gel is formed. This is the result of compression and depletion of the stars due to the action of the osmotic pressure from the linear homopolymers. The viscoelastic properties of the soft dense gel were studied with emphasis on the shear-induced yielding process, which involved localized breaking of elements with a size of the order of the correlation length. Based on these results, a phenomenological attempt was made at describing the universal rheological features of colloid/nonadsorbing polymer mixtures of varying softness. The star gel was found to undergo thermoreversible melting, despite the fact that conventional hard-sphere depletion gels are invariant to heating. This phenomenon is attributed to the hybrid internal microstructure of the stars, akin to a dry-to-wet brush transition, and is characterized by slow kinetics, on the time scale of the osmotic gel formation process. These results may be useful in finding generic features in colloidal gelation, as well as in the molecular design of new soft composite materials. V C 2014 The Society of Rheology. [http://dx

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

“…At larger strain amplitudes the onset of nonlinearity is marked by an increase of G", which eventually reaches a maximum before decreasing according to a power-law. At the same time, G' decreases with strain smoothly and eventually follows a power-law with exponent almost double compared to that of G'' as reported for different jammed systems [72], [75], [49], [71], [76]. This is not further discussed in this work.…”

“…Interestingly, the two yield strains appearing in the APS state at 10 rad/s are appreciably different from those characterizing the single yield point of the single (RG) and double (DG) glass states, suggesting that the very processes driving the yield of the suspensions in the APS state differ from those driving the yield of solid repulsive states [42], [49], [77]. Indeed, multiple yield processes are thought of as reflecting multiple constraining length scales (in our case dictated by bonds and cages) which occur in systems where interactions lead to additional length scale such as due to clustering in binary colloidal mixtures [77], [49], [42], attractive glasses [10], [71] or arrested phase separating systems alike. Given the yield strain behavior, the yield stress y(=Gpγy) expectedly follows the same qualitative trend observed for the plateau modulus Gp (Figure 9).…”

“…Once the mixtures enter in the basin of ergodic states, the characteristic time of the liquid (extracted from the terminal crossover of the moduli, i.e., r = 1/crossover) varies over three decades; it first decreases with cHS, then goes through a minimum and finally increases as the re-entrance is approached. This is a generic behavior for an ergodic phase between two glassy states [61], [62], [71], [49]. The re-entrant solid-like state has been discussed and identified as an arrested phase separation [61], [62] whose rheology (dynamics) is dominated by that of soft stars.…”

Transition from Confined to Bulk Dynamics in Symmetric Star–Linear Polymer Mixtures

“…This shrinkage of the stars upon adding linear chains is expected from simple osmotic considerations. 29,39,58 For all the mixtures, R decreases from a value R0 (L1000-free suspensions) to a value Rc corresponding to the collapsed stars. Figure 6.…”

“…Such a solid-like behavior is characteristic for aged suspensions. 39,45,47 Note that no systematic rheological investigation or light scattering characterization of the pure star suspensions (as done, for example, by Pellet and Cloitre for microgels 51 ) was carried out to identify possible distinct glassy and jammed regimes in the star polymers, as this goes beyond the scope of this work. For lower fractions (s = 0.9) the S362 suspensions exhibit a response typical for a viscoelastic liquid, with G"()>>G'() and respective frequency scaling of 1 and 2, whereas G'() is not resolved at the lowest frequencies where the stress  = G"() 0 is entirely dictated by the out-ofphase response of the system (Figure 1-A).…”