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

Abstract

Whereas mixtures of colloids and non-adsorbing polymers have been studied in great detail in the last two decades, binary colloidal mixtures have not received much attention. Yet, fragmental evidence from asymmetric mixtures of hard spheres indicates a wide-ranging, complex behavior from liquid to crystal to single glass and to double glass, and respective rich rheology. Recently, we addressed the question of softness by investigating a mixture of soft and virtually hard colloidal spheres. We found an unprecedented wealth of states including repulsive single glass (RG), liquid, arrested phase separation (APS) and double glass (DG). This is a consequence of the coupling of softness and osmotic forces due to the hard component. We now report on the rheology of the different states with emphasis on the nonlinear response during start-up of stress at constant rate, its relaxation upon flow cessation, and large amplitude oscillatory shearing. Distinct features are identified, whereas comparison with single-colloid (soft or hard) glasses reveals some phenomenological universalities in yielding, residual stresses and periodic intra-cycle stress response. In brief, the DG exhibits much larger yield and residual stresses as compared to the RG and APS, whereas the yield strain is the same for all states. Two-step yielding is unambiguously evidenced for the APS whereas both yield stress and strain exhibit a weak dependence on Péclet number. Large amplitude oscillatory tests reveal large value of the intrinsic nonlinear parameters, reflecting the role of colloidal interactions. Moreover, RG exhibits intra-cycle stress The following article has been accepted by Journal of Rheology. After it is published, it will be found at http://sor.scitation.org/journal/jor 2 overshoots, a feature that characterizes most of the soft glassy materials formed by interpenetrable particles and that vanishes as hard (nearly impenetrable) colloids are added in the mixtures. These results demonstrate the sensitivity of linear and nonlinear rheology to colloidal state transitions and, more importantly, the power of entropic mixing as a means to tailor the flow properties, hence performance and handling of soft composites.

Static and dynamic properties of block copolymer based grafted nanoparticles across the non-ergodicity transition

“…Such a value is much lower than that of a typical colloidal glass of (other) soft particles investigated in the literature. 18,19,29,[77][78][79] This simple observation is reminiscent of the behavior of hard sphere suspensions 80 which reveals that a colloidal crystal is characterized by lower dynamic moduli compared to the respective repulsive glass. Whereas this is a simple observation, it certainly motivates more investigations in the direction of linking the viscoelastic response of soft colloidal crystals and their respective glasses.…”

“…Examples of spherical soft colloidal systems are vesicles, dendrimers, 2 microgels, [3][4][5] block copolymer micelles, [6][7][8][9] polymer-grafted nanoparticles (PGNPs), [10][11][12][13][14] and star polymers. [15][16][17][18][19] Unlike hard-spheres, for which the phase diagram and associated dynamic properties have been exhaustively investigated, 20 the respective consequences of softness have not been fully explored. For instance, bridging the gap between ultrasoft stars and polymer-grafted nanoparticles by identifying similarities and distinct features in the behavior still presents formidable challenges.…”

Nanovesicles drive a tunable dynamical arrest of microparticles

“…Research to understand the physical subtleties behind the formation of vitried states, is still very active in the so matter scientic community. [1][2][3][4][5] Although it is hard to describe it in a few words, a succinct description of a vitrication process should include the fact that when a colloidal system vitries, either by adding mass, reducing volume or temperature, or by shear, there appears a prominent dynamical arrest of the colloids. This phenomenon is observed not only in hard-sphere mixtures, 1,2,5,6 but in so ones.…”

Particle Dynamics in Colloid–Polymer Mixtures with Different Polymer Architectures

“… 29 , 30 The difference in softness between the two types of particles results in distinctly unique interactions between the components, eliciting a diverse set of states both glassy and phase-separated, intrinsically relying on the penetrability of the constituent particles. 31 − 33 …”