Using a powerful combination of experiments and simulations we demonstrate how the microstructure and its time evolution are linked with mechanical properties in a frustrated, out-of-equilibrium, particle gel under shear. An intermediate volume fraction colloid-polymer gel is used as a model system, allowing quantification of the interplay between interparticle attractions and shear forces. Rheometry, confocal microscopy and Brownian dynamics reveal that high shear rates, fully breaking the structure, lead after shear cessation to more homogeneous and stronger gels, whereas preshear at low rates creates largely heterogeneous weaker gels with reduced elasticity. We find that in comparison, thermal quenching cannot produce structural inhomogeneities under shear. We argue that external shear has strong implications on routes towards metastable equilibrium, and therefore gelation scenarios. Moreover, these results have strong implications for material design and industrial applications, such as mixing, processing and transport protocols coupled to the properties of the final material.

We examine microstructural and mechanical changes which occur during oscillatory shear flow and reformation after flow cessation of an intermediate volume fraction colloidal gel using rheometry and Brownian Dynamics (BD) simulations. A model depletion colloid-polymer mixture is used, comprising of a hard sphere colloidal suspension with the addition of non-adsorbing linear polymer chains. Results reveal three distinct regimes depending on the strain amplitude of oscillatory shear. Large shear strain amplitudes fully break the structure which results into a more homogenous and stronger gel after flow cessation. Intermediate strain amplitudes densify the clusters and lead to highly heterogeneous and weak gels. Shearing the gel to even lower strain amplitudes creates a less heterogonous stronger solid. These three regimes of shearing are connected to the microscopic shear-induced structural heterogeneity. A comparison with steady shear flow reveals that the latter does not produce structural heterogeneities as large as oscillatory shear. Therefore oscillatory shear is a much more efficient way of tuning the mechanical properties of colloidal gels. Moreover, colloidal gels presheared at large strain amplitudes exhibit a distinct nonlinear response characterized largely by a single yielding process while in those presheared at lower rates a two step yield process is promoted due to the creation of highly heterogeneous structures.2

Viscoelastic phase separation (VPS) can produce a network structure of the minor phase, which needs to be stabilized for designing a heterogeneous structure with desired mechanical and electrical functions. In this work, we investigate the stabilization of the VPS-induced network structure in a dynamically asymmetric PS/PVME blend by incorporation of a SEBS-g-MA block copolymer or dimethyldichlorosilane modified nanosilica. The addition of SEBS-g-MA retards the volume shrinking process and slows down the kinetics of phase separation due to its localization at the PS/PVME interfaces. Consequently, in the later stage of VPS, phase inversion occurs at longer times with respect to the neat blend due to the decreased interfacial tension. In contrast, hydrophobic nanoparticles self-assemble in the bulk of PS-rich phase and restrain the dynamics of polymer chains enhancing the dynamic asymmetry of the system. The efficiency of nanoparticles in controlling the kinetics of phase separation is found to be superior compared to block copolymer-based compatibilizers indicating the significance of chain dynamics. Moreover, beyond a critical nanoparticle volume fraction, phase separation is pinned due to particle percolation within the PS-rich phase, yielding a kinetically trapped VPS-induced network structure.