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We investigate the complex flow behavior of a well-characterized colloidal star glass using linear and nonlinear rheology. The results are integrated into a generic state diagram which specifies the states of the glass for different initial conditions and mechanical histories: viscoelastic liquid and solid, homogeneous shear-thinned solution, and shear-banded material. Aging takes different forms which can be interpreted as kinetic pathways through the state diagram. Shear-banding appears to be an intrinsic mechanical instability which occurs when the solid state of the glass is shear-melted. Our results provide a straightforward methodology to fingerprint the material behavior of soft glassy materials undergoing rheological transitions which can be used as predictive tool to design systems with a desired rheological response.

We use time-resolved rheology to elucidate the slow dynamics and aging in highly concentrated suspensions of multiarm star polymers. The linear and nonlinear rheological properties exhibit a terminal regime corresponding to a well-defined maximal relaxation time. Terminal relaxation is driven by arm relaxation which speeds up the escape of stars from their cages. The fact that the system fully relaxes and flows at long times has important consequences. The yield stress only exists in the limited range of frequencies or shear rates where solid-like behavior is observed. Aging is controlled by the total time elapsed after flow cessation and not by the time elapsed from flow cessation to the beginning of the measurement as in other glassy materials. Our results, which demonstrate the importance of particle architecture with respect to glassy dynamics, should be generic for long hairy particles.

We present experimental evidence confirming the recently established rich dynamic state diagram of asymmetric binary mixtures of soft colloidal spheres. These mixtures consist of glassy suspensions of large star polymers to which different small stars are added at varying concentrations. Using rheology and dynamic light scattering measurements along with a simple phenomenological analysis, we show the existence of re-entrance and multiple glassy states, which exhibit distinct features. Cooperative diffusion, as a probe for star arm interpenetration, is proven to be sensitive to the formation of the liquid pockets which signal the melting of the large-star-glass upon addition of small stars. These results provide ample opportunities for tailoring the properties of soft colloidal glasses.

We attempt to elucidate the dynamics of multiarm star polymer solutions, which are representative of a large class of soft hairy colloids, over a wide range of concentrations. In addition to the usual β-relaxation (in-cage rattling) and α-relaxation (terminal cage escape), the relaxation of stars in the glassy state involves a mode associated with arm interpenetration. From linear and nonlinear rheological measurements, we establish a set of criteria for identifying the colloidal glass transition, including aging, yielding and elastic properties. Linear viscoelasticity and steady-shear measurements merge at low frequencies, indicating that terminal behavior is accessible, albeit at very long times. The transition from linear to nonlinear response is controlled by star elasticity and a balance between Brownian diffusion and flow advection. In the nonlinear regime, the flow curves collapse on a universal flow curve using a scaling that expresses a competition between solvent-mediated interactions and elastic forces. While applied to stars, our framework appears to be a generic tool for fingerprinting liquid-solid transitions in colloidal suspensions.

Recently, large-amplitude oscillatory shear has been studied in great detail with emphasis on its impact on the material response. Here we present a conceptually different, robust methodology based on viewing the stress waveforms as representing a sequence of physical processes. This novel approach provides the viscous and elastic contributions while overcoming the problems with infinite series encountered by Fourier transformation. Application to a soft colloidal star glass leads to the unambiguous determination and quantification of rate-dependent static and dynamic yield stresses, the rationalization of the response to strain sweeps and the post-yield regime by introducing the apparent cage modulus, and a connection to the steady-shear stress, all from a single-amplitude experiment. We propose that this approach is generic, but focus in this contribution only on a yield stress material which exhibits repeating cycles of (i) elastic extension, (ii) yielding, (iii) flow, and (iv) reformation. We show tha...