Rheological detection of caging and solid–liquid transitions in soft colloid–polymer mixtures

We examine experimentally the rheology of star–linear polymer mixtures with particular attention to transitions from solid (glass) to liquid state. Here, in addition to the well-known breaking of the cage that describes the glassy state, upon application of large strain deformation, the effects of depletion interactions due to the presence of linear polymers are considered. Hence, the system has a rich state diagram and two main parameters to control the softening and eventual melting of the solid: thermodynamic (osmotic effect leading eventually to star–star attraction) and shear fields. This provides additional tunability for this type of systems and contributes a great deal to the phenomenology of solid-to-liquid transitions. Moreover, it is reflected on the linear and nonlinear rheological properties. Analysis of the large-amplitude-oscillatory-shear data allows to extract the so-called apparent cage modulus of the glass, which serves as a sensitive indicator of the glass-to-liquid transition upon increasing linear polymer concentration. Same behavior has been observed for the yield stress as well. Furthermore, the aging of the glass, i.e. the time evolution of the original glassy system following pre-shear melting, is crucially important. The cage modulus, which essentially links the yield stress to the yield strain, increases with aging and depends on the strain amplitude differently for the fresh and aged systems, reflecting the interplay of shear and thermodynamic forces. Thus, we propose that the cage modulus is another sensitive generic indicator of solid-to-liquid transitions.

► Concept of cage modulus extended to colloid–polymer mixtures.
► Cage modulus sensitive to solid–liquid transitions.
► Yield stress equally sensitive to these transitions.
► Solid–liquid transitions tailored/monitored by both thermodynamic (depletion) and shear forces.
► Appropriate protocols needed to account for aging effects on yielding transitions and probed via cage modulus.