Abstract
We investigate, using simultaneous rheology and confocal microscopy, the time-dependent stress response and transient single-particle dynamics following a step change in shear rate in binary colloidal glasses with large dynamical asymmetry and different mixing ratios. The transition from solid-like response to flow is characterised by a stress overshoot, whose magnitude is linked to transient superdiffusive dynamics as well as cage compression effects. These and the yield strain at which the overshoot occurs vary with the mixing ratio, and hence the prevailing caging mechanism. The yielding and stress storage are dominated by dynamics on different time and length scales, the short-time in-cage dynamics and the long-time structural relaxation respectively. These time scales and their relation to the characteristic time associated with the applied shear, namely the inverse shear rate, result in two different and distinct regimes of the shear rate dependencies of the yield strain and the magnitude of the stress overshoot.
Highlights
A wide range of technical applications is based on glassy materials, including polymeric,[1] metallic[2] and colloidal systems.[3]
The pair distribution functions g(r) of the large particles in the quiescent state were determined by confocal microscopy (Fig. 1). They indicate an amorphous structure for all xs
The transition between caging by small and large particles, respectively, occurs at xs z 0.5.19,20 The degree of arrest is re ected in the dynamics at rest,[19,20] and, as shown here, under shear
Summary
A wide range of technical applications is based on glassy materials, including polymeric,[1] metallic[2] and colloidal systems.[3]. The glass state is driven by crowding: for f > 0.58 particles are permanently localised in cages formed by their neighbours, which they can only escape through activated processes.[4] Colloidal glasses melt and ow under application of shear.[5,6,7,8,9,10,11,12,13] Shear-induced melting is associated with an irreversible deformation of the cage[9,13] and the onset of diffusive dynamics.[8] It occurs via a transient regime in which the system yields. At yielding a stress overshoot is observed in the rheological response and re ects maximal cage distortion in the structure and a transient super-diffusive regime in the dynamics.[9,13,14,15]
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