ABSTRACT
Stress relaxation upon cessation of shear flow is known to be described by single-mode or multimode monotonic exponential decays. This is considered to be ubiquitous in nature. However, we found that, in some cases, the relaxation becomes anomalous in that an increase in the relaxing stress is observed. Those observations were made for physicochemically very different systems, having in common, however, the presence of self-associating units generating structures at large length scales. The nonmonotonic stress relaxation can be described phenomenologically by a generic model based on a redistribution of energy after the flow has stopped. When broken bonds are reestablished after flow cessation, the released energy is partly used to locally increase the elastic energy by the formation of deformed domains. If shear has induced order such that these elastic domains are partly aligned, the reestablishing of bonds gives rise to an increase of the overall stress.
ABSTRACT
The linear and nonlinear rheological behavior of two rod-like particle suspensions as a function of concentration is studied using small amplitude oscillatory shear, steady shear and capillary breakup extensional rheometry. The rod-like suspensions are composed of fd virus and its mutant fdY21M, which are perfectly monodisperse, with a length on the order of 900 nm. The particles are semiflexible yet differ in their persistence length. The effect of stiffness on the rheological behavior in both, shear and extensional flow, is investigated experimentally. The linear viscoelastic shear data is compared in detail with theoretical predictions for worm-like chains. The extensional properties are compared to Batchelor's theory, generalized for the shear thinning nature of the suspensions. Theoretical predictions agree well with the measured complex moduli at low concentrations as well as the nonlinear shear and elongational viscosities at high flow rates. The results in this work provide guidelines for enhancing the elongational viscosity based on purely frictional effects in the absence of strong normal forces which are characteristic for high molecular weight polymers.
ABSTRACT
We explore the relaxation behavior of model colloidal gels under steady shear flow by means of orthogonal superposition rheometry. Fumed silica and carbon black dispersions in Newtonian matrices are used as a model system. As shear rate increases, the frequency dependent orthogonal moduli of the gels shift along the frequency axis without changing their shape, which finally can be superimposed to yield a single master curve. This indicates that the shear rate tunes a master clock for overall relaxation modes in the sheared colloidal gels to produce a "time-shear rate superposition (TSS)", as temperature does in polymeric liquids to produce a time-temperature superposition (TTS). The horizontal shift factor required at each shear rate to obtain the master curve is found to be directly proportional to the suspension viscosity for all the cases. From this result, we suggest that the suspension viscosity determines the overall relaxation time of the particles in the flowing colloidal gel.
