Viscoplasticity and stratified flow of colloid suspensions

Abstract

Investigation of the rheology of concentrated colloid suspensions and direct observation of their flow allowed us to find several effects inherent to these media as typical soft matter. So, at low stress amplitudes, these colloids behave as mild gels with frequency independent elastic modulus and low mechanical losses. Meanwhile, suspensions demonstrate dualism of properties: at a given shear rate, they behave as viscoplastic media with clearly expressed yielding, while at a given low stress the pronounced Newtonian plateau is detected. The increase in shear rates and stresses leads to the sharp drop of the apparent viscosity, which usually is treated as the yielding effect. Transition through the yield stress is of a dynamic nature because the threshold stress depends on time and suspensions are thixotropic yielding materials. In the transient shear rate range, an unstable regime of deformation appears. It manifests itself either as deformation thickening up to jamming, or as the excitation of selfoscillations. The measuring of rheological properties in varying volume-to-surface ratio of a sample proves that flow of a suspension with high velocity at constant shear stress actually proceeds in a narrow layer inside the instrument gap. This conclusion has been confirmed by direct visual observations demonstrating that a flux is separated into three layers. A wide almost motionless layer is seen near a stationary surface. Near a moving surface, a narrow band with linear velocity profile is detected. Between them, a rather wide transient layer is observed and shear rate in this layer exceeds the average (global) shear rate by several times. Approximately, only a half of the total volume of a suspension is involved in flow. So, we observed a three-band flux of a suspension not described before. Shearing leads to an anisotropic structure of a solid phase.

Rheological properties of emulsions formed by polymer solutions and modified by nanoparticles

“…These data demonstrate the existence of a clear limit, i.e., the Newtonian plateau. So, this dispersion demonstrates a dualism of the rheological behavior (or viscosity bifurcation) at the low stress/rate limit depending on the mode of deformation [28][29][30].…”

“…Meanwhile, in many publications of the last decade (e.g., [29,[31][32][33][34][35][36]), it was shown that layer stratification (Bshear banding^) can take place in the flow of numerous multicomponent systems and emulsions, in particular. So, it seems reasonable to compare the above presented results with data obtained in different flow conditions bearing in mind to clear up whether we are dealing with the homogeneous flow or not.…”

Miscibility and rheological properties of epoxy resin blends with aromatic polyethers

“…All these data provide evidence that the chemical com position and structure of the system are instable and that not only linear but also crosslinked polymers may form during the interaction of the DGEBA with PC. Under these conditions, the effective vis cosity in the low stress region is labile and corre sponds to a transition state of the system [29][30][31].…”

Strain Dependence of the Uniaxial Compression Response of Vegetable Shortening

“…At the maximum stress, the shear plane becomes visible and is more pronounced as strain increases and the stress drops from its peak value. Shear banding develops where a localized region undergoes a large strain compared to the rest of the specimen; examples occur in polycrystalline materials [23,24], granular materials [25,26] and colloidal systems [27,28]. The drop in stress in the overshoot region is often associated with strain energy unloading from the low strain elastic regions adjacent to the shear band [23,29].…”

“…A number of materials exhibit structural reorganizations induced by applied stresses [26,28]. At the crystal level, crystallographic defects within the structure of the crystal can be induced to move by the applied stress.…”