Index of content:
Volume 59, Issue 6, November 2015
59(2015); http://dx.doi.org/10.1122/1.4930858View Description Hide Description
In order to deal with strong inhomogeneities of the polymer stress in shear banding of wormlike micelles, rheological models usually include stress diffusion terms. These terms lift the degeneracy of steady flow solutions by selecting the steady-state positions of interfaces between bands and the values of the total mechanical stress. They are also responsible for slow transients of interface kinetics under imposed step shear flow. In our previous experiments, we estimated the typical lengthscale associated with these terms to be of the order of several nanometers, while several other experiments obtained significantly larger estimates. We report here a new set of experiments that reconcile our previous work with the rest of the literature and obtain a diffusive lengthscale of order of a few micrometers. Surprisingly, we find that this lengthscale is a strong function of the applied shear rate, decreasing rapidly across the stress plateau. We perform numerical simulations of several constitutive equations used to model shear-banded flows and conclude that they cannot predict this behavior. We argue that the strong dependence of the diffusive lengthscale on the applied shear rate should be incorporated into the classical constitutive models to obtain quantitative predictions.
59(2015); http://dx.doi.org/10.1122/1.4931114View Description Hide Description
The micellar system composed of Cetylpyridinium Chloride-Sodium Salicylate (CPyCl-NaSal) in brine aqueous solutions has been studied by systematically changing the salt concentration, in order to investigate the rheology of the arising morphologies. In particular, the zero-shear viscosity and the linear viscoelastic response have been measured as a function of the NaSal concentration (with [CPyCl] = 100 mM). The Newtonian viscosity shows a nonmonotonic dependence upon concentration, passing through a maximum at NaSal/CPyCl ≈ 0.6, and eventually dropping at higher salt concentrations. The progressive addition of salt determines first a transition from a Newtonian to a purely Maxwell-like behavior as the length of the micelles significantly increases. Beyond the peak viscosity, the viscoelastic data show two distinct features. On the one hand, the main relaxation time of the system strongly decreases, while the plateau modulus remains essentially constant. Calculations based on the rheological data show that, as the binding salt concentration increases, there is a decrease in micelles breaking rate and a decrease in their average length. On the other hand, in the same concentration region, a low-frequency elastic plateau is measured. Such a plateau is considered as the signature of a tenuous, but persistent branched network, whose existence is confirmed by cryo-transmission electron microscopy images.
59(2015); http://dx.doi.org/10.1122/1.4931655View Description Hide Description
The shear rate-dependent rheological properties of soft to rigid colloidal suspensions are studied using computational models. We show that a contact force defined based on an elasto-hydrodynamic deformation theory captures an important rheological behavior of colloidal suspensions: While near hard-sphere particles exhibit a strong and continuous shear thickening the evolves to a constant viscosity state, soft suspensions undergo a second shear-thinning regime at high Péclet numbers when the hydrodynamic stresses become larger than the modulus of the colloidal particles. We measure N1 and N2 to be large and negative in the shear-thickening regime; however, for soft spheres at the onset of second shear-thinning N2 reduces in magnitude and eventually becomes positive. We show that for near hard-sphere suspensions, colloidal pressure, shear stress, and normal stress difference coefficients tend to diverge near the maximum packing fraction while .