A quest for shear banding in ideal and non ideal colloidal rods

Abstract

We assess the possibility of shear banding of semidilute rod-like colloidal suspensions under steady shear flow very close to the isotropic-nematic spinodal, using a combination of rheology, small angle neutron scattering, and laser Doppler velocimetry. Model systems are employed which allow for a length and stiffness variation of the particles. The rheological signature reveals that these systems are strongly shear thinning at moderate shear rates. It is shown that the longest and most flexible rods undergo the strongest shear thinning and have the greatest potential to form shear bands. Although we find a small but significant gradient of the orientational order parameter throughout the gap of the shear cell, no shear banding transition is tractable in the region of intermediate shear rates. At very low shear rates, gradient banding and wall slip occur simultaneously, but the shear bands are not stable over time.

Microstructural Understanding of the Length- and Stiffness-Dependent Shear Thinning in Semidilute Colloidal Rods

“…This points in the direction that a further increase in particle length as well as their stiffness could provoke flow instabilities such as those observed, e.g., for xanthane gum . It is known, however, that none of the systems under study here show flow instabilities in the long-time regime …”

“…23 These systems have proven to be ideally suited to study the flow behavior of rods. 18,[24][25][26][27][28] It is equally important to perform experiments that can relate the flow induced orientational ordering with the rheological behaviour of the different systems. In-situ small angle neutron scattering in combination with rheology (rheo-SANS) is particularly suited to obtain this relation 26,27,29 because of the high contrast of this technique.…”

“…18,[24][25][26][27][28] It is equally important to perform experiments that can relate the flow induced orientational ordering with the rheological behaviour of the different systems. In-situ small angle neutron scattering in combination with rheology (rheo-SANS) is particularly suited to obtain this relation 26,27,29 because of the high contrast of this technique. Moreover, performing the scattering experiments yields the full 3D orientational distribution.…”

“…The effective rod thickness that determines the range of inter-rod interactions is modified by surface coating and changing the ionic strength of the suspending medium . These systems have proven to be ideally suited to study the flow behavior of rods. ,− It is equally important to perform experiments that can relate the flow-induced orientational ordering with the rheological behavior of the different systems. In situ small-angle neutron scattering in combination with rheology (rheo-SANS) is particularly suited to obtain this relation ,, because of the high contrast of this technique.…”

“…These systems have proven to be ideally suited to study the flow behavior of rods. ,− It is equally important to perform experiments that can relate the flow-induced orientational ordering with the rheological behavior of the different systems. In situ small-angle neutron scattering in combination with rheology (rheo-SANS) is particularly suited to obtain this relation ,, because of the high contrast of this technique. Moreover, performing the scattering experiments yields the full 3D orientational distribution. ,, Thus, the core of this article is to determine the flow curves for all of the systems at varying concentrations and measuring the viscosity and orientational ordering as a function of the applied shear rate with rheo-SANS.…”