Natural clays, widespread in our soils, are today at the heart of a wide variety of industrial processes ranging from the pharmaceutical and cosmetics industries to the food industry. Researchers at Lorraine University embarked on an extensive investigation of the rheological properties (behavior under flow) of clay-based model systems. Their results, obtained using SAXS at SWING beamline, notably, have just been published in Langmuir.
Clay minerals are used especially as texturing agents because of their remarkable mechanical behavior. It is therefore important to identify the parameters that give the clay-based materials their particular mechanical properties.
To address this problem, groups from the LEMTA, LAEGO and LIEC laboratories at Lorraine University have established a protocol for developing aqueous suspensions of natural clays, in order to control many features, such as the size of the clay minerals in suspension, the repulsive interactions such minerals undergo, or particle concentrations.
Flow that depends on mechanisms at the microscopic level
Under flow, colloidal suspensions of natural clays may show different mechanical behaviors. For very low particle concentrations (less than 1% by volume) such suspensions have a wide range of rheological behaviors, from Newtonian fluids (where the viscosity does not depend on the applied shear) to a yield stress gels, where the viscosity is a decreasing function of the shear stress.
Shear-thinning of these suspensions is a result of structure formation at the scale of the clay sheets under the effect of the shear field. Clay minerals present, on average, in the form of platelets about 200 nm long and 1 nm thick, are oriented in the direction of flow, thereby decreasing the viscosity of the suspension. Thus, the rheology of these materials is directly related to the mechanisms involved on the microscopic level in these areas. It is for this reason that the Rheo-SAXS device available on the SWING beamline was adapted to their study. One such tool combines classical rheological measurements with those of small angle x-ray scattering.
Variations in viscosity of natural clay colloidal suspensions as a function of the shear stress (red symbols) and schematic representation of the increasing confinement of clay particles under the effect of shear. The increasing anisotropy of SAXS patterns indicates the orientation of clay particles in the shear field.
How the clay particles become oriented
In this study, researchers have therefore sought to characterize the orientation properties of different natural clays under flow: Idaho beidellite, Arizona and Wyoming montmorillonites and Australian nontronites. For each of these clays, the shear-thinning behavior of suspensions was studied for different particle sheet sizes and different concentrations of clay. These colloidal systems have an apparent complexity considering the electrostatic interactions present in these environments, but also the ill-defined morphology of colloids. Moreover, because of their very high aspect ratio (particle length / thickness > 100), the suspended particles are in a regime of multiple hydrodynamic interactions.
Following an extensive series of experiments, the results show that for these natural minerals, shear orientation results from competition between, on the one hand the hydrodynamic energy associated with the shear stress and, secondly, the Brownian thermal energy. Thus, for all the samples tested, the orientation field adopted by the colloids under shear was found to depend only on a single dimensionless number, the Peclet number, taking into account this competition.
In order to relate the orientation of clay particles to the shear-thinning of suspensions, an effective Quemada1 model, originally built for concentrated suspensions of hard spheres, has been adapted for use with suspensions of non-spherical particles. Instead of a volume fraction of hard spheres, an effective volume fraction was defined. By introducing into this relationship a dependence on the Peclet number, the researchers were then able to build a relationship not only between the viscosity of a suspension and particle concentration, but also their orientation.
This effective approach has shown that although, for a given flow regime (a Peclet number value), all colloids studied have the same orientational field, they do not seem to occupy the same effective volume in suspension. Indeed, it has been established that the smaller the particles in suspension are, the closer the effective volume they occupy is to that of the excluded volume sphere.
The results obtained in this study highlight an important aspect of the behavior of these colloidal suspensions of clay minerals. Indeed, it seems that the orientation of the particles in the flow alone is not sufficient to explain, in general, the rheological properties of these systems.
Such a conclusion leads to different perspectives, such as carrying out USAXS (ultra-small angle X-ray scattering) that provides information on the microscopic scale (unlike SAXS, which gives data on the nanoscale). Such an approach could show the existence of large-scale structures.
Another interesting avenue would be to make low-shear light scattering measurements to measure, not the orientation of the particles, but their characteristic rearrangement time.
These results therefore help to identify the origins of the mechanical properties of clay-based materials. It would be possible, ultimately, to “design" materials needed by industry (drilling fluid, cosmetic pastes ...) by adjusting the physico-chemical nature of the particles to be suspended, their shape, size or concentration, for example.
1. Such a phenomenological approach, takes the form of an equation expressing the viscosity of a suspension according to the particle volume fraction. This approach is particularly appropriate in the case of concentrated suspensions.