Yield stress thixotropic clay suspension: Investigation of structure by light, neutron, and x-ray scattering

The characteristic length scales of the structure and fractal behavior of a thixotropic colloidal suspension of synthetic clay were studied by using a combination of small-angle neutron and x-ray scattering and static light scattering. At the same time, macroscopic mechanical behavior at rest was characterized by means of rheometric measurements. Two characteristic length scales were detected in these yield stress suspensions of discotic texture. The first, measuring several tens of nanometres, is linked to a fractal dimension of 3. The second, of the order of 1 mu m, is linked to a fractal behavior of dimension D that increases with the particle volume fraction. Consequently, it is suggested that the structure of the dispersions at rest is composed of subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about 1 mu m. At larger length scales, these micrometer-sized aggregates are rearranged to form a continuous three-dimensional isotropic structure that has a fractal behavior of dimension D, which gives the gels their texture. The increase of this fractal dimension with the particle volume fraction, the ionic strength, and the gelation time is correlated to a hardening of the mechanical properties of the gels at rest. The gel state is reached above a volume fraction phi(nu)* for a given ionic strength and gelation time. In the gel phase,a critical volume fraction phi(nu c) separates two domains. Gels belonging to the domain phi(nu)*<phi(nu)<phi(nu c) have a fractal behavior of dimension D=1+/-0.05, suggesting an alignment of the micrometer-sized aggregates that leads to the formation of a mechanically weak fibrous structure. Gels belonging phi(nu)>phi(nu c) have a fractal dimension D=1.8+/-0.01, corresponding to a mechanically stronger structure consisting of zones of high and lower particle density. A scaling law enabled these fractal dimensions to be correlated with the effect of the volume fraction on the yield stress, In contrast to what is commonly assumed in relation to clay suspensions, it is suggested here that it is the large length scales, of the order of 1 mu m, associated with a fractal arrangement that governs the macroscopic mechanical behavior.