A yield Stress Thixotropic Clay Suspension: Investigations of Structure by Light, Neutron and X-ray Scattering
Résumé
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 micrometer, 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 micrometer. 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 fv* for a given ionic strength and gelation time. In the gel phase, a critical volume fraction fvc separates two domains. Gels belonging to the domain fv