Structuring dynamics in attractive gels
V. Grenard, T. Divoux, N. Taberlet et S. Manneville
Colloidals suspensions are formed of nano- or micro-particles in a fluid. Such suspensions exhibit a wide variety of rheological behaviours when the strength or the range of the interaction changes or when the concentration or temperature is varied. We are interested in colloidal gels made of carbon black particles (formed by incomplete combustion of fuel) in a mineral oil. Those gels are used to model the behavior of used motor oils where attractive interactions between carbon particles lead to an undesired viscosity increase and to the formation of a physical gel even at low particle concentration (as low as 2% in mass). When carbon black gels are sheared within a narrow gap (smaller than about 500 micron), the particles do not remain homogeneously distributed but they arrange into highly anisotropic structures perpendicular to the shear direction (as shown on the picture). A similar behavior has been reported in other attractive systems (flocculated emulsions, carbon nanotube suspensions and clay gels). Here we investigate this structuring experimentally through simultaneous rheological measurements and optical observations. We study the formation and stability of those shear-induced structures by varying the particle concentration, the gap width and the shear rate.