  We show that besides simple or thixotropic yield stress fluids there exists a
third class of yield stress fluids. This is illustrated through the rheological
behavior of a carbon black suspension, which is shown to exhibit a viscosity
bifurcation effect around a critical stress along with rheopectic trends, i.e.,
after a preshear at a given stress the fluid tends to accelerate when it is
submitted to a lower stress. Viscosity bifurcation displays here original
features: the yield stress and the critical shear rate depend on the previous
flow history. The most spectacular property due to these specificities is that
the material structure can be adjusted at will through an appropriate flow
history. In particular it is possible to tune the material yield stress to
arbitrary low values. A simple model assuming that the stress is the sum of one
component due to structure deformation and one component due to hydrodynamic
interactions predicts all rheological trends observed and appears to well
represent quantitatively the data.
