Vorticity Alignment and Negative Normal Stresses in Sheared Attractive Emulsions

Attractive emulsions near the colloidal glass transition are investigated by rheometry and optical microscopy under shear. We find that (i) the apparent viscosity $\eta$ drops with increasing shear rate, then remains approximately constant in a range of shear rates, then continues to decay; (ii) the first normal stress difference $N_1$ transitions sharply from nearly zero to negative in the region of constant shear viscosity; and (iii) correspondingly, cylindrical flocs form, align along the vorticity, and undergo a log-rolling movement. An analysis of the interplay between steric constraints, attractive forces, and composition explains this behavior, which seems universal to several other complex systems.