Negative normal stress and vorticity alignment in caged attractive emulsions
Left: trains of log-rolling droplets formed by a sheared attractive emulsion. Right: Alejandro Pena, Matteo Pasquali, and Alberto Montesi after co-discovering the effect on mayonnaise.
We discovered a new effect in attractive emulsions near the colloidal glass transition. We found that in a certain range of shear rates the viscosity of such emulsions is rate-independent; in this same range, the difference between stream-wise and cross-stream normal stresses switches from positive to negative, and trains of drops aligned along the vorticity axis form. This structure breaks at high shear rate, when the shear viscosity thins and the normal stress difference switches back to positive. The phenomenon is related to the interplay of gap confinement, shear forces in the continuous phase, and attractive forces between drops which cause flocculation; no structure formation is observed in repulsive emulsions. The negative normal stress phenomenon can be demonstrated with common emulsions such as mayonnaise.