Band formation on shearing in phase-separated polymer solutions

In a phase-separated mixture of two Newtonian polymer solutions sheared in a cone-plate geometry adapted for microscopic observation at a zero-velocity plane, shear-induced coalescence of droplets of the broken-up phase, followed by band formation, was observed. Initially wormlike structures developed into doughnut-shaped bands, disconnected from the walls of the geometry. The shear rate and composition inside the bands differed from that in the outside solution. The shear-shear rate instability preceding the band formation could be qualitatively described by a van der Waals-loop-shaped shear rate dependence of droplet deformation.