  Concentrated suspensions may shear-thin when the suspended particles form
planar sheets that slide over one another with less friction than if the
particles are randomly distributed. In a na\"ive model the suspension is
described by a mean effective viscosity, and particles that collide with each
other redistribute the mean density in the shearing direction. This leads to a
diffusion equation for the particle density. If the viscosity in the unthinned
state is a steeply increasing function of particle density the effective
diffusion coefficient is negative and the diffusion equation, meaningful only
on scales larger than the particle separation, is ill-posed. This singularity
corresponds to the formation of planar sheets of particles and defines a
critical particle density for the onset of shear thinning.
