  We study the rheology of confined suspensions of neutrally buoyant rigid
monodisperse spheres in plane-Couette flow using Direct Numerical Simulations.
We find that if the width of the channel is a (small) integer multiple of the
sphere's diameter, the spheres self-organize into two-dimensional layers that
slide on each other and the suspension's effective viscosity is significantly
reduced. Each two-dimensional layer is found to be structurally liquid-like but
their dynamics is frozen in time.
