  In directionally-dried colloidal dispersions regular bands can appear behind
the drying front, inclined at $\pm45^\circ$ to the drying line. Although these
features have been noted to share visual similarities to shear bands in metal,
no physical mechanism for their formation has ever been suggested, until very
recently. Here, through microscopy of silica and polystyrene dispersions, dried
in Hele-Shaw cells, we demonstrate that the bands are indeed associated with
local shear strains. We further show how the bands form, that they scale with
the thickness of the drying layer, and that they are eliminated by the addition
of salt to the drying dispersions. Finally, we reveal the origins of these
bands in the compressive forces associated with drying, and show how they
affect the optical properties (birefringence) of colloidal films and coatings.
