Dips and rims in dried colloidal films.

We describe a spatial pattern arising from the nonuniform evaporation of a colloidal film. Immediately after the film deposition, an obstacle is positioned above its free surface, minimizing evaporation at this location. In a first stage, the film dries everywhere but under the obstacle, where a liquid region remains. Subsequently, this liquid region evaporates near its boundaries with the dry film. This loss of water causes a flow of liquid and particles from the center of the obstructed region to its periphery. The final film has a dip surrounded by a rim whose diameter is set by the obstacle. This turns out to be a simple technique for structuring films of nanometric thickness.