  Evaporating suspensions of colloidal particles lead to the formation of a
variety of patterns, ranging from a left-over ring of a dried coffee drop to
uniformly distributed solid pigments left behind wet paint. To characterize the
transition between single rings, multiple concentric rings, broad bands, and
uniform deposits, we investigate the dynamics of a drying droplet via a
multiphase model of colloidal particles in a solvent. Our theory couples the
inhomogeneous evaporation at the evolving droplet interface to the dynamics
inside the drop, i.e. the liquid flow, local variations of the particle
concentration, and the propagation of the deposition front where the solute
forms an incompressible porous medium at high concentrations. A dimensionless
parameter combining the capillary number and the droplet aspect ratio captures
the formation conditions of different pattern types.

Soft Condensed Matter;Materials Science