Watching paint dry

Colloidal dispersions, such as those used in paints, ceramics, or in the production of photonic crystals, undergo a series of dramatic instabilities as they change from a liquid to a solid. A drying film may crystallize into beautiful artificial opals, or buckle, curl, crack, and deform into a shattered mess, all in response to capillary forces that may reach several atmospheres in pressure. We study the liquid-solid transition of a drying colloidal dispersion of charged latex spheres – paint. We show how solidification occurs in stages, as the material orders by long range forces, forming a glass, then aggregates into a solid film. For example, one can infer the nanoscopic response of the colloidal dispersion, by watching the colours change under the microscope. The flow of water during such a process breaks the symmetry of the film, and causes a series of unexpected, yet robust, features of drying, including a structural anisotropy of the dry film, birefringence, shear banding, and a preferred orientation of any paint cracks.