Dilational lateral stress in drying latex films.

Drying latex films usually experience tensile stress due to the reduction in volume. While an unconstrained film would shrink affinely in all three dimensions, a coating can only shrink along the vertical and therefore exerts tensile stress onto the substrate. Using an instrument capable of producing maps of the stress distribution, we found that dilational stress sometimes develops as well. The in-plane stress was monitored by spreading the latex dispersion on a flexible membrane. Usually, the membrane bends upward under the tensile stress exerted by the film, but it may also bend downward. Dilational stress was only found with samples showing a strong coffee stain effect, that is, samples in which there is a significant lateral flow from the center to the edge while the film dries. During drying, particles consolidate first at the edge because of the lower height in this region. Continued evaporation from the consolidated region results in a water flow toward the edge, exerting a force onto the latex particles. At the time, when the network is formed, any single sphere must be in a force-balance condition: the network must exert an elastic force onto the sphere which just compensates the viscous drag. Pictorially speaking, a spring (an elastic network) is created while an external force acts onto it. Once the flow stops, the drag force vanishes and the internal stress, which previously compensated the drag, expands the film laterally. This phenomenon can lead to buckling. Given that lateral flow of liquid while films dry is a rather common occurrence, this mode of structure formation should be widespread. It requires lateral flow in conjunction with elastic recovery of the particle network.