Illuminating the dynamics of drying suspensions in droplets and films

The drying of a suspension, e.g. in an inkjet printed droplet or a coating film, driven by evaporation or capillary imbibition into the substrate, leads to complex changes in the internal dynamics and mechanics of the system. As the particle concentration increases, diffusivities decrease, collective phenomena emerge, particle begin to interact, aggregate and coalesce. Often, these phenomena occur simultaneously without clear temporal separation, at a wide range of characteristic time scales, and in a spatially-inhomogeneous manner. Here we give an overview of recent advances in the development of an experimental tool, Laser Speckle Imaging, that can unravel many of these phenomena across a wide range of time scales, and in a spatially-resolved manner, even in complex and realistic coating formulations. We show how LSI can be used to pinpoint the coating open time and moment of film formation, give insight into subsurface transport and the emergence and propagation of cracks.