Imaging and Estimating the Surface Heterogeneity on a Droplet Containing Cosolvents

Cosolvents have numerous applications in many industries as well as scientific research. The shortage in the knowledge of the structures in a cosolvent system is significant. In this work, we display the spatial as well as the kinetic distribution of the cosolvents using droplets as paradigms. When an alcohol/water-containing sessile droplet evaporates on a substrate, it phase segregates into a water-enriched core and a thin alcohol prevailing shell. This is considered to be due to the different escaping rate of solvents out of the liquid−vapor (l−v) interfaces. In between the core and shell phases, there exists a rough and solid-like liquid−liquid (l−l) wall interface as marked by the fluorescent polystyrene spheres and imaged by a confocal microscope. Holes and patches of beads are observed to form on this phase boundary. The water-dispersed beads prefer to partition within the core. The shell prevails in the droplet during most of the drying and shrinks with the l−v boundary. By monitoring the morphological progression of the droplet, the composition of the cosolvent at the liquid−vapor interface is obtained.