Optical Anisotropy and Dimple Formation on Films Formed after Drying of Gelatinized Starch Solution Droplets

We study the microstructures in the drying droplets of gelatinized starch solutions on a flat substrate. Cryogenic scanning electron microscopy studies on the vertical cross-section of these drying droplets for the first time reveal a relatively thinner solid elastic crust of uniform thickness at the free surface, an intermediate mesh region below the crust, and an inner core of a cellular network structure made of starch nanoparticles. We find that the deposited circular films formed after drying are birefringent and azimuthally symmetric with a dimple at their center. We propose that the dimple formation in our sample occurs due to the evaporation-induced stress on the gel network structure in the drying droplet. The polarizing optical microscopic studies show that these films are optically uniaxial at their center and increasingly biaxial away from the center.