  We investigate deposit patterns and associated morphology formed after the
evaporation of an aqueous droplet containing mono- and bi-dispersed colloidal
particles. In particular, the combined effect of substrate heating and particle
diameter is investigated. We employ high-speed visualization, optical
microscopy and scanning electron microscopy to characterize the evaporating
droplets, particle motion, and deposit morphology, respectively. In the context
of mono-dispersed colloidal particles, an inner deposit and a typical ring form
for smaller and larger particles, respectively, on a nonheated surface. At
larger substrate temperature, a thin ring with inner deposit forms, explained
by the self-pinning of the contact line and advection of the particles from the
contact line to the center of the droplet due to Marangoni flow. In the context
of bi-dispersed colloidal particles, self-sorting of the colloidal particles
within the ring occurs at larger substrate temperature. The smaller particles
deposit at the outermost edge as compared to the larger diameter particles and
this preferential deposition in a stagnation region near the contact line is
due to the spatially-varying height of the liquid-gas interface above the
substrate. The sorting occurs at a smaller ratio of the diameter of the smaller
and larger particle. At the larger substrate temperature and a larger ratio,
the particles do not get sorted and mix into each other. Our measurements show
that there exists a critical substrate temperature as well as a diameter ratio
in order to achieve the sorting. We propose regime maps on substrate
temperature-particle diameter and substrate temperature-diameter ratio plane
for mono- and bi-dispersed solutions, respectively.
