  We experimentally investigate the profile and morphology of the ring-like
deposits obtained after evaporation of a sessile water droplet containing
polystyrene colloidal particles on a hydrophilic glass substrate. In
particular, the coupled effect of particle size and concentration are studied.
The deposits were qualitatively visualized under an optical microscope and
profile of the ring was measured by an optical profilometer. The profile of the
ring resembles a partial torus-like shape for all cases of particles size and
concentration. The cracks on the surface of the ring were found to occur only
at smaller particle size and larger concentration. We plot a regime map to
classify three deposit types - discontinuous monolayer ring, continuous
monolayer ring, and multiple layers ring - on particles concentration -
particle size plane. Our data shows a possible existence of a critical
concentration (particle size) for a given particle size (concentration) at
which the monolayer ring forms. For the larger particle sizes, the immersion
capillary forces between the particles dominate, aiding the formation of a
monolayer ring of the particles. The relative mass of the particles accumulated
in the ring is lesser in cases of the monolayer ring. We measure the width and
height of the ring and show that they scale with particle concentration by a
power law for the multiple layers ring. This scaling corroborates with an
existing continuum based theoretical model. We briefly discuss the effect of
the interaction of growing deposit with shrinking free surface on the ring
dimensions and profile. The present results aid understanding of the ring
formation process and will be useful in guiding the design of self-assemblies
of the colloidal particles formed by the evaporating droplets.
