  The pattern and profile of a dried colloidal deposit formed after evaporation
of a sessile water droplet containing polystyrene particles on a non-uniformly
heated glass are investigated experimentally. In particular, the effects of
temperature gradient across the substrate and particles size are investigated.
The temperature gradient was imposed using Peltier coolers, and side
visualization, infrared thermography, optical microscopy, and optical
profilometry were employed to collect the data. On a uniformly heated
substrate, a ring with an inner deposit is obtained, which is attributed to
axisymmetric Marangoni recirculation and consistent with previous reports.
However, the dimensions of the ring formed on a non-uniformly heated substrate
are significantly different on the hot and cold side of the substrate and are
found to be a function of the temperature gradient and particles size. In the
case of smaller particle size, the contact line on hot side depins and together
with twin asymmetric Marangoni recirculations, it results in a larger ring
width on the cold side as compared to the hot side. In contrast, the contact
line remains pinned in case of larger particles, and the twin asymmetric
Marangoni recirculations advect more particles on the hot side, resulting in a
larger ring width at the hot side. A mechanistic model is employed to explain
why the depinning is dependent on the particle size. A larger temperature
gradient significantly increases or decreases the ring width depending on the
particle size, due to a stronger intensity recirculation. A regime map is
proposed for the deposit patterns on temperature gradient-particle size plane
to classify the deposits.
