  The shape of deposits obtained from drying drops containing colloidal
particles matters for technologies such as inkjet printing, microelectronics
and bioassay manufacturing. In this work, the formation of deposits during the
drying of nanoliter drops containing colloidal particles is investigated
experimentally with microscopy and profilometry, and theoretically with an
inhouse finite-element code. The system studied involves aqueous drops
containing titania nanoparticles evaporating on a glass substrate. Deposit
shapes from spotted drops at different pH values are measured using a laser
profilometer. Our results show that the pH of the solution influences the dried
deposit pattern, which can be ring-like or more uniform. The transition between
these patterns is explained by considering how DLVO interactions such as the
electrostatic and van der Waals forces modify the particle deposition process.
Also a phase diagram is proposed to describe how the shape of a colloidal
deposit results from the competition between three flow patterns: a radial flow
driven by evaporation at the wetting line, a Marangoni recirculating flow
driven by surface tension gradients, and the transport of particles towards the
substrate driven by DLVO interactions. This phase diagram explains three types
of deposits commonly observed experimentally, such as a peripheral ring, a
small central bump, or a uniform layer. Simulations and experiments are found
in very good agreement.
