Evaporation of Sessile Drops Containing Colloidal Rods: Coffee-Ring and Order–Disorder Transition

Liquid
drops containing insoluble solutes when dried on solid substrates
leave distinct ring-like deposits at the periphery or along the three-phase
contact line–a phenomena popularly known as the coffee-ring
or the coffee stain effect. The formation of such rings as well as
their suppression is shown to have applications in particle separation
and disease diagnostics. We present an experimental study of the evaporation
of sessile drops containing silica rods to elucidate the structural
arrangement of particles in the ring, an effect of the addition of
surfactant and salt. To this end, the evaporation of aqueous sessile
drops containing model rod-like silica particles of aspect ratio ranging
from ∼4 to 15 on a glass slide is studied. We first show that
when the conditions such as (1) solvent evaporation, (2) nonzero contact
angle, (3) contact line pinning, (4) no surface tension gradient driven
flow, and (5) repulsive particle–particle/particle–substrate
interactions, that are necessary for the formation of the coffee-ring
are met, the suspension drops containing silica rods upon evaporation
leave a ring-like deposit. A closer examination of the ring deposits
reveals that several layers of silica rods close to the edge of the
drop are ordered such that the major axis of the rods are oriented
parallel to the contact line. After the first few layers of ordered
arrangement of particles, a random arrangement of particles in the
drop interior is observed indicating an order–disorder transition
in the ring. We monitor the evolution of the ring width and particle
velocity during evaporation to elucidate the mechanism of the order–disorder
transition. Moreover, when the evaporation rate is lowered, the ordering
of silica rods is observed to extend over large areas. We demonstrate
that the nature of the deposit can be tuned by the addition of a small
quantity of surfactant or salt.