  The transport and aggregation of particles in suspensions is an important
process in many physicochemical and industrial processes. In this work, we
study the transport of particles in an evaporating binary droplet.
Surprisingly, the accumulation of particles occurs not only at the contact line
(due to the coffee-stain effect) or at the solid substrate (due to
sedimentation), but also at a particular radial position near the liquid-air
interface, forming a "ring", which we term as the Marangoni ring. The formation
of this ring is primarily attributed to the solutal Marangoni flow triggered by
the evaporation dynamics of the water-glycerol droplet. Experiments and
simulations show fair agreement in the volume evolution and the general
structure of the solutal Marangoni flow, that is, the Marangoni vortex.
Experiments show that the location of the Marangoni ring is strongly correlated
with the Marangoni vortex. However, finite element numerical simulations fail
to describe the particle distribution seen in the experiments. Interestingly,
the particles not only accumulate to form the Marangoni ring but also assemble
as colloidal crystals close to the liquid-air interface, yielding iridescence.
The formation of the colloidal crystals in the experiments is strong evidence
that non-hydrodynamic interactions, which are not represented in the
simulations, also play a significant role in our system.

Fluid Dynamics