  Drying of particle suspensions is an ubiquitous phenomenon with many natural
and practical applications. In particular, in unidirectional drying, the
evaporation of the solvent induces flows which accumulate particles at the
liquid/air interface. The progressive build-up of a dense region of particles
can be used, in particular, in the processing of advanced materials and
architectures while the development of heterogeneities and defects in such
systems is critical to their function. A lot of attention has thus been paid to
correlate the flow and particles dynamics to the ordering of particles.
However, dynamic observation at the particle scale and its correlation with
local particle ordering are still missing. Here we show by measuring the
particle velocities with high frame rate laser scanning confocal microscopy
that the ordering of weakly Brownian particles during directional drying in a
Hele-Shaw cell opened on one side depends on the particle velocity. Under the
ambient and experimental conditions presented in the following, the particle
velocities accumulate in two branches. A higher degree of ordering is found for
the branch of faster particle velocity which we explain by an increase in the
pressure drop which drags the particles into a denser packing as the flow
velocity increases. This counter-intuitive behaviour is the opposite to what is
found with Brownian particles, which can reorganize by Brownian motion into
denser packing during drying, as long as the flow velocity is not too high.
These results show that different kinetic conditions can be used to obtain
dense, defect-free regions of particles after drying. In particular, it
suggests that rapid, directional drying could be used to control the
crystallinity of particle deposits.
