  The process of colloidal drying gives way to particle self-assembly in
numerous elds including photonics or biotechnology. Yet, the mechanisms and
conditions driving the nal particle arrangement in dry colloidal layers remain
elusive. Here, we examine how the drying rate selects the nanostructure of
thick dried layers in four dierent suspensions of silica nanospheres. Depending
on particle size and dispersity, either an amorphous arrangement, a crystalline
arrangement, or a rate-dependent amorphous-to-crystalline transition occurs at
the drying surface. Amorphous arrangements are observed in the two most
polydisperse suspensions while crystallinity occurs when dispersity is lower.
Counter-intuitively in the latter case, a higher drying rate favors ordering of
the particles. To complement these measurements and to take stock of the bulk
properties of the layer, tests on the layer porosity were undertaken. For all
suspensions studied herein, faster drying yields denser dry layers. Crystalline
surface arrangement implies large bulk volume fraction ($\sim$ 0.65) whereas
amorphous arrangements can be observed in layers with either low (down to
$\sim$ 0.53) or high ($\sim$ 0.65) volume fraction. Lastly, we demonstrate via
targeted additional experiments and SAXS measurements, that the packing
structure of the layers is mainly driven by the formation of aggregates and
their subsequent packing, and not by the competition between Brownian diusion
and convection. This highlights that a second dimensionless ratio in addition
to the Peclet number should be taken into account, namely the aggregation over
evaporation timescale.
