  The evaporation of drops of colloidal suspensions plays an important role in
numerous contexts, such as the production of powdered dairies, the synthesis of
functional supraparticles, and virus and bacteria survival in aerosols or drops
on surfaces. The presence of colloidal particles in the evaporating drop
eventually leads to the formation of a dense shell that may undergo a shape
instability. Previous works propose that, for drops evaporating very fast, the
instability occurs when the particles form a rigid porous solid, constituted of
permanently aggregated particles at random close packing. To date, however, no
measurements could directly test this scenario and assess whether it also
applies to drops drying at lower evaporation rates, severely limiting our
understanding of this phenomenon and the possibility of harnessing it in
applications. Here, we combine macroscopic imaging and space- and time-resolved
measurements of the microscopic dynamics of colloidal nanoparticles in drying
drops, measuring the evolution of the thickness of the shell and the spatial
distribution and mobility of the nanoparticles. We find that, above a threshold
evaporation rate, the drop undergoes successively two distinct shape
instabilities. While the second instability is due to the permanent aggregation
of nanoparticles, as hypothesized in previous works on fast-evaporating drops,
we show that the first one results from a reversible glass transition of the
shell, unreported so far. We rationalize our findings and discuss their
implications in the framework of a unified state diagram for the drying of
colloidal drops.
