  The evaporation of multi-component droplets is relevant to various
applications but challenging to study due to the complex physicochemical
dynamics. Recently, Li (2018) reported evaporation-triggered segregation in
1,2-hexanediol-water binary droplets. In this present work, we added 0.5 wt%
silicone oil into the 1,2-hexanediol-water binary solution. This minute
silicone oil concentration dramatically modifies the evaporation process as it
triggers an early extraction of the 1,2-hexanediol from the mixture.
Surprisingly, we observe that the segregation of 1,2-hexanediol forms plumes,
rising up from the rim of the sessile droplet towards the apex during the
droplet evaporation. By orientating the droplet upside down, i.e., by studying
a pendant droplet, the absence of the plumes indicates that the flow structure
is induced by buoyancy, which drives a Rayleigh-Taylor instability (i.e.,
driven by density differences & gravitational acceleration). From micro-PIV
measurement, we further prove that the segregation of the non-volatile
component (1,2-hexanediol) hinders the evaporation near the contact line, which
leads to a suppression of the Marangoni flow in this region. Hence, on long
time scales, gravitational effects play the dominant role in the flow
structure, rather than Marangoni flows. We compare the measurement of the
evaporation rate with the diffusion model of Popov (2005), coupled with
Raoult's law and the activity coefficient. This comparison indeed confirms that
the silicone-oil-triggered segregation of the non-volatile 1,2-hexanediol
significantly delays the evaporation. With an extended diffusion model, in
which the influence of the segregation has been implemented, the evaporation
can be well described.
