hide
Free keywords:
-
Abstract:
The evaporation of multicomponent droplets is relevant to various applications but
challenging to study due to the complex physicochemical dynamics. Recently, Li et al.
(Phys. Rev. Lett., vol. 120, 2018, 224501) reported evaporation-triggered segregation in
1,2-hexanediol–water binary droplets. In this present work, we added 0.5 wt % silicone
oil to 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 droplet evaporation. By orientating the droplet upside down, i.e. by studying
a pendent 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
and gravitational acceleration). From micro particle image velocimetry 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, rather than Marangoni
flows, play the dominant role in the flow structure. We compare the measurement of the
evaporation rate with the diffusion model of Popov (Phys. Rev., vol. 71, 2005, 036313),
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.
