  We experimentally investigate the dissolution of microscale sessile alcohol
droplets in water under the influence of impermeable vertical confinement. The
introduction of confinement suppresses the mass transport from the droplet to
bulk medium in comparison with the non-confined counterpart. Along with a
buoyant plume, flow visualization reveals that the dissolution of a confined
droplet is hindered by a newly identified mechanism - levitated toroidal
vortex. The morphological changes in the flow due to the vortex-induced
impediment alters the dissolution rate, resulting in enhancement of droplet
lifetime. Further, we have proposed a modification in the key non-dimensional
parameters (Rayleigh number $Ra^{'}$ (signifying buoyancy) and Sherwood number
$Sh^{'}$ (signifying mass flux)) and droplet lifetime $\tau_{c}^{'}$, based on
the hypothesis of linearly stratified droplet surroundings (with revised
concentration difference $\Delta C^{'}$), taking into account the geometry of
the confinements. We show that experimental results on droplet dissolution
under vertical confinement corroborate universal scaling relations $Sh^{'} \sim
Ra^{' 1/4}$ and $\tau_{c}^{'} \sim \Delta C^{'-5/4}$. We also draw attention to
the fact that the revised scaling law incorporating the geometry of
confinements proposed in the present work can be extended to other known
configurations such as droplet dissolution inside a range of channel
dimensions, as encountered in a gamut of applications such as micro-fluidic
technology and biomedical engineering.
