Investigation of buoyancy-driven convection in non-axisymmetric evaporating droplets confined on hollow micropillars
Date of Award
Master of Science (MS)
Sessile droplet evaporation is ubiquitous in natural and industrial processes, including mammal perspiration, transpiration by plants, and industrial spray cooling. Although extensive evaporation-related research has examined convection effects inside droplets, most studies have been based on hemispherical droplets. Hence the underlying mechanisms of evaporation from non-axisymmetric droplets are not well understood. Here, we investigate the mechanisms of evaporation from non-axisymmetric droplets with sizes ranging from 100 μm to 8 mm. An experimentally validated finite element model simulates different transport mechanisms during evaporation. For non-axisymmetric droplets larger than 4 mm, we observe a non-uniform recirculation pattern in the azimuthal direction, leading to non-uniform temperature distributions not observed in hemispherical droplets. Analysis of the local thermal resistance shows that the recirculation patterns change with different sizes and azimuthal angles, which is caused by the competing effects of capillary force and buoyant force. For 8 mm droplets, the conduction resistance peak shifts from r/R = 0.9 to r/R = 0.63 as the azimuthal angle increases from 0° to 60°, indicating that the recirculation vortex is moving away from the contact line region. In the droplet size range from 8 mm to 4 mm, at an azimuthal angle equal to 60°, the conduction resistance peak shifts from r/R = 0.63 to r/R = 0.55, indicating that buoyancy-driven flow is becoming less dominant over capillary flow. For 2 mm and smaller droplets, the absence of a recirculation vortex shows the dominance of capillary flow over buoyancy flow. These findings offer new insights into evaporation from non-axisymmetric droplets, revealing for the first time how the convection pattern changes.
Damena Agonafer/Mechanical Engineering and Material Science
David Peters, Guy Genin, Damena Agonafer