Evaporation of sessile drops is of interest in academic and industrial research due to applications such as ink-jet printing, biological and chemical assays, thin-film coatings, DNA depositions, efficient electronic cooling. Understanding these sessile drops’ evaporation dynamics involves a comprehensive investigation of contact line dynamics, thermal field, and flow field. The interplay between heat conduction in the drop and evaporative cooling due to latent heat release can result in significant temperature differences within the drop leading to internal and external convective flows. The internal fluid motion can be either buoyant convection (due to density gradient) or Marangoni convection (due to surface tension gradient).
In this study, by synchronous utilization of infrared thermography and Particle Image Velocimetry, we probe internal flow characteristics in evaporating water drops on hydrophobic substrates. For drops evaporating on non-heated substrates, an axisymmetric thermal field results in an axisymmetric velocity field inside the drop due to the dominance of buoyancy. However, for drops evaporating on heated substrates, two counter-rotating vortices are identified when the thermal field shows a convective cell at the liquid-vapor interface due to the Marangoni flow. In this regime, the flow is non-axisymmetric and highly transient. Eventually, during the evaporation, a transition to a buoyancy-dominated flow regime is observed. This transition coincides with the disappearance of the convective cell leading to an axisymmetric thermal field. Consequently, after the transition, the flow becomes axisymmetric, which is similar to previous observations. Interestingly, the bulk of the flow is directed towards the substrate when Marangoni flow is dominant. In contrast, the flow reverses and is towards the apex at the center of the drop during buoyancy dominated flow regime. This directionality of the flow field is further explained through analytical calculations. We observe higher velocities of O(mm/s) when the flow is dominated by Marangoni flow, whereas the velocities are observed to be O(µm/s) when buoyancy is dominant. The present study’s novel and exciting outcome is the hitherto unreported correlation between the contact line dynamics, thermal field, and internal flow field. Lastly, proof-of-concept experiments are conducted where the physical mechanisms governing the evaporation of sessile drops can be utilized to sort particles and mix liquids on a micro-scale.