Transient effects and mass convection in sessile droplet evaporation: The role of liquid and substrate thermophysical properties

In this paper, a transient droplet evaporation model based on Arbitrary Lagrangian-Eulerian (ALE) frame, which tracks the liquid-gas interface evolution and fully couples heat and mass transfer in solid, liquid and gas domains, was used to study the spontaneous evaporation of water, methanol and 3MP droplet (with low to high volatility) on PTFE substrates with small thermal conductivities and Al substrates with large thermal conductivities. It was found that because of the fully coupled transportation processes in droplet evaporation, the duration of transient periods determined according to the judgment criterion based on whether droplet or substrate are very close to each other. The larger the droplet volatility and the smaller the substrate thermal diffusivity, the longer the period of transient stage (normalized) of droplet evaporation, with 33.3% of the droplet lifetime for 3MP on a PTFE substrate. The larger the volatility and the substrate thermal conductivity, the larger the contribution of convective mass transfer for total evaporation, with the under-predictions of diffusive model of 41% for 3MP droplet on Al substrates. The percentage of contribution of natural convection is 29-64% of total contribution of convective mass transport, which points out the necessity of considering natural convection in the modelling of convective mass transfer. The main characteristics of flow field with and without buoyancy flow under the different combination of substrate and liquid material and their roles on mass transfer are discussed.