This paper investigates the linear and non-linear instabilities during evaporation of liquid layers and droplets by means of two-phase 3D direct numerical simulations. The interface is open to the atmosphere under the consideration that vapour diffusion is the rate- limiting mechanism for evaporation. In both configurations, the vapour-liquid interface is prone to travelling thermal instabilities, i.e., hydrothermal waves (HTWs), due to the presence of temperature gradients along the interface. We have already shown in our recent work 7 that under saturated conditions (negligible evaporation) the HTWs additionally give rise to interface deformations of similar features, i.e., physical waves. We have also demonstrated 8 that phase change plays a dual role through its effect on these instabilities: the latent energy required during the evaporation process tends to inhibit the HTWs while the accompanying level reduction enhances the physical waves by minimizing the role of gravity. The dynamics of the gas phase are also discussed. We have also established that the HTW-induced convective patterns in the gas along with the travelling nature of the instabilities have a significant impact on the local evaporation flux and the vapour distribution above the interface. The Marangoni effect plays a major role in the vapour distribution generating a vacuum effect in the warm region and vapour accumulations at the cold boundary capable of inverting the phase change, i.e., the capillary flow can lead to local condensation. These results provide evidence of the inefficiencies of the traditional phase change models based on pure vapour diffusion to capture the dynamics of thermocapillary flows. To conclude, we also present results of a parallel investigation focusing on three-dimensional phenomena on evaporating sessile drops placed on heated substrates.
- hydrothermal waves
- liquid layers
- thermocapillary instabilities