Experimental study on evaporation of droplets in microgravity and in the presence of electric field

One of the key limiting factors in the evaporation rate of sessile droplets is the evacuation of the generated vapour away from the liquid-air interface. In the absence of convection, the main driving mechanism is the vapour diffusivity: as mass diffusion is quite a slow process, this generally leads to small evaporation rates. For liquids with a heavy vapour, natural convection induced by density gradients in the vapour cloud can increase the evaporation rate significantly on ground. Nevertheless, this effect is absent in space and one therefore has to find alternative mechanisms to accelerate evaporation. The electric field may induce some convection and influence evaporation. This work describes experiments regarding evaporation of droplets in microgravity conditions performed during a sounding rocket campaign in June 2019. Microgravity allows to exclude the contribution of natural convection that otherwise prevails on the other effects. The electric field produces a clear change in the vapour concentration distribution surrounding the droplet and an increase of the evaporation rate with respect the case with no electric field. Moreover, in the absence of electric field, the Marangoni convection in the liquid has an effect on the vapour cloud too, preventing the pure diffusive behaviour.