How coatings with hydrophobic particles may change the drying of water droplets: incompressible surface versus porous media effects

There is no clear statement on the role of particles in the drying of liquid marbles, which are liquid drops coated with hydrophobic solid particles. While some works report a similar drying time for liquid marbles and bare water drops others observe a faster evaporation of either liquid marbles or of bare water drops. To provide insight into the subject, we report water drying experiments in different configurations. We first focus on the drying of flat water surfaces coated with a single or several layers of hydrophobic micronic particles. Quite surprisingly, surfaces coated with a single layer of densely packed particles dry at the same speed as the bare surfaces. However, when coated with several layers of particles, the drying rate per unit surface area is significantly diminished. This effect is quantitatively explained by considering vapor diffusion through the porous media formed by the stacking of micronic particles above the interface. Then, we consider the drying of curved interfaces which are liquid marbles, i.e. drops coated with one monolayer of micronic particles. Those systematically dry faster than pure drops of the same initial volume. As the presence of a single layer of particles does not significantly affect the drying rate, this speed-up effect is attributed to the conservation of the surface area of the coated drop during the drying. Our quantitative experiments and understanding of the drying of liquid marbles therefore support the different results found in the literature: liquid marbles coated with one monolayer of fine solid particles do dry faster than water drops, while those coated with several layers - that may be formed by aggregates of nanoparticles - experience slower drying.