Some dynamics of vanishing droplets
Par François Boulogne, Laboratoire de Physique des Solides, CNRS, Université Paris-Sud, Orsay, France
Mardi 15 Janvier 2019, 14h00, Salle des séminaires (215), 2e étage, Bâtiment A4N
Drops can appear by condensation of water vapor on small dusts or on surfaces. The same drops can also disappear, often by evaporation in the ambient atmosphere. Although these phenomena are ubiquitous, the description of evaporation started about a century ago, and is still an active field of research with recent significant progresses that will be summarized. For example, under certain conditions, the convective flow of vapor has been revealed recently from observations on the evaporation kinetics compared to purely diffusive predictions. This situation requires some modeling and some progress can be achieved by using analogies with heat transfer.
Evaporation also received a tremendous interest after Deegan’s work on the so called « coffee stain effect », which is a transport of solute toward the contact line of the drop during its evaporation. Indeed, not only the evaporation can be used to remove a solvent and leave a deposit of particles as I will consider the absorption of the liquid in a substrate. Compared to evaporation, absorption leads to a different internal flow field carrying the particles, which forms a nearly uniform deposition pattern in constrast to the coffee stain effect. These results suggest that boundary conditions or environmental conditions can lead to interesting dynamics of vanishing droplets, crucial for a better understanding of climate change, coating processes, transpiration and cooling effects.