Influence of the relative humidity influence on the salt deposit after the evaporation of a saline sessile droplet
Résumé
We report experimental observations of salt deposits after complete evaporation of a sessile droplet containing sodium chloride, and we provide quantitative
and qualitative information to the understanding of the complex mechanisms
involved in the evaporation problem.
We define precisely two regimes as function of the initial concentration of
solute : coffee-ring and Marangoni driven flow, the last one is well determined
by the surface tension variation as function of salt concentration.
We propose a solutal instability mechanism in agreement with our experimental results and those found in literature. For the solute crystallization and
final deposits we observe two phases : the first phase without crystal development from the initial time to 65 % of the evaporation time, and the second
one where the chemical process of crystallization appears. In the second phase
we provide quantitative data of the crystallization process using the normalized
average distance between crystals as a function of relative humidity and the
initial salt concentration