Intense jet flow with symmetric vortices induced by saline concentration gradient at free surface of a drying saline droplet

The evolution of the flow paradigm of an evaporating saline droplet is investigated experimentally. The evaporation of the sessile saline droplet can be divided into two regimes by crystallization, and each regime features a unique flow pattern. By seeding fluorescent particles into the saline droplet, we use PIV (particle image velocimetry) method to find that in regime I before crystallization, a flow circulation emerges which consists of the outward surface flow and inward base flow. More significantly, in regime II, the commence of crystallization triggers intense jet flow away from the crystal surface, accompanied with symmetric vortices. By comparing the magnitudes of the possible flows emerging in the drying saline droplet, we demonstrate that the jet flow with vortices is intrinsically solutal Marangoni flow given rise by the saline concentration gradient at the free surface.