Individual trajectories were evaluated to obtain the statistics of the particle velocities. We observed a fast-rotating convection current along the NAPL–water interface with a maximum velocity of approximately 1,000 μm s−1 after 10 min. The fluid motion showed a persistent movement in the form of a rolling cell for at least 99 h, but a decreasing rotation speed over time. We attributed the convective flow dynamics to three mechanisms following different kinetic rates: (a) a short-lived Marangoni flow, (b) a medium-lived dissolution-driven flow, and (c) a long-lived evaporation-driven flow. Upon initial contact between water and NAPLs, the differences in surface tension caused a rapid Marangoni flow along the interface, which died out quickly as the surface tensions were equilibrated. The Marangoni flow was superseded by a dissolution-driven flow as the NAPLs dissolved in the aqueous phase. The dissolution-driven flow dissipated according to a first-order rate law and died out when the liquids were mutually saturated. Evaporation of water and NAPLs caused a long-term but slow convective flow. The interaction of these three mechanisms caused enhanced mixing during multiphase transport.
Full article at https://doi.org/10.1002/vzj2.20209