Observation of the Localized Interfacial Evolution Preceding Marangoni Convection

Mass transfer–induced Marangoni convection in volatile binary liquids is commonly associated with the amplification of interfacial concentration disturbances, yet the localized evolution preceding the first visible convective cell remains difficult to quantify experimentally. Here, ethanol–water desorption in a confined quasi–two–dimensional cell with a 2 mm liquid thickness was investigated using quantitative Schlieren imaging. The apparent transient concentration field and interfacial concentration profiles were reconstructed to resolve the earliest observable stage of Marangoni onset. The early behavior depended strongly on the initial ethanol mass fraction. Low–concentration cases mainly exhibited Rayleigh plume structures, high–concentration cases developed Marangoni cellular structures too rapidly for reliable early–stage tracking, whereas intermediate–concentration cases provided a resolvable window before Marangoni cell formation. For an initial ethanol mass fraction of 8 wt.%, a localized interfacial onset site appeared before the first visible Marangoni convective cell. This event initiated two counter–propagating spreading fronts, enriched the swept interfacial region, and was followed shortly by visible Marangoni cellular structures within the redistributed region. The apparent surface tension gradient field exhibited a transient evolution, with an initial increase, followed by a decrease during spreading, and a subsequent increase upon front interaction. These results provide experimental reference data for the pre–cellular interfacial redistribution sequence associated with perturbation–driven Marangoni onset in confined ethanol–water desorption systems.