Merging of two droplets at an air-water interface driven by surface tension gradient

Abstract

This study reports the unique phenomena of merging of two oil droplets floating on the free surface of water. For the first time, the coalescence has been initiated through the creation of a surface tension gradient by depositing the drop of surfactant at the free surface of water. The experiment reveals that droplets over water and surfactant-laden water surfaces do not merge in the absence of an external gradient, and we experimentally delineate the necessary conditions under which the coalescence of droplets will occur on the imposition of a surface tension gradient. Due to the presence of a surface tension gradient, the droplets experience a substitutional reduction in size even before coalescence, and for obvious reasons, the droplet close to the point of administration of the surfactant suffers a higher reduction in size; it also has a higher initial velocity. We studied the time-dependent variation in the neck radius (rn) formed upon the initial contact of two droplets of both identical and non-identical sizes. The final size of rn is proportional to the diameter of the droplet at the inception of the coalescence when the droplets are of identical size. For droplets of non-identical size, the final value of rn never exceeds the initial diameter of the smaller droplet. During the initial period of coalescence, rn grows with time (t) as rn≈t, but at the later stage, the rate of growth reduces as rn≈tn, where n is between 0.3 and 0.4. The relationship of rn with time in these two stages is similar to that reported in literature. However, the limit of rn for the transformation of one phase to another is significantly different in the present case compared to the other reported results.