  When two sessile drops of the same liquid touch, they merge into one drop,
driven by capillarity. However, the coalescence can be delayed, or even
completely stalled for a substantial period of time, when the two drops have
different surface tensions, despite being perfectly miscible. A temporary state
of non-coalescence arises, during which the drops move on their substrate, only
connected by a thin neck between them. Existing literature covers pure liquids
and mixtures with low surface activities. In this paper, we focus on the case
of large surface activities, using aqueous surfactant solutions with varying
concentrations. It is shown that the coalescence behavior can be classified
into three regimes that occur for different surface tensions and contact angles
of the droplets at initial contact. However, not all phenomenology can be
predicted from surface tension contrast or contact angles alone, but strongly
depends on the surfactant concentrations as well. This reveals that the merging
process is not solely governed by hydrodynamics and geometry, but also depends
on the molecular physics of surface adsorption.
