Delayed coalescence of surfactant containing sessile droplets

Abstract

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.

Asymmetric coalescence of two droplets with different surface tensions is caused by capillary waves

“…Indeed, unequal-sized droplets show different collision outcomes when compared to collisions between droplets of equal size [18,19]. Moreover, when two droplets of different surface tensions coalesce, they show fundamentally different dynamics than their equal-surface tensions counterpart-they exhibit intricate phenomena such as "delayed coalescence" [20][21][22][23][24] and have enhanced internal mixing [25].…”

Wetting and coalescence: beyond single-phase flows

“…This fundamentally changes the coalescence dynamics-for example, Marangoni flow can hinder the coalescence of drops of different surface tensions in a process known as "delayed coalescence", which keeps the drops temporarily separated, see Fig. 1.9b [46,49,52] A similar phenomenon was observed for the coalescence between drops with different surfactant concentrations [53]. Such phenomena are also expected to play a role in the coalescence of ink drops, since their complex composition gives rise to intricate dynamics.…”

“…Coalescence (Fig. 1.6b), like break-up, has been an active field of research in recent years [27,[43][44][45][46][47][48][49][50][51][52][53][54]. Note the singularity at the connection of the thread to the drop.…”

“…The RH was controlled using a home-built apparatus (for details see Ref. [53]), and was constantly monitored along with temperature T during the measurement using a sensor (Honeywell HIH6130) in the setup. Example measurements of the time evolution of θ for φ = 0.08 and φ = 1 are shown in Fig.…”

“…Indeed, unequal-sized droplets show different collision outcomes when compared to collisions between droplets of equal size [62,208]. Moreover, when two droplets of different surface tensions coalesce, they show fundamentally different dynamics than their equal-surface tensions counterpart-they exhibit intricate phenomena such as 'delayed coalescence' and have enhanced internal mixing [46,49,52,53,64,209].…”