Laser induced fluorescence studies on the distribution of surfactants during drop/interface coalescence

Abstract

The spatiotemporal distribution of fluorescent surfactants on the merging interfaces during the coalescence of an aqueous drop with an organic/aqueous flat interface was studied experimentally with high-speed laser induced fluorescence. The aqueous phase was a 46% glycerol solution, while the organic phase was a 5 cSt silicone oil. A fluorescently tagged surfactant was used at a concentration of 0.001 mol/m 3 in the aqueous phase. To vary the concentration of surfactants on the interfaces, the drop and the flat interface were left to stand for different times before the coalescence experiments (different interface ages). It was found that when a drop rested on the interface, the surfactants adsorbed on the interfaces were swept outwards by the draining liquid film between the drop and the flat interface and reached a peak value at 0.75R h away from the centre of the film, where R h is the horizontal drop radius. After the film rupture, the concentration of the surfactants at the tip of the meniscus increased. Once the film had retracted, the concentration of the surfactants peaked at the meniscus at the bottom of the drop. As the liquid in the drop started to merge with its homophase, the drop formed a cylinder from the upward capillary waves on the drop surface. The surfactant concentration was found to be low at the top of the liquid cylinder as the interface was stretched by the convergence of the capillary waves. Subsequently, the cylinder began to shrink and the top part of the drop acquired a high surfactant concentration.

Role of surfactant-induced Marangoni stresses in drop-interface coalescence

“…Recently, the experimental work performed by Dong et al. (2019) suggested that the presence of surfactants induces the rupture of the interface (i.e. hole formation) in an off-axis location at high Bond numbers.…”

“…Current understanding of the coalescence dynamics in such surfactant-laden systems came from Dong, Weheliye & Angeli (2019), who experimentally showed for the first time surfactant concentration profiles for systems characterised by high Bond numbers. Additionally, they suggested that surfactants have a strong effect on the interfacial dynamics inducing interfacial rupture (i.e.…”

Surfing of drops on moving liquid–liquid interfaces

“…A stopwatch with an accuracy of 0.01s was used to measure the rest times. It was previously found that about 50 runs were sufficient to capture the distribution of the rest times of the drops with a stationary interface (Dong et al 2017(Dong et al , 2019). In the current experiments, around 200 runs were conducted for each set of conditions to reduce uncertainties caused by the moving interface.…”

“…When a drop coalesces with a stationary interface, a meniscus is created immediately after the film rupture which expands rapidly until the liquid in the drop completely merges with the bulk homophase. The expansion of the meniscus is resisted by the viscosity of the fluids at the initial stages, and later by the inertia of the liquids Aarts & Lekkerkerker (2008); Murano & Okumura (2018); Dong et al (2019). It was found, however, that when drops coalesce with moving interfaces, the trapped film, before it fully drains, can break in drop strings where different size drops form connected with thin threads.…”

“…In as early as 1881, Reynolds discussed this phenomenon and indicated that impurities of the surface might lead to coalescence (Reynolds 1881). In contrast, later studies found an increase of the drop rest time when detergents or surfactants were added (Yeo & Matar 2003;Dai & Leal 2008;Dong et al 2017Dong et al , 2019. In fact, surfactants are widely used to stabilize emulsions or dispersions (Sajjadi et al 2002;Li et al 2013).…”