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/m3 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 0.75Rh away from the centre of the film, where Rh 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.
Highlights
The phenomenon of drop coalescence is often encountered in emulsions and dispersions in many industrial fields, including oil production and processing,1–3 milk production,4 drug encapsulation,5,6 and liquid jet atomization.7,8 The rate of drop coalescence plays a key role in controlling the stability of emulsions
Other studies supported that the Marangoni backflow was not strong enough to suppress the outward flow in the film22,23 and the only role of the Marangoni stresses was to suppress the inner circulation in the drops and make the interface rigid
Giribabu and Ghosh23 further derived a stochastic model to estimate the drop rest times based on the uneven distribution of surfactants along the interface, and the results were consistent with previous experimental data
Summary
The phenomenon of drop coalescence is often encountered in emulsions and dispersions in many industrial fields, including oil production and processing, milk production, drug encapsulation, and liquid jet atomization. The rate of drop coalescence plays a key role in controlling the stability of emulsions. In drop coalescence with a liquidliquid interface, many scientists experimentally observed that the rest times increased considerably when surfactants were present.. Hodgson and Woods suggested that the surfactants altered the draining behaviour of the thin film In their experiments, the thinning of the film was found to be relatively symmetrical in the absence of surfactants, while a dimpled film, with a thicker part at the centre and a thinner barrier ring at the circumference, commonly occurred when surfactants were present. Giribabu and Ghosh further derived a stochastic model to estimate the drop rest times based on the uneven distribution of surfactants along the interface, and the results were consistent with previous experimental data. Even though there is disagreement on how surfactants affect the film drainage, the uneven distribution of the surfactants on the interface is generally accepted by most researchers
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