Abstract
In this work the coalescence of an aqueous drop with a flat aqueous-organic interface was investigated in a thin gap Hele-Shaw cell. Different concentrations of a nonionic surfactant (Span 80) dissolved in the organic phase were studied. We present experimental results on the velocity field inside a coalescing droplet in the presence of surfactants. The evolution of the neck between the drop and the interface was studied with high-speed imaging. It was found that the time evolution of the neck at the initial stages of coalescence follows a linear trend, which suggests that the local surfactant concentration at the neck region for this stage of coalescence can be considered quasiconstant in time. This neck expansion can be described by the linear law developed for pure systems when the surfactant concentration at the neck is assumed higher than in the bulk solution. In addition, velocity and vorticity fields were computed inside the coalescing droplet and the bulk homophase using a high-speed shadowgraphy technique. The significant wall effects in the Hele-Shaw cell in the transverse axis cause the two vertical velocity components towards the singularity rupture point, from the drop and from the bulk homophase, to be of the same order of magnitude. This movement together with the neck expansion creates two pairs of counteracting vortices in the drop and in the bulk phase. The neck velocity is the average of the advection velocities of the two counteracting vortex pairs on each side of the neck. The presence of the surfactant slows down the dynamics of the coalescence, affects the propagation direction of the pair of vortices in the bulk phase, and reduces their size faster compared to the system without surfactant.
Highlights
When two bodies of the same liquid come into contact within a second immiscible liquid, a thin film of the surrounding immiscible phase is formed between them
A similar geometrical modeling has been used by Yokota and Okumura [23] to describe the dynamics of droplet coalescence for different viscosity fluids in a Hele-Shaw cell
The coalescence of an aqueous drop with a liquid-liquid flat interface was investigated in a Hele-Shaw cell for different concentrations of an oil soluble nonionic surfactant
Summary
When two bodies of the same liquid come into contact within a second immiscible liquid, a thin film of the surrounding immiscible phase is formed between them. This film will gradually drain until it ruptures and the coalescence of the two bodies is initiated. The study of coalescence directly in the complex environment where drops appear (i.e., dispersions, emulsions, and separators) can prove challenging. Investigating a controlled system of either an isolated drop coalescing with the flat surface of a bath of the same liquid (homophase), or with another drop, allows detailed observations to be made. This study will focus on coalescence between a droplet and a flat interface
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