Dong, Teng; (2020) Drop coalescence with liquid/liquid interfaces in the presence of surfactants. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The dissertation describes the experimental investigations on the effect of surfactants on drop coalescence with liquid/liquid interfaces. Different coalescence events are carried out in two unique-designed vessels at different dimensions. Optical methods including high-speed shadowgraphy and Particle Image Velocimetry (PIV) are applied to detect the drop surface behaviour and the drop inner dynamics. Planar Laser Induced Fluorescence (PLIF) is also utilized to detect the spatio-temporal distribution of surfactants on the coalescing drop surface. A novel flow channel is developed to investigate the surfing drops at moving liquid/liquid interfaces at last. Initially, the total coalescence of drops with the liquid/liquid interfaces, both packed with surfactants at concentrations up to the Critical Micelle Concentration (CMC), are studied. It is found that the increase of the surfactant concentration promotes the deformation of the interface before the film that separates the drop from the interface drains and ruptures. After the film rupture, two counter-rotating vortices appear inside the drop, which move faster at low surfactant concentrations. The intensities of the two counter-rotating vortices significantly decrease at increasing surfactant concentration. In the second part of the dissertation, the experimental results on the partial coalescence of drops are presented. The coalescence map based on the dimensionless numbers Oh=µ_d/(ρ_mσD)^1/2 and Bo = (ΔρgD^2)/σ is plotted to distinguish the partial coalescence region, which is found to reduce in the presence of surfactants. The size ratios of the daughter to the mother drop are measured and plotted against the Bo and Oh as well. In the gravity dominant regime, the surfactants have a negligible effect on the drop ratio, while in the inertia-capillary regime the drop size ratio decreases with increasing surfactant concentration. The velocity fields inside the partial coalescing drop are acquired. In the surfactant-free system, it is found that the inward motion of the fluids at the upper part of the drop favours the generation of a liquid cylinder in the early stages. The pressure gradient created by the downward stream at the bottom of the liquid cylinder is believed to cause the pinch-off of the cylinder and the formation of the secondary droplet. The surfactants tend to make the coalescence non-symmetric resulting in total coalescence. The spatiotemporal distributions of surfactants on the merging interfaces are presented in the following part. It is found that when a drop rests on an interface, the surfactants that have been adsorbed on the interfaces are swept outwards by the draining liquid film between the drop and the flat interface and reach a peak value at about 0.75R away from the center, where R is the horizontal drop radius. After the film ruptures, the surfactant concentration at the tip of the retreating meniscus continues to increase. Once the film has retracted to the drop sides, the concentration of the surfactants peaks at the meniscus at the bottom of the drop. The variation of the surfactant concentration along the merging interfaces in the later stages is presented as well. In the end, the delayed coalescence of drops with moving liquid-liquid interfaces is experimentally investigated. Drops are released onto the moving interfaces at velocities from 0 cm/s up to 3.4 cm/s. Drop coalescence is found to be largely delayed at increasing interface speed, which is attributed to the lubrication pressure developed in the draining film. Numerical simulations are conducted for a half-pendent drop levitating on a moving liquid-liquid interface. The results indicate that the minimum pressure appears at the front bottom of the moving drop, where most of the film ruptures are observed to take place. Also, the pressure in the film is calculated based on the local curvature of the drop surface and the tangential velocities on the drop surface and on the interface in the film region. It is found that the lubrication pressure increases with the interface speed. The PLIF technique is used to measure the drop shape and the film thickness between the drop and the interface. A dimple structure in the film is more likely to form at lower interface speeds, while the film tends to be flat when the interfaces move with high speed. During coalescence of drops with moving interfaces, in some cases when films are thicker, the liquid in the film cannot completely drain out but form drops-on-string resulting in the oil entrainment in the aqueous phase.

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