Coalescence stability of water-in-oil drops: Effects of drop size and surfactant concentration

Abstract

We study the effects of (1) drop size; (2) surfactant chain-length and concentration; and (3) viscosity of the oil phase on the stability of water drops, pressed by gravity towards planar oilwater interface. The experimental results show that at low surfactant concentrations (around and below the CMC) the drop lifetime is controlled by the drainage time of the oily film, viz. by the time for reaching the critical film thickness at which this film ruptures. The small drops coalesce before the formation of a planar film with the large interface and, as a consequence, their lifetime rapidly decreases with the increase of drop diameter. In contrast, the bigger drops coalesce after formation of a plane-parallel film and their lifetime increases with the drop diameter. Therefore, the drop lifetime passes through a minimum when varying the drop diameter. The obtained results at low surfactant concentrations and for small drops are described well by theoretical models, available in the literature. The lifetime of the respective larger drops is shorter, as compared to the theoretical predictions, due to the faster thinning of the oil films which are with uneven thickness. The increase of surfactant concentration above the CMC leads to a significant increase in the stability of the small drops. Interestingly, the stability of the intermediate in size drops remains low for all surfactants and oils studied. At higher surfactant concentrations, the stability of the large water drops increases significantly when Span 80 is used, while it remains rather low for drops stabilized by Span 20. The results obtained with single water drops are in a good agreement with the stability of the respective batch water-in-oil emulsions.

Lifetime of Surface Bubbles in Surfactant Solutions

“…Finally, bubbles having an equivalent radius corresponding to Bo 5 have a smaller lifetime, which can be rationalized by a too small "rigidity" parameter, Λ = Ma/Bo 0.05. It should be noted that, experimental and theoretical studies on the lifetime of drops beneath a free surface have shown that, for large drops, Bo 1, the lifetime increases with their size [37,38,39] while, as seen in this study and the one of Nguyen et al (2013), the lifetime of bubbles decreases with their size for Bo 1 (see Fig 6a of Nguyen et al (2013)). In the case of drops, the inner liquid viscosity might not be negligible when compared to the outer phase viscosity.…”

“…In that respect, the drainage of a liquid film on the top of a bubble is expected to have a different dynamics when compared to drops. For example, while Nguyen et al (2013) found that the lifetime of a large bubble decreases as the size of the bubble increases, studies on drops showed the opposite [37], [38], [39].…”

Coalescence of Binary Droplets in the Transformer Oil Based on Small Amounts of Polymer: Effects of Initial Droplet Diameter and Collision Parameter

“…The results indicated that the coalescence of droplets could be accelerated by changing operating parameters such as temperature and electric field. The investigation of the size distribution of liquid droplets and droplet coalescence stability is of high importance in water-oil two-phase dispersed flow [ 20 ]. Fortelný et al reviewed the applicability of droplet diameter distribution in the processing of immiscible polymer blends [ 21 ].…”

Study of Parameters Affecting the Coalescence of Dimethyl Disulfide Drops in a Merox Unit

“…The validity of the model was demonstrated by applying it to a range of model problems. In the same year, Politova et al (2017) studied the effects of drop size and viscosity of the oil phase on the stability of water drops moving towards a planar oil-water interface. They found small drops coalescence to occur before formation of a planar film at the interface.…”