Coalescence of Water Drops at an Oil–Water Interface Loaded with Microparticles and Surfactants

Abstract

This work investigates the coalescence of water droplets settled on a water−oil interface in the presence of microparticles and surfactant. The successive stages of the coalescence process, including interstitial film formation, drainage, rupture, and retraction, are analyzed in detail. This leads us to distinguish between contrasted situations depending on the nature of the surfactant and its affinity with the microparticles. Hydrophilic particles have been previously shown to promote coalescence by means of a bridging mechanism. In that case, coalescence is a deterministic process that lasts the time required for the drainage to make the film thickness equal to the size of the particles. However, the present study shows how surfactants can totally change the effect of the particles upon coalescence. When surfactant both stabilizes the water−oil interface and adsorbs onto the particles, the bridging mechanism is inhibited and the coalescence becomes a random process. Since molecular forces between facing film interfaces are not attractive, thermal fluctuations are required to initiate the formation of a hole in the adsorbed surfactant layer. Provided the surfactant concentration in the bulk is large enough to ensure that the interfaces are close to saturation, the coalescence is delayed by a stochastic time interval and the drop coalescence becomes a Poisson process. These results shed a new light on the mechanisms of droplet coalescence in complex industrial applications where surfactant and particles are present, either purposely added or present as uncontrolled contaminants. Figure 1. Schematic of the experimental setup.

Effect of a Surfactant Mixture on Coalescence Occurring in Concentrated Emulsions: The Hole Nucleation Theory Revisited

“…For concentrated emulsion systems having high amounts of surfactant, the thickness of the interstitial thin film between two droplets fully covered by surfactant molecules can reach the size of a molecule . In such a regime under which the rupture of a thin film is governed by thermal fluctuations, − coalescence is expected to be a stochastic phenomenon with a probability for the fusion of two droplets that increases with their contact area. − This coalescence scenario has recently been experimentally confirmed in 2D foams . The coalescence rate per unit contact area, ω, is therefore a key parameter for emulsion science because it determines the stability of such emulsion systems.…”

Physical mechanisms for droplet size and effective viscosity asymmetries in turbulent emulsions

“…We hypothesize that the possible reason for the asymmetry of the droplet size between O/W and W/O emulsions is the presence of surface-active contaminants. These surface-active contaminants are found widely on the liquid-liquid interface in practical environments, which is focused on by various studies related to interfacial phenomena (de Gennes 2001;De Malmazet et al 2015;Calvo et al 2019). On the one hand, these surface-active contaminants can modify the interfacial properties, yielding the change of the droplet size in the emulsion (Bazazi & Hejazi 2020;Manikantan & Squires 2020).…”

Evaluation of the Hemocompatibility and Anticancer Potential of Poly(ε-Caprolactone) and Poly(3-Hydroxybutyrate) Microcarriers with Encapsulated Chrysin

“…It is well-known that the surface area to volume ratio of micro- For comparative purposes, Table 2 presents the data reported in the literature on cytotoxic IC 50 values of chrysin-loaded nano-formulations in several cancer cell lines. In most studied cases, nano-formulated chrysin showed lower IC 50 values compared to free chrysin against various cancer cell lines, such as AGS, T47D, and MCF-7 [41,63,64], whereas in our case, both types of micro-formulated chrysin showed higher IC 50 values, suggesting that the chrysin released from MCs was more slowly taken up by cells, potentially due to the slow release of chrysin, the limited degradation rates of the employed biopolymers [39], and the micro-dimensions of the produced carriers that affect cellular uptake to a certain degree. It is well-known that the surface area to volume ratio of microparticles is relatively low compared to that of nanoparticles [70].…”

“…The PVA used was 87–90% hydrolyzed, which is a degree of hydrolysis that ensures the optimum solubility of PVA in water [ 48 ]. In general, the addition of a PVA surfactant as a stabilizing and emulsifying agent enhances the stability of the dispersed phase droplets formed during the process of emulsification via the emerging interactions between the hydroxyl groups in its structure with the aqueous phase and the vinyl chain with the organic phase, thus inhibiting microsphere flocculation and coalescence [ 49 , 50 ]. Furthermore, the addition of the highly hydrophilic PVA limits the hydrophobic nature of the produced micro-formulations, promoting the formation of more amphiphilic species [ 31 ].…”