Effect of Surfactants on Drop Stability and Thin Film Drainage

Colloid & Polymer Science, 1986

A system of two emulsion droplets is examined as they mutually approach at small separations. The mass transfer of difffusion-controlled surfactants towards the interface is regarded. The cases of a surfactant soluble in only one of the phases as well as in all of them are analysed. Quantitative estimates are presented for the tangential mobility of the droplet/thin layer interface. Different regimes of mass transfer and flow in the drops and a creeping flow and various regimes of mass transfer in the thin layer between them are considered.

Proceedings of the …, 2002

The understanding of the coalescence process between two drops is essential in determining the stability of liquid-liquid systems. By solving the equation governing the evolution of the continuous phase film trapped between two drops colliding at constant velocity coupled with that describing the surfactant interfacial concentration in the lubrication approximation, we show that the interface is rendered immobile due to the presence of a small amount of surfactant. Film rupture is therefore delayed due to the Marangoni effect retarding the drainage of the film. In this study, the effects of the viscosity ratio, surface diffusivity, approach velocity and the van der Waals interaction force are reported.

Journal of Colloid and Interface Science, 1995

stability against flocculation and coalescence. The stability A simple theoretical model which allows the study of the conof suspensions containing solid particles are usually treated figuration and the interaction energy of a doublet of flocculated in the framework of the Derjaguin-Landau-Verwey-Over-Brownian droplets was recently proposed (Denkov et al., Phys. beek (DLVO) theory (1-3) which accounts for the electro-Rev. Lett. 71, 3226 (1993)). The model assumes that the doublet static and van der Waals interactions between the particles.

Journal of colloid and interface science, 2012

Here we present the exact solution of two approaching spherical droplets problem, at small Reynolds and Peclet numbers, taking into account surface shear and dilatational viscosities, Gibbs elasticity, surface and bulk diffusivities due to the presence of surfactant in both disperse and continuous phases. For large interparticle distances, the drag force coefficient, f, increases only about 50% from fully mobile to tangentially immobile interfaces, while at small distances, f can differ several orders of magnitude. There is significant influence of the degree of surface coverage, θ, on hydrodynamic resistance β for insoluble surfactant monolayers. When the surfactant is soluble only in the continuous phase the bulk diffusion suppresses the Marangoni effect only for very low values of θ, while in reverse situation, the bulk diffusion from the drop phase is more efficient and the hydrodynamic resistance is lower. Surfactants with low value of the critical micelle concentration (CMC) m...

Journal of Fluid Mechanics, 1999

A theory is developed for the hydrodynamic interactions of surfactant-covered spherical drops in creeping flows. The surfactant is insoluble, and flow-induced changes of surfactant concentration are small, i.e. the film of adsorbed surfactant is incompressible.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2000

The stability of partially mobile drainage thin liquid film formed between two slightly deformed approaching bubbles or drops is studied. The intervening film is assumed to be thermodynamically unstable. The material properties of the interfaces (surface viscosity, Gibbs elasticity, surface and bulk diffusion) are taken into account. To examine the stability of the thin film we consider the coupling between the drainage and the disturbance flows. The velocity and pressure distributions due to the drainage flow are obtained by using the lubrication approximation. The disturbance flow is examined by imposing small perturbations on the film interfaces and liquid flow. The long wave approximation is applied. We solved the linear problem for the evolution of the fluctuations in the local film thickness, interfacial velocity and pressure. The linear stability analysis of the gap region allows us to calculate the critical thickness, at which the system becomes unstable. Quantitative explanation of the following effects is proposed, (i) the increase of critical thickness with the increase of the interfacial mobility; (ii) the role of surface viscosity, compared with that of the Gibbs elasticity; (iii) the significant destabilization of the gap region with the decreasing droplet radius in the case of buoyancy driven motion. The analytical expressions for critical thickness in the case of negligible surface viscosity and tangentially immobile interfaces are presented. : S 0 9 2 7 -7 7 5 7 ( 0 0 ) 0 0 6 2 1 -X

2018

Film drainage and rupture during coalescence between two drops approaching each other under a constant force in the presence of insoluble surfactants affected by the van der Waals forces is studied numerically. The mathematical problem is based on the coupled equations of flow in each phase and the convection-diffusion equation governing the surfactant distribution at the interface as well as the related Marangoni effects. The latter are subject to the boundary conditions in the limit of gentle interactions (small-deformations) for which the drops are nearly spherical, except in the near-contact region, where a flattened thin film forms. The finite difference method is used to discretize the lubrication equation in the gap and the interfacial diffusion-convection equation while the boundary integral method is employed to solve the flow in the drops. In this work, a parametric study is carried out by a numerical simulation for Peclet numbers (Pe) ranging from 102 to 104, initial dime...

Journal of Colloid and Interface Science, 2000

The deformation, drainage, and rupture of an axisymmetrical film between colliding drops in the presence of insoluble surfactants under the influence of van der Waals forces is studied numerically at small capillary and Reynolds numbers and small surfactant concentrations.

1996

The effects of alkyl chain length and concentration of surfactant the radius of the particle or of the curvature for the mica on the yield stress-pH behavior of a 57 wt% ZrO 2 suspension sheet, A and B are proportionality constants, and l 1 and l 2 were evaluated. An alkyl chain length n of between 5 to 12 carbons are characteristic decay lengths of a short-and a long-range was used. For chain length n § 7, a much larger maximum yield component, respectively (9-11). l 1 is on the order of about stress (at zero zeta potential) than that without additive was re-25 Å and l 2 is about 250 Å. The short-range component corded. The increase in the maximum yield stress is as much as was shown to be insensitive to monovalent salt concentration sixfold. Generally, a higher surfactant concentration leads to a (3, 4). higher yield stress. The increase in the maximum yield stress is The measured SFA force F(H) is related to the free enquite insensitive to surfactant chain length. Ionic strength appears to have no effect on the yield stress-pH behavior of suspensions ergy of interaction per unit area (E f (H)) via the Derjaguin with a surfactant. The data are consistent with the observation of approximation, a long-range attractive (hydrophobic) force measured in surface force experiments.

International Journal of Mineral Processing, 2005

The properties of thin liquid films (TLF) are of paramount significance for colloidal disperse systems, and a number of industrial processes, including froth flotation. In flotation, the bubble-particle attachment is controlled by the thinning and rupture of the intervening liquid film between an air bubble and a mineral particle. The froth evolution and its transient stability are also a function of the drainage and rupture of liquid films between air bubbles. Surface-active substances (surfactants) are used as flotation reagents to control the behavior of the liquid films. This paper presents a review of our research in the area of surfactant adsorption, surface forces and liquid films. It mainly focuses on the validation, application and extension of the Stefan-Reynolds theory on the liquid drainage. The extension of the Stefan-Reynolds theory comprises surface forces (disjoining pressure), surface tension variation, caused by the adsorption and diffusion of surfactants. Both the experimental and theoretical results are mostly related to the free (foam) films formed between two bubbles but can be principally extended to emulsion films between two oil drops and wetting films between an air bubble and a solid surface. D