Elena Mileva

Foam films as instrumentation in the study of amphiphile self-assembly

Abstract A survey on recent experimental investigations of microscopic foam films containing self-assembled amphiphilic structures is presented. A specific advantage of the microscopic film techniques is that the fine control of system parameters allows the estimation of the consecutive changes of film properties for low surfactant content and extremely small concentration changes. This gives a unique possibility to reach amphiphile quantities when initial onset of self-assembly is to be observed. The film characteristics are investigated via microinterferometric method, which operates with the measuring cell of Scheludko-Exerowa. The experimental set is additionally improved by including video-recording and consecutive image analysis. The results show the following: (1) Unstable black patterns (dots and spots) are observed; they have very short lifetimes and the films, which contain them rupture quickly. (2) Several of the kinetic characteristics of the films display a sharp change within a narrow surfactant concentration range. The experiments are interpreted on the basis of the assumption that a series of smaller self-assembled aggregates (premicelles) with various geometries exist at the interfaces and inside the thin film. The proposed theoretical scheme puts forward a mechanism connecting the formation of unstable black patterns (dots and spots) with the reorganization and destruction of the existing surfactant assemblies both in the bulk of the film and on the interfaces. The results suggest that the observed unstable black formations may serve as indicators for the presence of surfactant structures in amphiphilic solutions and the microscopic foam-film techniques has a serious potential as a prospective instrumentation in the study of amphiphilic self-assemblies.

Black dots as a detector of self-assembly in thin liquid films

Abstract Experiments with different types of amphiphilic substances exhibit two stable tendencies. (1) The surface tension/concentration curves (Δσ/Cs) display well-defined kink and plateau sectors. (2) Free thin liquid films (foam films) from solutions with concentrations in the plateau regions drain until ‘black dots’ appear and then rupture almost immediately. A general theoretical scheme is developed that suggests a mechanism connecting the formation of black dots with the onset and the reorganization of smaller self-assembled aggregates (premicelles) in the bulk of the solution and on the interfaces. The results show that the black dots in free thin liquid films may serve as an indicator of the presence and reorganization of self-assembled structures in amphiphilic solutions.

Mass transfer of surfactants and hydrodynamic interaction at small separations in emulsion systems 1. Scaling concepts

Abstract A model investigation of the coupling of the mass transfer of surfactants and the fluid flows in emulsion systems is presented. The case of close approach of two emulsion droplets, when a thin liquid layer is formed in the narrowest parts between the fluid interfaces, is analysed. New scaling parameters are proposed, based on the introduction of the surface Peclet number Pes. The increase in this surface analogue of the bulk Peclet number results in an increased mobility of the interfaces and in a reduced influence of the surfactant. The validity of the lubrication theory model for the drainage of the thin layers is outlined in terms of the tangential mobility and the Gibbs elasticity of the interfaces. The influence of surfactants on the tangential mobility and the velocity of coalescence is discussed.

Trajectories of fine particles in a boundary layer at a plate

ABSTRACT The disturbance of the bounary layer on a plate by tiny solid spheres is studied. The range of the particle dimensions is

Mass transfer of surfactants and hydrodynamic interaction at small separations in emulsion systems 2. Potential theory

(L being the length of the plate and Rei is the background Reynolds number). Two model cases are analysed -- viscous and inertial-viscous. The forces that act on the solids inside the layer flow are calculated. The trajectories of particles with different radii are presented. The basic result is that fine solids are retained inside the boundary layer. This entrapment is maximal for particles whose dimensions are in the interval

Entrapment efficiencies of hydrodynamic boundary layers on rising bubbles

Abstract Scaling parameters are applied in order to propose a physically adequate asymptotic model for the coupling of the mass transfer of soluble surfactants and the fluid flows in emulsion systems. The hydrodynamic interaction at small gap widths is studied when thin liquid layers are formed. The classical potential theory is used for the numerical calculations. Expressions are obtained for the tangential mobility of the fluid flow interfaces and the drag force in the cases of low and high viscosities of the disperse phase.

Interaction of a solid particle with boundary layer on a bubble

Flotation and separation practice shows that fine hydrophilic solids are often drawn into the froth product. The occurrence of this unwanted event in the classical froth flotation has led to the idea of using it for the separation by size of ground materials. Thus, a method for the extraction of hydrophilic fines by foaming of a suspension was proposed. The aim of the present study is to relate this phenomenon to the residence time of the particles in the vicinities of the rising bubbles. Dynamic interactions of fine solids with rising bubbles cause perturbations in the background flow field. A procedure for the mathematical modeling of these disturbance effects is proposed. The initial idea is that the particles lag behind the background bubble-driven flows. A key point is the possibility of classifying fine entities according to a general criterion, containing only parameters of the outer flow. The basic result is that there exists a range of particle and bubble dimensions for which this entrapment is optimal. The proposed model investigation gives a concise explanation for the observed capture of fine solids in many flotation and separation processes.

Fine particles inside boundary layers on rising bubbles

Abstract The hydrodynamic interaction of a solid particle with a boundary layer on a rising bubble is analyzed. A scaling procedure, based on the concept that the particle perturbs the background flow only in the close vicinity of its surface, is proposed. Two basic types of disturbance are studie: viscous interaction and inertial effects. Various subcases are specified. They are defined according to a universal criterion for the dimensions of the entrapped solids with regard to a dynamic distance, comprising characteristic parameters only of the boundary-layer flow region. Various sets of asymptotic equations that govern the hydrodynamic interaction are obtained. For the small and medium size particles with radii R p R B R B 3 4 , the leading order effect is the viscous interaction in the direction across the boundary layer thinkness. The tangential viscous and the inertial terms appear in higher approximations. The larger solids with R p ϵ [ R B R B 3 4 , R B R B 1 2 ] are dominated by an inertial type of perturbation.

DISTURBANCE FLOW FIELD OF A SOLID PARTICLE IN A BOUNDARY LAYER ALONG A PLATE

Abstract The aim of the present study is to find the individual paths of fine hydrophilic particles inside boundary layers on rising bubbles. Single solids with different dimensions are treated on the basis of the analytical model developed previously [E. Mileva, L. Nikolov, Colloids Surf. A132 (1998) 95]. Explicit formulae for the forces that act on these particles are presented. The obtained trajectories show that the bubbles do retain fine particles of certain dimensions. In general, smaller bubbles are more efficient in retaining micron species. Larger bubbles predominantly eject the already entrained solids. The proposed model investigation gives an explanation for the observed entrapment of fine solids in some separation processes.

Combined Sum Frequency Generation and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on the Interfacial Water Structure

ABSTRACT The perturbation flow field that appears as a result of the presence of a spherical solid inside a boundary layer along a plate is quantitatively analyzed. The problem of the hydrodynamic interaction is reduced to the solution of a partial differential equation for the longitudinal disturbance velocity. The results are presented in an analytical form. A general overview of the disturbance flow field is presented as a function of the increasing effective dimensions of the particle. It was established that there exists an optimum range of particles dimensions