Hydrodynamics of thin liquid films: Retrospective and perspectives.

This review presents a summary of the results in the domain of microscopic liquid film hydrodynamics for several decades of experimental and theoretical research. It mainly focuses on the validation, application and further development of the Stefan-Reynolds theory on the liquid drainage, based on the accumulated knowledge of surface forces, surface tension caused by the surfactant adsorption, and diffusion of surfactants. Liquid films are of primary significance for colloidal disperse systems, and diverse industrial processes. The transient stability of the froth phase and the froth drainage is a function of the drainage and rupture of liquid films between air bubbles. 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. Both the experimental and theoretical results are mostly related to the foam liquid films between two bubbles, but can be principally generalized for emulsion films, formed in another liquid, as well as wetting films between a bubble and a solid surface.

Equilibrium and dynamic osmotic behaviour of aqueous solutions with varied concentration at constant and variable volume.

Osmosis is essential for the living organisms. In biological systems the process usually occurs in confined volumes and may express specific features. The osmotic pressure in aqueous solutions was studied here experimentally as a function of solute concentration (0.05-0.5 M) in two different regimes: of constant and variable solution volume. Sucrose, a biologically active substance, was chosen as a reference solute for the complex tests. A custom made osmotic cell was used. A novel operative experimental approach, employing limited variation of the solution volume, was developed and applied for the purpose. The established equilibrium values of the osmotic pressure are in agreement with the theoretical expectations and do not exhibit any evident differences for both regimes. In contrast, the obtained kinetic dependences reveal striking divergence in the rates of the process at constant and varied solution volume for the respective solute concentrations. The rise of pressure is much faster at constant solution volume, while the solvent influx is many times greater in the regime of variable volume. The results obtained suggest a feasible mechanism for the way in which the living cells rapidly achieve osmotic equilibrium upon changes in the environment.

Streaming potential effect on the drainage of thin liquid films stabilized by ionic surfactants.

Dynamic effects originating from the electric double layers (EDL) are studied in thin liquid films (TLF) containing ionic and nonionic surfactants. To account for such effects, the EDL are to be incorporated into the differential equations describing the TLF drainage. Numerical simulations in the literature have shown that foam films containing ionic surfactants can drain at a slower rate than that predicted by the Reynolds equation (V(Re)) which postulates rigid planar film surfaces. However, the physical reason of the trend has remained unclarified, and the numerical results have not been validated by any experimental data. In the present study, experiments on the drainage of planar foam films were conducted with the anionic surfactant sodium dodecylsulfate (SDS) in the presence of additional electrolyte (0.02 M NaCl) and with the cationic tetrapentylammonium bromide (TPAB). The obtained results are in accord with the numerical simulations from the literature (V/V(Re) < 1). Such behavior was observed already in our preceding experiments on planar TLF with SDS without added electrolyte. These results were compared to the data of the experiments with TLF containing nonionic surfactant, and differences in the drainage pattern between ionics and nonionics were established. A new theoretical model was developed to account for the dynamic effects arising from EDL. According to the present model, the liquid outflow drags the bulk charges of EDL toward the film border, thus generating streaming potential (as in capillary tubes), which in turn brings the charges back toward the center to maintain the state of zero total electrical current. This creates reverse convection of the liquid near the surfaces, resulting in a velocity of film drainage smaller than V(Re). The present theory predicts kinetic dependence closer to the experiment than the Reynolds equation. The limitations of this new model are specified: it is valid for high ionic strength or low value of the surface potential.

Effect of ionic surfactants on drainage and equilibrium thickness of emulsion films.

This paper presents new theoretical and experimental results that quantify the role of surfactant adsorption and the related interfacial tension changes and interfacial forces in the emulsion film drainage and equilibrium. The experimental results were obtained with plane-parallel microscopic films from aqueous sodium dodecyl sulphate solutions formed between two toluene droplets using an improved micro-interferometric technique. The comparison between the theory and the experimental data show that the emulsion film drainage and equilibrium are controlled by the DLVO interfacial forces. The effect of interfacial viscosity and interfacial tension gradient (the Marangoni number) on the film drainage is also significant.

A novel technique for improving interferometric determination of emulsion film thickness by digital filtration.

Determination of the thickness of emulsion films by using the film interferometric images is usually less accurate than that of foam films, due to the close values of the refractive indices of the liquid film and adjacent liquid phases (hence, low contrast and high level of noise at high magnification). A new technique was developed to improve the thickness determination by obtaining the interferometric images without directly filtering the illuminating light, as is usually done in the classical Scheludko interferometric technique. The new method then uses digital filtration during the off-line image post-processing to obtain monochromatic interferometric images required for the thickness determination. The technique was tested with foam films stabilised by sodium dodecyl sulfate and successfully applied to determine thickness of toluene-water-toluene emulsion films using the green and red digital filters. Results for emulsion film thickness determined by either the green or red digital filtration are comparable, thus validating the new technique developed here for emulsion films.

Critical thickness of microscopic thin liquid films.

This paper outlines the progress achieved during the four decades of research on the spontaneous destruction of the thinning microscopic liquid films through rupture or black spot formation at the so-called critical thickness. Although most of both experimental and theoretical results are primarily related to the foam films that form between gas bubbles, in many respects they can be principally generalized for emulsion films, as well as the wetting films confined between a bubble and a solid surface. The paper focuses on the validation, application and extension of the theory of the phenomenon. The experimental results are analysed with respect to the frequently observed deviations from the widely used model of a planar circular film with tangentially immobile surfaces. The applicability of the new theory of accelerated drainage due to spatial variation in thickness is expressed. The effects of surface tension, surface mobility, variation of the film size, and spatial thickness heterogeneity on the critical thickness are compared.

Effect of interactions between the adsorbed species on the properties of single and mixed-surfactant monolayers at the air/water interface.

Experimental data on surface tension available from the literature and generated in the present study are analyzed to estimate the applicability of adsorption models, based on the Frumkin equation, to nonionic and ionic surfactants and their mixtures. Optimization programs based on the least-squares method in media of Delphi V and Pascal VII are used. The effect of interactions between the adsorbed species on surface tension is considered in all cases. The results are compared to those obtained with the simpler Szyszkowski equation, employed in numerous studies of nonionic surfactants, when interactions are neglected. Cases where the Frumkin model can be successfully employed with ionic surfactants and mixtures are presented and the conditions of its applicability are analyzed. Related characteristic quantities (maximum adsorption, standard free energy of surfactant adsorption, energy of interaction between adsorbed species, standard free energy of counterion adsorption, degree of coverage by surfactant/counterion associates) are established as a function of: The properties of an adsorption layer from a mixture of nonionic and ionic surface-active species are compared to those of the single surfactants.