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ABSTRACT: Solutions of surfactant-polymer mixtures often exhibit different foaming properties, compared to the solutions of the individual components, due to the strong tendency for formation of polymer-surfactant complexes in the bulk and on the surface of the mixed solutions. A generally shared view in the literature is that electrostatic interactions govern the formation of these complexes, for example between anionic surfactants and cationic polymers. In this study we combine foam tests with model experiments to evaluate and explain the effect of several polymer-surfactant mixtures on the foaminess and foam stability of the respective solutions. Anionic, cationic, and nonionic surfactants (SDS, C(12)TAB, and C(12)EO(23)) were studied to clarify the role of surfactant charge. Highly hydrophilic cationic and nonionic polymers (polyvinylamine and polyvinylformamide, respectivey) were chosen to eliminate the (more trivial) effect of direct hydrophobic interactions between the surfactant tails and the hydrophobic regions on the polymer chains. Our experiments showed clearly that the presence of opposite charges is not a necessary condition for boosting the foaminess and foam stability in the surfactant-polymer mixtures studied. Clear foam boosting (synergistic) effects were observed in the mixtures of cationic surfactant and cationic polymer, cationic surfactant and nonionic polymer, and anionic surfactant and nonionic polymer. The mixtures of anionic surfactant and cationic polymer showed improved foam stability, however, the foaminess was strongly reduced, as compared to the surfactant solutions without polymer. No significant synergistic or antagonistic effects were observed for the mixture of nonionic surfactant (with low critical micelle concentration) and nonionic polymer. The results from the model experiments allowed us to explain the observed trends by the different adsorption dynamics and complex formation pattern in the systems studied.
Langmuir 03/2012; 28(11):4996-5009. · 4.19 Impact Factor
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ABSTRACT: Foam and emulsion jamming at low shear rates is explained by considering the thinning dynamics of the transient films, formed between neighboring bubbles and drops. After thinning gradually to a critical thickness, these films undergo an instability transition, which leads to the formation of very thin "black films" providing strong adhesion between the dispersed particles. Analysis shows that such film thickness instability occurs only if the contact time between particles is sufficiently long-an explicit expression for the respective critical shear rate is derived and compared to experimental data.
Physical Review Letters 09/2009; 103(11):118302. · 7.37 Impact Factor
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ABSTRACT: This experimental study is focused on the process of bubble breakup in steadily sheared foams, at constant shear rate or constant shear stress. Two different types of surfactants were used and glycerol was added to the aqueous phase, to check how the bubble breakup depends on the surface modulus and on bulk viscosity of the foaming solutions. The experiments show that bubble breakup in foams occurs above a well defined critical dimensionless stress, tau[over]CR identical with(tauCRR/sigma) approximately 0.40, which is independent of surfactant used, solution viscosity, and bubble volume fraction (varied between 92 and 98%). Here tauCR is the dimensional shear stress, above which a bubble with radius R and surface tension sigma would break in sheared foam. The value of the critical stress experimentally found by us tau[over]CR approximately 0.40, is about two orders of magnitude lower than the critical stress for breakup of single bubbles in sheared Newtonian liquids, tau[over]CR approximately 25. This large difference in the critical stress is explained by the strong interaction between neighboring bubbles in densely populated foams, which facilitates bubble subdivision into smaller bubbles. A strong effect of bubble polydispersity on the kinetics of bubble breakup (at similar mean bubble size) was observed and explained. Experiments were also performed with hexadecane-in-water emulsions of drop volume fraction 83%<or=Phi<or=95% to study drop breakup in concentrated emulsions. Qualitatively similar behavior was observed to that of foams, with the critical dimensionless stress for drop breakup being lower, tau[over]CR approximately 0.15, and practically independent of the drop volume fraction and viscosity ratio (varied between 0.01 and 1). This critical stress is by several times lower than the critical stress for breakage of single drops in sheared Newtonian fluids at comparable viscosity ratio, which evidences for facilitated drop subdivision in concentrated emulsions. To explain the measured low values of the critical stress, a different type of capillary instability of the breaking bubbles and drops in concentrated foams and emulsions is proposed and discussed.
Physical Review E 11/2008; 78(5 Pt 1):051405. · 2.26 Impact Factor
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ABSTRACT: A new class of surfactant mixtures is described, which is particularly suitable for studies related to foam dynamics, such as studies of foam rheology, liquid drainage from foams and foam films, and bubble coarsening and rearrangement. These mixtures contain an anionic surfactant, a zwitterionic surfactant, and fatty acids (e.g., myristic or lauric) of low concentration. Solutions of these surfactant mixtures exhibit Newtonian behavior, and their viscosity could be varied by using glycerol. Most importantly, the dynamic surface properties of these solutions, such as their surface dilatational modulus, strongly depend on the presence and on the chain-length of fatty acid(s). Illustrative results are shown to demonstrate the dependence of solution properties on the composition of the surfactant mixture, and the resulting effects on foam rheological properties, foam film drainage, and bubble Ostwald ripening. The observed high surface modulus in the presence of fatty acids is explained with the formation of a surface condensed phase of fatty acid molecules in the surfactant adsorption layer.
Langmuir 10/2008; 24(18):9956-61. · 4.19 Impact Factor
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ABSTRACT: In a recent Letter [N. D. Denkov, Phys. Rev. Lett. 100, 138301 (2008)] we calculated theoretically the macroscopic viscous stress of steadily sheared foam or emulsion from the energy dissipated inside the transient planar films, formed between neighboring bubbles or drops in the shear flow. The model predicts that the viscous stress in these systems should be proportional to Ca 1/2, where Ca is a capillary number and n=1/2 is the power-law index. In the current paper we explain our model in detail and develop it further in several aspects: First, we extend the model to account for the effects of viscous friction in the curved meniscus regions, surrounding the planar films, on the dynamics of film formation and on the total viscous stress. Second, we consider the effects of surface forces (electrostatic, van der Waals, etc.) acting between the surfaces of the neighboring bubbles or drops and show that these forces could be important in emulsions, due to the relatively small thickness of emulsion films (often comparable to the range of action of surface forces). In contrast, the surface forces are usually negligible in steadily sheared foams, because the dynamic foam films are thicker than the extent of surface forces, except for foams containing micrometer-sized bubbles and/or at very low shear rates. Third, additional consideration is made for bubbles or drops exhibiting high surface viscosity, for which we demonstrate an additional contribution to the macroscopic viscous stress, created by the surface dissipation of energy. The new upgraded model predicts that the energy dissipation at the bubble or drop surface leads to power-law index n<1/2 , whereas the contribution of the surface forces leads to n>1/2 , which explains the rich variety of foam or emulsion behaviors observed upon steady shear. Various comparisons are made between model predictions and experimental results for both foams and emulsions, and very good agreement is found.
Physical Review E 07/2008; 78(1 Pt 1):011405. · 2.26 Impact Factor
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ABSTRACT: We present a model for the viscous friction in foams and concentrated emulsions, subject to steady shear flow. First, we calculate the energy dissipated due to viscous friction inside the films between two neighboring bubbles or drops, which slide along each other in the flow. Next, from this energy we calculate the macroscopic viscous stress of the sheared foam or emulsion. The model predictions agree well with experimental results obtained with foams and emulsions.
Physical Review Letters 04/2008; 100(13):138301. · 7.37 Impact Factor
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ABSTRACT: The aim of this paper is to present a short overview of the main mechanisms operative in the formation and stabilization of emulsions by solid particles and, on this basis, to make comparisons between solid particles, surfactants and globular proteins as emulsifiers. When available, simple quantitative relations are presented, with the respective numerical estimates and discussion of the applicability of these relations to particle-stabilized systems. Non-obvious similarities between the different types of emulsifiers are outlined in several cases in which the description of the system can be performed at a phenomenological level. Examples are presented for the process of emulsification, where we show that several simple theoretical expressions, derived originally in the studies of surfactants and protein emulsifiers, can be successfully applied to particle-stabilized emulsions. In contrast, for the phenomena in which the detailed mechanisms of particle adsorption and film stabilization are important, the differences between the various emulsifiers prevail, thus making it impossible to use the same theoretical description. The most important specific characteristics of the solid particles which strongly affect their behavior are the high barrier to particle adsorption, high desorption energy and strong capillary forces between particles trapped in liquid films, which all originate in the relatively large particle size (as compared to the size of surfactant and protein molecules). The capillary mechanism of stabilization of liquid films by solid particles is reviewed in some detail, to emphasize its specific features and to demonstrate the applicability of several simple expressions for approximate estimates. Interestingly, we found that the hypothesis for some exceptionally high coalescence stability of the particle-stabilized emulsions is not supported by the experimental data available in literature. On the other hand, the particles are able to completely arrest the process of Ostwald ripening in foams and emulsions, and this effect can be easily explained with the high desorption energy of the particles and the resulting capillary effects.
Physical Chemistry Chemical Physics 04/2008; 10(12):1608-27. · 3.57 Impact Factor
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ABSTRACT: Here, we investigate water-in-oil (W/O) emulsions that are stabilized by polystyrene latex particles with sulfate surface groups. The particles, which play the role of emulsifier, are initially contained in the disperse (water) phase. The existence of such emulsions formally contradicts the empirical Bancroft rule. Theoretical considerations predict that the drop diameter has to be inversely proportional to the particle concentration, but should be independent of the volume fraction of water. In addition, there should be a second emulsification regime, in which the drop diameter is determined by the input mechanical energy during the homogenization. The existence of these two regimes has been experimentally confirmed, and the obtained data agree well with the theoretical model. Stable W/O emulsions have been produced with hexadecane and tetradecane, while, in the case of more viscous and polar oils (soybean and silicone oil), the particles enter into the oily phase, and Pickering emulsions cannot be obtained. The formation of stable emulsions demands the presence of a relatively high concentration of electrolyte that lowers the electrostatic barrier to particle adsorption at the oil-water interface. Because the attachment of particles at the drop surfaces represents a kind of coagulation, it turns out that the Schulze-Hardy rule for the critical concentration of coagulation is applicable also to emulsification, which has been confirmed with suspensions containing Na(+), Mg(2+), and Al(3+) counterions. The increase of the particle and electrolyte concentrations and the decrease of the volume fraction of water are other factors that facilitate emulsification in the investigated system. To quantify the combined action of these factors, an experimental stability-instability diagram has been obtained.
Langmuir 06/2006; 22(11):4968-77. · 4.19 Impact Factor
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ABSTRACT: The process of exhaustion of mixed PDMA-silica antifoams was studied by several experimental methods. It was found that the globules in an active fast antifoam are deformable and have a low entry barrier. FTT experiments proved that the process of antifoam exhaustion is related to a gradual segregation of the oil from the silica. Ellipsometric measurements showed that the thickness of the spread oil layer decreases in the course of exhaustion.
Langmuir 02/2003; 19(7):3084-3089. · 4.19 Impact Factor
