Here we study the factors affecting the foam generation in a planetary mixer with a series of surfactant solutions, having different dynamic surface tensions, surface dilatational moduli and bulk viscosities. The foam generation in this device consists of three well defined periods: (1) Induction period during which very slow increase of the foam volume is observed. The duration of this period depends significantly on the shear rate during foaming and on the volume of the surfactant solution; (2) Fast increase of foam volume; the rate of this process depends primarily on the shear rate and dynamic surface tension of the solutions; (3) Plateau region in which the foam volume remains constant. The experimental results show that the processes of air entrapment ends when a certain critical (dimensionless) shear stress of the foam is reached. Depending on the bulk and surface properties of the surfactant solutions, this critical stress is achieved for foams with different air volume fractions and mean bubble sizes. Thus, when solutions with higher bulk viscosity and/or higher surface modulus are used, the critical stress is reached at lower air volume fraction and with smaller bubbles. Power-law equations are shown to describe very well the effects of the foaming shear rate and solution viscosity on the final foam volume and mean bubble size.

We study the effects of (1) drop size; (2) surfactant chain-length and concentration; and (3) viscosity of the oil phase on the stability of water drops, pressed by gravity towards planar oil-water interface. The experimental results show that at low surfactant concentrations (around and below the CMC) the drop lifetime is controlled by the drainage time of the oily film, viz. by the time for reaching the critical film thickness at which this film ruptures. The small drops coalesce before the formation of a planar film with the large interface and, as a consequence, their lifetime rapidly decreases with the increase of drop diameter. In contrast, the bigger drops coalesce after formation of a plane-parallel film and their lifetime increases with the drop diameter. Therefore, the drop lifetime passes through a minimum when varying the drop diameter. The obtained results at low surfactant concentrations and for small drops are described well by theoretical models, available in the literature. The lifetime of the respective larger drops is shorter, as compared to the theoretical predictions, due to the faster thinning of the oil films which are with uneven thickness. The increase of surfactant concentration above the CMC leads to a significant increase in the stability of the small drops. Interestingly, the stability of the intermediate in size drops remains low for all surfactants and oils studied. At higher surfactant concentrations, the stability of the large water drops increases significantly when Span 80 is used, while it remains rather low for drops stabilized by Span 20. The results obtained with single water drops are in a good agreement with the stability of the respective batch water-in-oil emulsions.

Water-oil-water (oily) liquid films appear between the droplets in water-in-oil emulsions. Here we report results from systematic experiments, aimed to clarify how several important factors affect the stability and drainage of such oily films, in the presence of the nonionic oil-soluble surfactant Span 80. These results reveal that: (1) At Span 80 concentrations around the CMC, the film lifetime coincides with the duration of film drainage. When the oily films reach a critical thickness of around 40 nm, unstable thinner black spots of thickness 1 h. This effect is explained with a mass transfer of molecules between the aqueous and the oil phases which keeps the oil films out of equilibrium for a long period. These results can be used to explain and control the formation and stability of water-in-oil emulsions, in the presence of nonionic oil-soluble surfactants.

Emulsions are disperse systems in which one liquid is dispersed in the form of small droplets within another (immiscible) liquid. These droplets are with typical size range between several hundred nanometers and several millimetres. Many everyday consumer products are emulsions, such as many foods, pharmaceutical drugs and paints. Emulsions play also an important role in various technological processes, such as extraction and water purification from organic contaminants. The size of the dispersed drops in these systems is of crucial importance for their properties and for their efficiency upon application.The disruption of big droplets in smaller ones usually occurs in so-called turbulent flow. The latter flow is generated by passing the two immiscible liquids through a device with special geometry, called “homogenizer”, with a high linear speed. Usually, the liquids are forced to make multiple passes through the homogenizer, as smaller and smaller drops are formed after each pass.

We study the effect of two cationic polymers, with trade names Jaguar C13s and Merquat 100, on the rheological properties of foams stabilized with a mixture of anionic and zwitterionic surfactants (sodium lauryloxyethylene sulfate and cocoamidopropyl betaine). A series of five cosurfactants are used to compare the effect of these polymers on foaming systems with high and low surface dilatational moduli. The experiments revealed that the addition of Jaguar to the foaming solutions leads to (1) a significant increase of the foam yield stress for all systems studied, (2) the presence of consecutive maximum and minimum in the stress vs shear rate rheological curve for foams stabilized by cosurfactants with a high surface modulus (these systems cannot be described by the Herschel-Bulkley model anymore), and (3) the presence of significant foam-wall yield stress for all foaming solutions. These effects are explained with the formation of polymer bridges between the neighboring bubbles in slowly sheared foams (for inside foam friction) and between the bubbles and the confining solid wall (for foam-wall friction). Upon addition of 150 mM NaCl, the effect of Jaguar disappears. The addition of Merquat does not noticeably affect any of the foam rheological properties studied. Optical observations of foam films, formed from all these systems, show a very good correlation between the polymer bridging of the foam film surfaces and the strong polymer effect on the foam rheological properties. The obtained results demonstrate that the bubble-bubble attraction can be used for efficient control of the foam yield stress and foam-wall yield stress, without significantly affecting the viscous friction in sheared foams.