Thin liquid films

The surface, film and foam properties of two nonionic surfactants, dodecyl sucrose ester (L1695) and dodecyl polyoxyethylene ether (Brij L23), were studied at four different temperatures between 25 and 60 °C and three surfactant concentrations (0.01%, 0.1%, and 1% wt.). The impact of 6 M urea was also assessed to determine the role of hydrogen bonds for the observed trends. The foams were generated using two methods: the fast foaming method (Bartsch test), producing foams with smaller bubbles, and the slow foaming method (foam rise method), yielding foams with bigger bubbles. For Brij L23, an increase in temperature resulted in a decrease in the critical micellar concentration (CMC), reduced surfactant adsorption on the air-water interface, weakened electrostatic repulsion between the foam film surfaces and significantly decreased foam stability. For L1695, an increase in temperature increased surfactant adsorption, maximized the foamability at 40 °C, and did not affect the foam stability for foams with small bubbles. However, the temperature increase leads to decreased stability at low concentrations for foams with bigger bubbles. The addition of 6 M urea, resulted in increased adsorption without any effect on foam stability for L1695, whereas it decreased the adsorption and foam stability for Brij L23. The comparison of relative foamability vs. dynamic surface coverage revealed that a lower threshold surface coverage is required to increase the foamability of L1695 (≈ 80%) compared to Brij L23 (≈ 95%). This difference is explained by the action of weak electrostatic repulsion and the adsorption of sucrose diesters on the bubble surfaces when L1695 surfactant is used. The higher stability of L1695 foams under all studied conditions is attributed to the formation of a denser adsorption layer due to the adsorption of sucrose diesters. The diesters prevent the formation of weak spots within the foam films, even at high temperatures. This work contributes to the advancement of the foam field by demonstrating that a mixture of sucrose mono and diester surfactants can be highly effective in forming stable foams at higher temperatures and in the presence of urea. Both factors (higher temperature and 6 M urea) have negative effect on Brij L23 foams, while they have no significant effect on L1695 foams with smaller bubbles.

Interactions across incredibly thin layers of fluids, known as thin films, underpin many important processes involving colloids, such as wetting-dewetting phenomena. Often in these systems, thin films are composed of complex fluids that contain dispersed components, such as spherical micelles, giving rise to oscillatory structural forces due to preferential layering under confinement. Modelling of thin film dynamics involving Derjaguin-Landau-Verwey-Overbeek (DLVO) type forces has been widely reported using the Stokes-Reynolds-Young-Laplace (SRYL) model, and we hypothesize that this theory can be extended to a concentrated micellar system by including an oscillatory structural force term in the disjoining pressure.

We study the drainage behaviour of thin films comprising sodium dodecyl sulfate (SDS) micelles across a range of concentrations and interaction conditions between an air bubble and a mica disk using a custom-built dual-wave interferometry apparatus.

Early-stage film behaviour is dominated by hydrodynamics, which can be well reproduced by the SRYL model. However, experimental profiles drain significantly faster than predicted, transitioning into a structural force dominated phase characterised by four types of film ripping instabilities that we term ‘waving’, ‘ridging’, ‘webbing’, and ‘hole-sheeting’. These instabilities were mapped according to SDS concentration and approach velocity, providing insight into the interplay between structural forces and hydrodynamic conditions.

The surface, film, and foam properties of six polyvinyl alcohols (PVA) with different degrees of hydrolysis (DH) and molecular weights were studied and compared with the properties of nonionic Brij 35 and anionic SLES. Four different foaming methods were employed: the fast foaming method (Bartsch test), intermediate tests (shake test and Ultra Turrax), and slow foaming method (foam rise method) to assess the foamability at various bulk concentrations. The foamability data obtained from different foaming tests, utilizing various surfactant and polymeric concentrations, and differing foaming times, were shown to follow a universal master curve when plotted as relative foamability vs. scaled concentration. A new simple theoretical equation was derived to describe this universal curve, allowing for foamability prediction. The threshold surfactant concentration required to achieve 50% of the maximal foam volume under given conditions (used for scaling the bulk concentration) was found to decrease with foaming time and increase from slow foaming methods to fast foaming methods. When the experimental data are plotted against surface coverage, the results for PVA solutions exhibit intermediate behavior between nonionic surfactants, where a threshold surface coverage of 95% is required to achieve 50% of maximal foamability and anionic surfactants, where 30% surface coverage is sufficient to reach 50% of maximal foamability due to the action of electrostatic repulsion. This intermediate behavior observed in PVA solutions is attributed to the presence of a long-range steric repulsion arising from the adsorption of PVA molecules onto the bubble surfaces. This work advances the foam field by showing that the approach developed in Petkova et al. 2020 can be used for polymeric molecules and by deriving a new equation for foamability which is expected to be applicable for wide range of systems.

The effects of antifoam and surfactant concentration on the foamability of solutions of an anionic (SLES) and nonionic (Brij 35) surfactants and a series of polyvinyl alcohols with 88% and 98% degree of hydrolysis and molecular masses between 31 and 205 kDa, were studied. Three methods which differ in the way of air incorporation were used for foaming – Bartsch test, shake test and Ultra Turrax. Mixed silicone oil-silica particles antifoam was studied. The antifoam was introduced in the foaming solution as pre-dispersed in organic solvent or as antifoam-in-water emulsion. It was shown that the antifoam is very active in the fast foaming methods (Bartsch and shake tests) for the slow adsorbing polymers PVA and has no any activity in the slow foaming method (Ultra Turrax) for the fast adsorbing surfactants with electrostatic stabilization (SLES). The efficiency of pre-dispersed in organic solvent antifoam is much higher as compared to that of emulsified antifoam, due to the faster segregation of the silica particles and silicone oil in the emulsified antifoam. The antifoam efficiency increases with antifoam concentration and with lowering the surfactant concentration. In a given foaming method, the antifoam efficiency is the highest in PVA solutions with 98% DH, intermediate for PVA with 88% DH and Brij 35, and the lowest for SLES solutions. At a certain degree of hydrolysis, the molecular mass of PVA has no significant effect on the antifoam activity. Good correlation between the antifoam efficiency and the stability of the pseudo emulsion film formed between the antifoam globule and the bubble surface is established, showing that the electrostatic repulsion is more efficient to prevent the entering of the antifoam globules on the air-water interface, as compared to the steric repulsion.

Bubble-particle attachment is a fundamental element of the flotation process, and its occurrence depends on the stability of the thin liquid film separating these objects. In this study, the effect of surface wettability on the stability of liquid film and subsequent attachment was investigated by flotation of glass microspheres (70–110 μm) with controlled contact angles (9–89°) in a Hallimond tube. Additionally, pH was varied to determine the electrostatic interactions between the bubble and particles. The results showed that for the flotation process to take place, the minimum (critical) value of the contact angle must be exceeded. This value was found to be ca. 25° when the electrostatic interactions were either attractive or weakly repulsive and as high as 62° for strong repulsive electrostatic interactions. These findings highlight the role of attractive hydrophobic interactions, whose importance increases with increasing surface contact angle in the context of stability of thin liquid films.

In this paper we present a literature review on the role of surface-active components in emulsions and foams. Models are presently unable to accurately predict scenarios involving multiphase flows with complex chemistry, and this is a major obstacle for many petroleum production/transport systems. This paper highlights some of the most important physical phenomena that are relevant for understanding and ultimately predicting the flow behaviour of complex fluid systems. Some of the main topics addressed are fluid interfaces with surfactants, droplet coalescence and breakup, emulsions, and foams.