On the rupture of thin films made from aqueous surfactant solutions

Vertical concentration gradients of soluble surfactants in the rupture of thin liquid films

HYPOTHESIS

Surfactant-laden thin liquid films can rupture due to van der Waals forces, and being able to accurately predict the rupture time is important for applications involving coatings, foams, and emulsions. A common simplification in modeling film rupture is to assume that diffusion along the film thickness is so rapid that the surfactant concentration can be replaced by an averaged value. However, we hypothesize that vertical concentration gradients can develop as a result of surfactant adsorption at the interface, potentially rendering the vertical-averaging (VA) approximation inaccurate under certain conditions. Simulations: We assess the accuracy and limitations of this approximation by performing calculations with a lubrication-theory-based model that explicitly accounts for surfactant concentration gradients along the film thickness for a film on a horizontal solid substrate. Linear stability analysis and nonlinear simulations are performed to understand the role of vertical concentration gradients on film rupture.

FINDINGS

Results show that when surfactant diffusion is slow relative to advection and adsorption, substantial surfactant vertical concentration gradients can emerge. These gradients slow down adsorption and increase stabilizing Marangoni stresses, leading the VA approximation to underestimate the rupture time. Significant deviations in predicted rupture time are also observed when the initial bulk and surfactant concentrations are not in equilibrium, which is common in industrial applications.

Thin liquid films stabilized by plant proteins: Implications for foam stability

HYPOTHESIS

Plant-based proteins offer a sustainable solution for stabilizing multiphase food materials like edible foams and emulsions. However, challenges in understanding and engineering plant protein-stabilized interfaces persist, mostly because of the commonly poorer functionality and complex composition of the respective protein isolates. We hypothesize that part of the limited understanding is related to the lack of experimental data on the length-scale of the thin liquid film that separates two neighboring bubbles. By conducting such experiments, we aim to better understand the mechanisms by which plant proteins stabilize foams, a critical material in food applications.

EXPERIMENTS

In this study, we employ the dynamic thin film balance method to study the equilibrium properties and dynamic drainage behavior of foam thin liquid films stabilized by proteins derived from two main plant protein sources, yellow peas and rapeseeds, to investigate potential differences in film stabilization.

FINDINGS

Our thin film results provide new insights into the general foam stabilization mechanism of the two plant proteins. Most studies in this field focus on the impact of surface rheological parameters on stability of plant protein-based foam. We show that for such foams the half-life scales linearly with film thickness, the latter being closely related to the steric and electrostatic interactions developed across the respective films in equilibrium. Our study demonstrates the value of thin film studies in complementing traditional methods for studying protein-stabilized interfaces and facilitates an understanding of foam stabilization mechanisms that are universal among various surface-active species.

A novel micro-aqueous cold extraction of salmon head oil to reduce lipid oxidation and fishy odor: Comparison with common methods

Traditional heat extraction (HE) has a low efficiency (75.2 wt%) and induces lipid oxidation of PUFAs. The novel micro-aqueous cold (<25 °C) extraction (MAE) was applied to extract salmon head oil. The recovery rate was 93.4 wt% at oil volume fraction Φ = 74 %. The extraction mechanism was agitation-induced droplet coalescence at an unstable and close-packing state (Φ = 74 %), increasing the portions of the large-sized droplets (>50 μm) from 2.8 vol% to 91.7 vol%. The MAE reduced the oil oxidation level and odor intensity compared to HE, although the lipid profile differed slightly. The HE head oil had more key fishy odor compounds, including hexanal (0.98 mg/kg), 3-methyl-butanal (0.25 mg/kg), 1-penten-3-ol (0.49 mg/kg), and 2-ethylfuran (0.19 mg/kg). The MAE oil had only 2-methyl-butanal (0.10 mg/kg) and 1-penten-3-ol (0.47 mg/kg). Overall, micro-aqueous extraction has great potential to replace industrial heat extraction with a better product quality.

Intrinsic dynamics of emulsions: Experiments in microgravity on the International Space Station

HYPOTHESIS

In order to understand the basic mechanisms affecting emulsion stability, the intrinsic dynamics of the drop population must be investigated. We hypothesize that transient ballistic motion can serve as a marker of interactions between drops. In 1G conditions, buoyancy-induced drop motion obscures these interactions. The microgravity condition onboard the International Space Station enable this investigation.

EXPERIMENTS

We performed Diffusing Wave Spectroscopy (DWS) experiments in the ESA Soft Matter Dynamics (SMD) facility. We used Monte Carlo simulations of photon trajectory to support data analysis. The analysis framework was validated by ground-based characterizations of the initial drop size distribution (DSD) and the properties of the oil/water interface in the presence of surfactant.

FINDINGS

We characterized the drop size distribution and found to be bi-disperse. Drop dynamics shows transient ballistic features at early times, reaching a stationary regime of primarily diffusion-dominated motion. This suggests different ageing mechanisms: immediately after emulsification, the main mechanism is coalescence or aggregation between small drops. However at later times, ageing proceeds via coalescence or aggregation of small with large drops in some emulsions. Our results elucidate new processes relevant to emulsion stability with potential impact on industrial processes on Earth, as well as enabling technologies for space exploration.

Peanut de-oiling at room temperature by micro-aqueous hydration: Co-destabilization driven by oleosome coalescence and protein aggregation

The peanut de-oiling industry currently lacks efficient processing technologies for de-oiling at low or room temperatures. A novel method, micro-aqueous extraction (MAE), offers over 93 % de-oiling efficiency at room temperature and is also effective for other oilseeds like sesame, camellia, and rapeseed. Despite its effectiveness, the exact mechanism behind oleosomes destabilization at a critical hydration level or oil volume fraction (φ ∼ 0.75) is not fully understood. This study investigates how MAE affects peanut oleosome size, paste stability, and the interfacial properties of surfactant proteins. Results showed that micro-aqueous hydration and agitation caused small droplets (85.6 vol% < 10 μm) to coalesce into larger droplets (90.0 vol% > 30 μm) due to press-induced rupture of the liquid film. Simultaneously, agitation decreased water mobility and protein intrinsic fluorescence, while increasing paste viscosity, leading to protein aggregation. This aggregation further promoted oleosome coalescence. Additionally, hydration and agitation weakened the ability of membrane proteins to stabilize oleosomes by increasing interfacial tension and decreasing dilatational storage modulus. The insights into the peanut oleosome destabilization mechanism for MAE provide a foundation for scaling up the process, with the potential to replace current hot and cold pressing techniques.

Effect of CO2 Concentration on the Performance of Polymer-Enhanced Foam at the Steam Front

This study examines the impact of CO2 concentration on the stability and plugging performance of polymer-enhanced foam (PEF) under high-temperature and high-pressure conditions representative of the steam front in heavy oil reservoirs. Bulk foam experiments were conducted to analyze the foam performance, interfacial properties, and rheological behavior of CHSB surfactant and Z364 polymer in different CO2 and N2 gas environments. Additionally, core flooding experiments were performed to investigate the plugging performance of PEF in porous media and the factors influencing it. The results indicate that a reduction in CO2 concentration in the foam, due to the lower solubility of N2 in water and the reduced permeability of the liquid film, enhances foam stability and flow resistance in porous media. The addition of polymers was found to significantly improve the stability of the liquid film and the flow viscosity of the foam, particularly under high-temperature conditions, effectively mitigating the foam strength degradation caused by CO2 dissolution. However, at 200 °C, a notable decrease in foam stability and a sharp reduction in the resistance factor were observed. Overall, the study elucidates the effects of gas type, temperature, and polymer concentration on the flow and plugging performance of PEF in porous media, providing reference for fluid mobility control at the steam front in heavy oil recovery.

Comparison of Methods Used to Investigate Coalescence in Emulsions

We present a study of moderately stable dilute emulsions. These emulsions are models for water contaminated by traces of oil encountered in many water treatment situations. The purification of water and the elimination of oil rely on the emulsion stability. Despite actively being studied, the topic of emulsion stability is still far from being fully understood. In particular, it is still unclear whether experimental methods accessing different length scales lead to the same conclusions. In the study presented in this paper, we have used different methods to characterize the emulsions, such as centrifugation and simple bottle tests, as well as investigations of the collision of single macroscopic oil drops at an oil-water interface. We studied different emulsions containing added polymer or surfactant. In the case of added polymer, centrifugation and single drop experiments led to opposite trends in stability when the polymer concentration is varied. In the case of added surfactant, both centrifugation and single drop experiments show a maximum stability when the surfactant concentration is increased, whereas bottle tests show a monotonous increase in stability. We propose tentative interpretations of these unexpected observations. The apparent contradictions are due to the fact that different methods require different drop sizes or different drop concentrations. The puzzling decrease in emulsion stability at a higher surfactant concentration observed with some methods, however, remains unclear. This coalescence study illustrates the fact that different results can be obtained when different experimental methods are used. It is therefore advisable not to rely on a single method, especially in the case of emulsions of limited stability for reasons explained in the paper.

PFAS removal from landfill leachate by ozone foam fractionation: System optimization and adsorption quantification

Landfills are the primary endpoint for the disposal of PFAS-laden waste, which subsequently releases PFAS to the surrounding environments through landfill leachate. Ozone foam fractionation emerges as a promising technology for PFAS removal to address the issue. This study aims to (i) assess the effectiveness of the ozone foam fractionation system to remove PFAS from landfill leachate, and (ii) quantify equilibrium PFAS adsorption onto the gas-water interface of ozone bubbles, followed by a comparison with air foam fractionation. The results show that ozone foam fractionation is effective for PFAS removal from landfill leachate, with more than 90 % long-chain PFAS removed. The identified operating conditions provide valuable insights for industrial applications, guiding the optimization of ozone flow rates (1 L/min), dosing (43 mg/L) and minimizing foamate production (4 % wettability). The equilibrium modelling reveals that the surface excess of air bubbles exceeds that of ozone bubbles by 20-40 % at a corresponding PFAS concentration. However, the overall removal of PFAS from landfill leachate by ozone foam fractionation remains substantial. Notably, ozone foam fractionation generates foamate volumes 2 - 4 times less, resulting in significant cost savings for the final disposal of waste products and reduced site storage requirements.

Stability and thinning of liquid jets in the presence of soluble surfactants

The dynamics of many multiphase fluid systems involve the thinning and eventual break up of a slender fluid filament or a liquid jet. The interfacial instability that controls the rate of jet thinning depends on the relative magnitudes of capillary, viscous, and inertial stresses. Surfactants add an additional layer of physicochemical dynamics by reducing the surface tension of the interface and introducing reverse Marangoni flows in response to surface concentration gradients. Surfactants may also introduce an intrinsic surface rheology that affects jet thinning. Quantifying these effects has been a significant problem in chemical physics and a topic of key research interest. Recent studies have shown that insoluble surfactants delay thread thinning and suppress instabilities in Newtonian jets. However, the role of surfactant solubility in liquid jet stability is still unknown. In this work, we use linear stability analysis to quantitatively show the stabilizing effects of Marangoni stresses, surfactant adsorption and desorption time, and intermolecular forces upon adsorption. We highlight the seemingly indistinguishable way in which various surfactant properties result in the same outcome. We also identify a surface dissipative contribution that arises from the interplay of Marangoni flows with finite adsorption and desorption, which acts as an "apparent" surface viscosity. We verify predictions of our linear stability results against numerical simulations and conclude by noting that tuning surface activity and kinetics of adsorbed surfactants or particles can potentially suppress droplet formation, which is of significant impact in the printing industry and in the control of the spread of aerosols.

Non-equilibrium molecular simulations of thin film rupture

The retraction of thin films, as described by the Taylor-Culick (TC) theory, is subject to widespread debate, particularly for films at the nanoscale. We use non-equilibrium molecular dynamics simulations to explore the validity of the assumptions used in continuum models by tracking the evolution of holes in a film. By deriving a new mathematical form for the surface shape and considering a locally varying surface tension at the front of the retracting film, we reconcile the original theory with our simulation to recover a corrected TC speed valid at the nanoscale.

A Comparative Study on CO2-Switchable Foams Stabilized by C22- or C18-Tailed Tertiary Amines

The CO2 aqueous foams stabilized by bioresource-derived ultra-long chain surfactants have demonstrated considerable promising application potential owing to their remarkable longevity. Nevertheless, existing research is still inadequate to establish the relationships among surfactant architecture, environmental factors, and foam properties. Herein, two cases of ultra-long chain tertiary amines with different tail lengths, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and N-oleicamidopropyl-N,N-dimethylamine (UC18AMPM), were employed to fabricate CO2 foams. The effect of temperature, pressure and salinity on the properties of two foam systems (i.e., foamability and foam stability) was compared using a high-temperature, high-pressure visualization foam meter. The continuous phase viscosity and liquid content for both samples were characterized using rheometry and FoamScan. The results showed that the increased concentrations or pressure enhanced the properties of both foam samples, but the increased scope for UC22AMPM was more pronounced. By contrast, the foam stability for both cases was impaired with increasing salinity or temperature, but the UC18AMPM sample is more sensitive to temperature and salinity, indicating the salt and temperature resistance of UC18AMPM-CO2 foams is weaker than those of the UC22AMPM counterpart. These differences are associated with the longer hydrophobic chain of UC22AMPM, which imparts a higher viscosity and lower surface tension to foams, resisting the adverse effects of temperature and salinity.

Repulsive, but sticky - Insights into the non-ionic foam stabilization mechanism by superchaotropic nano-ions

HYPOTHESIS

The superchaotropic Keggin polyoxometalate α-SiW₁₂O₄₀^4- (SiW) was recently shown to stabilize non-ionic surfactant (C_18:1E₁₀) foams owing to electrostatic repulsion that arises from the adsorption of SiW-ions to the foam interfaces. The precise mechanism of foam stabilization by SiW however remained unsolved.

EXPERIMENTS

Imaging and conductimetry were used on macroscopic foams to monitor the foam collapse under free drainage and small angle neutron scattering (SANS) at a given foam height allowed for the tracking of the evolution of film thickness under quasi-stationary conditions. Thin film pressure balance (TFPB) measurements enabled to quantify the resistance of single foam films to external pressure and to identify intra-film forces.

FINDINGS

At low SiW/surfactant ratios, the adsorption of SiW induces electrostatic repulsion within foam films. Above a concentration threshold corresponding to an adsorption saturation, excess of SiW screens the electrostatic repulsion that leads to thinner foam films. Despite screened electrostatics, the foam and single foam films remain very stable caused by an additional steric stabilizing force consistent with the presence of trapped micelles inside the foam films that bridge between the interfaces. These trapped micelles can serve as a surfactant reservoir, which promotes self-healing of the interface leading to much more resilient foam films in comparison to bare surfactant foams/films.

Polyelectrolyte/Surfactant Mixtures: A Pathway to Smart Foams

This review deals with liquid foams stabilized by polyelectrolyte/surfactant (PS) complexes in aqueous solution. It briefly reviews all the important aspects of foam physics at several scales, from interfaces to macroscopic foams, needed to understand the basics of these complex systems, focusing on those particular aspects of foams stabilized by PS mixtures. The final section includes a few examples of smart foams based on PS complexes that have been reported recently in the literature. These PS complexes open an opportunity to develop new intelligent dispersed materials with potential in many fields, such as oil industry, environmental remediation, and pharmaceutical industry, among others. However, there is much work to be done to understand the mechanism involved in the stabilization of foams with PS complexes. Understanding those underlying mechanisms is vital to successfully formulate smart systems. This review is written in the hope of stimulating further work in the physics of PS foams and, particularly, in the search for responsive foams based on polymer-surfactant mixtures.

Investigating pore-opening in hydrogel foams at the scale of free-standing thin films

Controlling the pore connectivity of polymer foams is key for most of their applications, ranging from liquid uptake, mechanics, and acoustic/thermal insulation to tissue engineering. Despite its importance, the scientific phenomena governing the pore-opening processes remain poorly understood, requiring tedious trial-and-error procedures for property optimisation. This lack of understanding is partly explained by the high complexity of the different interrelated, multi-scale processes which take place as the foam transforms from an initially fluid foam into a solid foam. To progress in this field, we take inspiration from long-standing research on liquid foams and thin films to develop model experiments in a microfluidic "Thin Film Pressure Balance". These experiments allow us to investigate isolated thin films under well-controlled environmental conditions reproducing those arising within a foam undergoing cross-linking and drying. Using the example of alginate hydrogel films, we correlate the evolution of isolated thin films undergoing gelation and drying with the evolution of the rheological properties of the same alginate solution in bulk. We introduce the overall approach and use a first set of results to propose a starting point for the phenomenological description of the different types of pore-opening processes and the classification of the resulting pore-opening types. This article is protected by copyright. All rights reserved.

Large-Scale Dewetting via Surfactant-Laden Droplet Impact

The dewetting phenomenon of a liquid film in the presence of a surfactant exists in various natural, industrial, and biomedical processes but still remains mysterious in some specific scenarios. Here, we investigate the dewetting behavior of water films initiated by surfactant-laden droplet impact and show that the maximum dewetting diameter can even reach more than 50 times that of the droplet size. We identify the S-type variation of the dewetting area and demonstrate its correlation to the dynamic surface tension reduction. From a viewpoint of energy conversion, we attribute the dewetting to the released surface energy caused by the surfactant addition and establish a linear relation between the maximum dewetting and the surfactant concentration in the film, i.e., d_max² ∝ c_film, which agrees well with the experiments. These results may advance the physics of liquid film dewetting triggered by surfactant injection, which shall further guide practical applications.

Drainage via stratification and nanoscopic thickness transitions of aqueous sodium naphthenate foam films

Sodium naphthenates (NaNs), found in crude oils and oil sands process-affected water (OSPW), can act as surfactants and stabilize undesirable foams and emulsions. Despite the critical impact of soap-like NaNs on the formation, properties, and stability of petroleum and OSPW foams, there is a significant lack of studies that characterize foam film drainage, motivating this study. Here, we contrast the drainage of aqueous foam films formulated with NaN with foams containing sodium dodecyl sulfate (SDS), a well-studied surfactant system, in the relatively low concentration regime (c/CMC < 12.5). The foam films exhibit drainage via stratification, displaying step-wise thinning and coexisting thick-thin regions manifested as distinct shades of gray in reflected light microscopy due to thickness-dependent interference intensity. Using IDIOM (interferometry digital imaging optical microscopy) protocols that we developed, we analyze pixel-wise intensity to obtain thickness maps with high spatiotemporal resolution (thickness <1 nm, lateral ∼500 nm, time ∼10 ms). The analysis of interference intensity variations over time reveals that the aqueous foam films of both SDS and NaN possess an evolving, dynamic, and rich nanoscopic topography. The nanoscopic thickness transitions for stratifying SDS foam films are attributed to the role played by damped supramolecular oscillatory structural disjoining pressure contributed by the confinement-induced layering of spherical micelles. In comparison with SDS, we find smaller concentration-dependent step size and terminal film thickness values for NaN, implying weaker intermicellar interactions and oscillatory structural disjoining pressure with shorter decay length and periodicity.

Emulsification and emulsion stability: The role of the interfacial properties

In this review, we highlight and discuss the effects of interfacial properties on the major mechanisms governing the aging of emulsions: flocculation, coalescence and Ostwald ripening. The process of emulsification is also addressed, as it is well recognized that the adsorption properties of emulsifiers play an important role on it. The consolidated background on these phenomena is briefly summarised based on selected literature, reporting relevant findings and results, and discussing some criticalities. The typical experimental approaches adopted to investigate the above effects are also summarised, underlining in particular the role of adsorption at the droplet interface. Attention is paid to different types of surface-active species involved with emulsion production, including solid particles. The latter being of increasing interest in a wide variety of emulsions-related products and technologies in various fields. The possibility to stop the long term aging caused by Ostwald ripening in emulsions is also discussed, quantifying under which conditions it may occur in practice.

Scaling Laws in the Dynamics of Collapse of Single Bubbles and 2D Foams

Avalanches of rupturing bubbles play an important role in the dynamics of collapse of macroscopic liquid foams. We hypothesized that the occurrence of cascades of rupturing bubbles in foams depends, at least in part, on the power released during the rupture of a bubble. In this paper, we present results on the dynamics of single bubble bursting obtained by analyzing the pressure wave (sound) emitted by the bubble when collapsing. We found that the released energy varies linearly with bubble size, the frequency of the emitted sound follows a power law with exponent 3/2 (compatible with the Helmholtz resonator model) and the duration of a rupturing event seems to be independent of bubble size. To correlate the dynamics of individual bubbles with the dynamics of foams, we studied the occurrence of avalanches on bubble rafts and found that the phenomenon appears to be a self-organized criticality (SOC) process. The distribution functions for the size of the avalanches are a power law with exponents between 2 and 3, depending on the surfactant concentration. The distribution of times between ruptures also follows a power law with exponents close to 1, independently of the surfactant concentration.

Physical models of infant mortality: implications for defects in biological systems

Reliability engineering concerned with failure of technical inanimate systems usually uses the vocabulary and notions of human mortality, e.g., infant mortality vs. senescence mortality. Yet, few data are available to support such a parallel description. Here, we focus on early-stage (infant) mortality for two inanimate systems, incandescent light bulbs and soap films, and show the parallel description is clearly valid. Theoretical considerations of the thermo-electrical properties of electrical conductors allow us to link bulb failure to inherent mechanical defects. We then demonstrate the converse, that is, knowing the failure rate for an ensemble of light bulbs, it is possible to deduce the distribution of defects in wire thickness in the ensemble. Using measurements of lifetimes for soap films, we show how this methodology links failure rate to geometry of the system; in the case presented, this is the length of the tube containing the films. In a similar manner, for a third example, the time-dependent death rate due to congenital aortic valve stenosis is related to the distribution of degrees of severity of this condition, as a function of time. The results not only validate clearly the parallel description noted above, but also point firmly to application of the methodology to humans, with the consequent ability to gain more insight into the role of abnormalities in infant mortality.