Drainage of single Plateau borders: direct observation of rigid and mobile interfaces

The peak viscosity of decaying foam with natural drainage and coarsening

Studying the change in foam viscosity during foam decay, a spontaneous and inevitable process, is of fundamental and practical interest across many applications, ranging from the froth in a cup of coffee to the carbon sequestration in deep geological reservoirs. However, standard rheological measurements impose several experimental constraints, such as the narrow sample confinement and the long initial setup time, interfering with the natural conditions for foam decay. Here, we perform fast and in situ measurements on decaying foam immediately after its generation in a wide column, measuring the viscosity by vibrational probes and measuring the foam structure by optical imaging. We successfully capture the changes during the transition from the drainage-dominated stage to the coarsening-dominated stage. The viscosity reaches its maximum at the crossover point, elucidating the competing effects of drainage and coarsening. The viscosity peaks magnitude and position are influenced by the gas solubility and diffusion coefficient. The phenomena are quantitatively explained by the film-shearing model. Our findings provide the foundation for enhancing foam stability and performance, improving the efficiency of foam-based applications.

Characterizing Aqueous Foams by In Situ Viscosity Measurement in a Foam Column

Foam characterization is essential in many applications of foams, such as cleaning, food processing, cosmetics, and oil production, due to these applications' diversified requirements. The standard characterization method, the foam column test, cannot provide sufficient information for in-depth studies. Hence, there have been many studies that incorporated different characterization methods into a standard test. It should be enlightening and feasible to measure the foam viscosity, which is both of practical and fundamental interest during the foam column test, but it has never been done before. Here, we demonstrate a method to characterize aqueous foams and their aging behaviors with the simultaneous measurement of foam viscosity and foam height. Using a vibration viscometer, we integrate foam column experiments with in situ foam viscosity measurements. We studied the correlation among the foam structure, foam height, and foam viscosity during the foam decay process. We found a drastic decrease in foam viscosity in the early foam decay, while the foam height remained unchanged, which is explained by coarsening. This method is much more sensitive and time-efficient than conventional foam-height-based methods by comparing the half-life. This method successfully characterizes the stability of foams made of various combinations of surfactants and gases.

The Influence of Kaolinite and Quartz on Stability of Coal Froths - A Rheology and Structure Study

Kaolinite and quartz are the common gangue minerals found in raw coal; however, their effects on stability of coal froths and subsequent settling of coal flotation products have not been investigated. In this study, in the coal froths batch settling tests, the amount of froth floating on top of water was 275, 325, 355, and 405 mL for coal concentrates generated with 0, 20, 40, and 60 wt % kaolin Q38, respectively, while that was almost the same (300-306 mL) for coal froth concentrates generated with 0, 20, 40, and 60 wt % quartz added in flotation, respectively, which turned out that the kaolinite could increase the stability of coal froth, while quartz could not. To investigate the mechanism, oscillatory rheology and scanning electron microscopy (SEM) were applied. The results of the oscillatory rheology suggested that the structural strength in coal froth was strengthened with the addition of kaolinite. In addition, images of Plateau borders by SEM illustrated that the addition of kaolinite in flotation increased the size of Plateau borders and generated network structures in the Plateau borders. However, as a comparison, the addition of quartz did not cause an obvious change for the oscillatory rheology and SEM results of coal froth. Based on the results, it can be concluded that network structures were generated in the Plateau border of coal froth with the addition of kaolinite, which increased its structural strength and retarded the drainage in froth. As a result, the stability of the coal froth increased.

Surface viscosity in simple liquids

The response of Newtonian liquids to small perturbations is usually considered to be fully described by homogeneous transport coefficients like shear and dilatational viscosity. However, the presence of strong density gradients at the liquid/vapor boundary of fluids hints at the possible existence of an inhomogeneous viscosity. Here, we show that a surface viscosity emerges from the collective dynamics of interfacial layers in molecular simulations of simple liquids. We estimate the surface viscosity to be 8-16 times smaller than that of the bulk fluid at the thermodynamic point considered. This result can have important implications for reactions at liquid surfaces in atmospheric chemistry and catalysis.

A new model to describe small-angle neutron scattering from foams

The modelling of scattering data from foams is very challenging due to the complex structure of foams and is therefore often reduced to the fitting of single peak positions or feature mimicking. This article presents a more elaborate model to describe the small-angle neutron scattering (SANS) data from foams. The model takes into account the geometry of the foam bubbles and is based on an incoherent superposition of the reflectivity curves arising from the foam films and the small-angle scattering (SAS) contribution from the plateau borders. The model is capable of describing the complete scattering curve of a foam stabilized by the standard cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) with different water contents, i.e. different drainage states, and provides information on the thickness distribution of liquid films inside the foam. The mean film thickness decreases with decreasing water content because of drainage, from 28 to 22 nm, while the polydispersity increases. These results are in good agreement with the film thicknesses of individual horizontal foam films studied with a thin-film pressure balance.

Foaming of Binary Mixtures: Link with the Nonlinear Behavior of Surface Tension in Asymmetric Mixtures

The lifetimes of single bubbles or foams that are formed in mixtures of liquids can be several orders of magnitude larger than the ones formed in pure liquids. We recently demonstrated that this enhanced stability results from differences between bulk and interfacial concentrations in the mixture, which induce a thickness dependence of the surface tension in liquid films, and thus a stabilizing Marangoni effect. Concentration differences may be associated with nonlinear variations of surface tension with composition and we further investigate their link with foamability of binary mixtures. We show that, for asymmetric binary mixtures, that is, made of molecules of very different sizes, strong nonlinearities in surface tension can be measured, that are associated with large foam lifetimes. When the molecules that occupy the largest surface areas have the smallest surface tension, the surface tension of the mixture varies sublinearly with composition, reflecting an enrichment in this species at the interface with air, as classically reported in the literature. In contrast, when they exhibit the largest surface tension, superlinear variations of surface tension are observed, despite a similar enrichment. We discuss these variations in light of a simple thermodynamic model for ideal mixtures and we demonstrate why foam stability is enhanced for both sublinear and superlinear surface tension variations, thus, shedding new light on foamability without added surfactants.

Numerical Modeling of Flow through Foam Nodes within the Dry and Wet Limits

We present a numerical simulation using three-dimensional microscale models to illustrate flow dynamics through different foam geometries. These were designed to represent the flow with various liquid volume fractions throughout the Plateau border (PB) and node system within the "dry" limit and throughout the two nodes and PB system within the "wet" limit. Most surfaces in the models involve a gas-liquid interface. Here, the stress-balance boundary condition was applied to achieve coupling between the surface and bulk. The three-dimensional Navier-Stokes equation along with the continuity equation was solved using the finite volume approach, and a qualitative evaluation of flow velocities in different foam geometries was obtained. The numerical results were validated against the available experimental results for foam permeabilities in the nodes and PBs. Discrepancies were expected between the simulated and empirical values as the latter values were obtained by considering only the viscous losses in the PBs. Furthermore, the scaled resistance to flow for varying gas-liquid interface mobilities and liquid fractions was studied. The individual geometrical characteristics of the node and PB components were compared to investigate the PB- and node-dominated flow regimes numerically. Additionally, more accurate information was obtained for comparing the average flow velocities within the node-PB and the two-node-PB systems, providing a better understanding of the effect of a gas-liquid interface on foam flow. We eventually show that when the foam geometry is correctly described, the relative effect of the geometrical factors of the PB and node components of system models can be evaluated, allowing a numerical flow simulation with a unique parameter-the Boussinesq number (B_o).

Foam fractionation for effective removal of Pseudomonas aeruginosa from water body: Strengthening foam drainage by artificially inducing foam evolution

Lack of microbial contamination is of great significance to drinking water safety and water reclamation. In this work, foam fractionation was employed to remove Pseudomonas aeruginosa (P. aeruginosa) from aqueous solution and dodecyl dimethyl betaine (BS12) was used as the collector. Since the attachment of strain cells on the bubble surface would impede the reflux of interstitial liquid in the plateau borders (PBs), a novel strategy in strengthening foam drainage was developed through artificially inducing foam evolution. Two gas distributors with different pore diameters had been mounted at the bottom of the column for regulating the radial distribution of bubble size in the foam phase. Experimental results indicated that gas diffuse and bubble coarsening could be significantly promoted by increasing the size difference among the adjacent bubbles. Bubble coalescence contributed to broadening the width of plateau borders, thereby avoiding the borders blockage by strain cells. During bubble coalescence, surfactant molecules would be partially shifted from the surface of small bubble towards that of large bubble due to the molecule density difference. The increase in surface excess of surfactant molecules on gas-liquid interface was conducive to improving foam stability. Under the suitable conditions of air flow rates of gas distributor with 0.125 mm of pore diameter 75 mL/min and gas distributor with 0.425 mm of pore diameter 125 mL/min, BS12 concentration 0.1 g/L, and P. aeruginosa concentration 2.0 × 10⁴ CFU/mL, the removal percentage and enrichment ratio of P. aeruginosa were 99.6% and 10.6, respectively. This work is expected to provide some new light for strengthening foam drainage in the presence of solid particles and to facilitate the industrialization of foam fractionation in water treatment.

Understanding Frothing of Liquid Mixtures: A Surfactantlike Effect at the Origin of Enhanced Liquid Film Lifetimes

The formation of froth in mixtures of liquids is well documented, particularly in oil mixtures. However, in nonvolatile liquids and in the absence of surface-active molecules, the origin of increased liquid film lifetimes had not been identified. We suggest a stabilizing mechanism resulting from the nonlinear variations of the surface tension of a liquid mixture with its composition. We report on experimental lifetimes of froths in binary mixtures and show that their variations are well predicted by the suggested mechanism. We demonstrate that it prescribes the thickness reached by films before their slow drainage, a thickness which correlates well with froth lifetimes for both polar and nonpolar liquids.

Trapping of swimming microalgae in foam

Massive foam formation in aquatic environments is a seasonal event that has a significant impact on the stability of marine ecosystems. Liquid foams are known to filter passive solid particles, with large particles remaining trapped by confinement in the network of liquid channels and small particles being freely advected by the gravity-driven flow. By contrast, the potential role of a similar retention effect on biologically active particles such as phytoplankton cells is still relatively unknown. To assess if phytoplankton cells are passively advected through a foam, the model unicellular motile alga Chlamydomonas reinhardtii (CR) was incorporated in a bio-compatible foam, and the number of cells escaping the foam at the bottom was measured in time. Comparing the escape dynamics of living and dead CR cells, we found that dead cells are totally advected by the liquid flow towards the bottom of the foam, as expected since the diameter of CR remains smaller than the typical foam channel diameter. By contrast, living motile CR cells escape the foam at a significantly lower rate: after 2 hours, up to 60% of the injected cells may remain blocked in the foam, while 95% of the initial liquid volume in the foam has been drained out of the foam. Microscopic observation of the swimming CR cells in a chamber mimicking the cross-section of foam internal channels revealed that swimming CR cells accumulate near channels corners. A theoretical analysis based on the probability density measurements in the micro chambers has shown that this trapping at the microscopic scale contributes to explain the macroscopic retention of the microswimmers in the foam. At the crossroads of distinct fields including marine ecology of planktonic organisms, fluid dynamics of active particles in a confined environment and the physics of foam, this work represents a significant step in the fundamental understanding of the ecological consequences of aquatic foams in water bodies.

Crossover between Athermal Jamming and the Thermal Glass Transition of Suspensions

The non-Newtonian flow behavior of thermal and athermal disordered systems of dispersed uniform particles at high densities have strikingly similar features. By investigating the flow curves of yield-stress fluids and colloidal glasses having different volume fractions, particle sizes, and interactions, we show that both thermal and athermal systems exhibit power-law scaling with respect to the glass and jamming point, respectively, with the same exponents. All yield-stress flow curves can be scaled onto a single universal curve using the Laplace pressure as the stress scale for athermal systems and the osmotic pressure for the thermal systems. Strikingly, the details of interparticle interactions do not matter for the rescaling, showing that they are akin to usual phase transitions of the same universality class. The rescaling allows us to predict the flow properties of these systems from the volume fraction and known material properties.

A review of aqueous foam in microscale

In recent years, significant progress has been achieved in the study of aqueous foams. Having said this, a better understanding of foam physics requires a deeper and profound study of foam elements. This paper reviews the studies in the microscale of aqueous foams. The elements of aqueous foams are interior Plateau borders, exterior Plateau borders, nodes, and films. Furthermore, these elements' contribution to the drainage of foam and hydraulic resistance are studied. The Marangoni phenomena that can happen in aqueous foams are listed as Marangoni recirculation in the transition region, Marangoni-driven flow from Plateau border towards the film in the foam fractionation process, and Marangoni flow caused by exposure of foam containing photosurfactants under UV. Then, the flow analysis of combined elements of foam such as PB-film along with Marangoni flow and PB-node are studied. Next, we contrast the behavior of foams in different conditions. These various conditions can be perturbation in the foam structure caused by injected water droplets or waves or using a non-Newtonian fluid to make the foam. Further review is about the effect of oil droplets and particles on the characteristics of foam such as drainage, stability and interfacial mobility.

Electroosmosis near surfactant laden liquid-air interfaces

Generation of an electroosmostic (EO) flow near a liquid-gas interface covered with ionic surfactants is experimentally investigated. A combination of microscopic flow measurements with a molecular characterization of the interface by second harmonic generation (SHG) shows that under an electrical forcing, although a liquid flow is generated below the free surface, the surfactants remain immobile. The zeta potential was then determined and compared to the surfactant surface coverage. This combination of experimental techniques opens the route to simultaneously probe the liquid flow near a soapy interface and the corresponding surfactant repartition affecting the hydrodynamic boundary condition.

Universal rescaling of flow curves for yield-stress fluids close to jamming

The experimental flow curves of four different yield-stress fluids with different interparticle interactions are studied near the jamming concentration. By appropriate scaling with the distance to jamming all rheology data can be collapsed onto master curves below and above jamming that meet in the shear-thinning regime and satisfy the Herschel-Bulkley and Cross equations, respectively. In spite of differing interactions in the different systems, master curves characterized by universal scaling exponents are found for the four systems. A two-state microscopic theory of heterogeneous dynamics is presented to rationalize the observed transition from Herschel-Bulkley to Cross behavior and to connect the rheological exponents to microscopic exponents for the divergence of the length and time scales of the heterogeneous dynamics. The experimental data and the microscopic theory are compared with much of the available literature data for yield-stress systems.

Surface conductivity measurements in nanometric to micrometric foam films

Foam films (a liquid lamella in air covered by surfactants) are tools of choice for nanofluidic characterization as they are intrinsically nanometric. Their size is indeed fixed by a balance between external pressure and particular molecular interactions in the vicinity of interfaces. To probe the exact nature of these interfaces, different characterizations can be performed. Among them, conductivity in confined systems is a direct probe of the electrostatic environment in the vicinity of the surface. Therefore, we designed a dedicated experiment to measure this conductivity in a cylindrical bubble coupled to interferometry for film thickness characterization. We then show that this conductivity depends on the surfactant nature. These conductivity measurements have been performed in an extremely confined system, the so called Newton black foam films. Unexpectedly in this case, a conductivity close to surface conductivity is recovered.

Foam clogging

To what extent are aqueous foams prone to clogging? Foam permeability is measured as a function of particulate loading (trapped hydrophilic particles) under conditions where the particle to bubble size ratio is allowed to increase when the number of particles per bubble is fixed. In addition to experiments performed on the foam scale, we investigated experimentally and numerically the hydrodynamic resistance of a single foam node loaded with one particle. It is shown that, with respect to solid porous media, aqueous foams clog more efficiently due to two reasons: (i) the deformation of interfaces allows for larger particles to be incorporated within the interstitial network and (ii) the interfacial mobility contributes to lowering of the reduced permeability.

Surface shear inviscidity of soluble surfactants

Foam and emulsion stability has long been believed to correlate with the surface shear viscosity of the surfactant used to stabilize them. Many subtleties arise in interpreting surface shear viscosity measurements, however, and correlations do not necessarily indicate causation. Using a sensitive technique designed to excite purely surface shear deformations, we make the most sensitive and precise measurements to date of the surface shear viscosity of a variety of soluble surfactants, focusing on SDS in particular. Our measurements reveal the surface shear viscosity of SDS to be below the sensitivity limit of our technique, giving an upper bound of order 0.01 μN·s/m. This conflicts directly with almost all previous studies, which reported values up to 10(3)-10(4) times higher. Multiple control and complementary measurements confirm this result, including direct visualization of monolayer deformation, for SDS and a wide variety of soluble polymeric, ionic, and nonionic surfactants of high- and low-foaming character. No soluble, small-molecule surfactant was found to have a measurable surface shear viscosity, which seriously undermines most support for any correlation between foam stability and surface shear rheology of soluble surfactants.

Surface shear rheology of saponin adsorption layers

Saponins are a wide class of natural surfactants, with molecules containing a rigid hydrophobic group (triterpenoid or steroid), connected via glycoside bonds to hydrophilic oligosaccharide chains. These surfactants are very good foam stabiliziers and emulsifiers, and show a range of nontrivial biological activities. The molecular mechanisms behind these unusual properties are unknown, and, therefore, the saponins have attracted significant research interest in recent years. In our previous study (Stanimirova et al. Langmuir 2011, 27, 12486-12498), we showed that the triterpenoid saponins extracted from Quillaja saponaria plant (Quillaja saponins) formed adsorption layers with unusually high surface dilatational elasticity, 280 ± 30 mN/m. In this Article, we study the shear rheological properties of the adsorption layers of Quillaja saponins. In addition, we study the surface shear rheological properties of Yucca saponins, which are of steroid type. The experimental results show that the adsorption layers of Yucca saponins exhibit purely viscous rheological response, even at the lowest shear stress applied, whereas the adsorption layers of Quillaja saponins behave like a viscoelastic two-dimensional body. For Quillaja saponins, a single master curve describes the data for the viscoelastic creep compliance versus deformation time, up to a certain critical value of the applied shear stress. Above this value, the layer compliance increases, and the adsorption layers eventually transform into viscous ones. The experimental creep-recovery curves for the viscoelastic layers are fitted very well by compound Voigt rheological model. The obtained results are discussed from the viewpoint of the layer structure and the possible molecular mechanisms, governing the rheological response of the saponin adsorption layers.

Recirculation model for liquid flow in foam channels

Although extensively studied in the past, drainage of aqueous foams still offers major unaddressed issues. Among them, the behaviour of foam films during drainage has great significance as the thickness of the films is known to control the Ostwald ripening in foams, which in turn impacts liquid drainage. We propose a model relating the films' behavior to the liquid flow in foam channels. It is assumed that Marangoni-driven recirculation counterflows take place in the transitional region between the foam channel and the adjoining films, and the Gibbs elasticity is therefore introduced as a relevant parameter. The velocity of these counterflows is found to be proportional to the liquid velocity in the channel. The resulting channel permeability is determined and it is shown that Marangoni stresses do not contribute to rigidify the channel's surfaces, in strong contrast with the drainage of horizontal thin liquid films. New experimental data are provided and support the proposed model.

Permeability of aqueous foams

We perform forced-drainage experiments in aqueous foams and compare the results with data available in the literature. We show that all the data can be accurately compared together if the dimensionless permeability of the foam is plotted as a function of liquid fraction. Using this set of coordinates highlights the fact that a large part of the published experimental results corresponds to relatively wet foams (epsilon approximately 0.1). Yet, most of the foam drainage models are based on geometrical considerations only valid for dry foams. We therefore discuss the range of validity of the different models in the literature and their comparison to experimental data. We propose extensions of these models considering the geometry of foam in the relatively wet-foam limit. We eventually show that if the foam geometry is correctly described, forced drainage experiments can be understood using a unique parameter --the Boussinesq number.

Instrument and methods for surface dilatational rheology measurements

We describe an instrument combining the advantages of two methods, axisymmetric drop shape analysis for well-deformed drops and capillary pressure tensiometry for spherical drops, both used for measuring the interfacial tension and interfacial rheological parameters. The rheological parameters are the complex interfacial elasticity, and the surface elasticity and viscosity of Kelvin-Voigt and Maxwell rheological models. The instrument is applicable for investigation of the effect of different types of surfactants (nonionic, ionic, proteins, and polymers) on the interfacial rheological properties both of air/water and oil/water interfaces, and of interfaces between liquids with equal mass densities. A piezodriven system and a specially designed interface unit, implemented in the instrument, ensure precise control for standard periodic waveforms of surface deformation (sine, square, triangle, and sawtooth) at a fixed frequency, or produce surface deformation at constant rate. The interface unit ensures accurate synchronization between the pressure measurement and the surface control, which is used for real-time data processing and feedback control of drop area in some of the applications.

Surfactant mixtures for control of bubble surface mobility in foam studies

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.

Foam: a multiphase system with many facets

Liquid foams are an extreme case of multiphase flow systems: capable of flow despite a very high dispersed phase volume fraction, yet exhibiting many characteristics of not only viscoelastic materials but also elastic solids. The non-trivial, well-defined geometry of foam bubbles is at the heart of a plethora of dynamical processes on widely varying length and time scales. We highlight recent developments in foam drainage (liquid dynamics) and foam rheology (flow of the entire gas-liquid system), emphasizing that many poorly understood features of other materials have precisely defined and quantifiable analogues in aqueous foams, where the only ingredients are well-known material parameters of Newtonian fluids and bubble geometry, together with subtle but important information on the surface mobility of the foam. Not only does this make foams an ideal model system for the theorist, but also an exciting object for experimental studies, in which dynamical processes span length scales from nanometres (thin films) to metres (foam continuum flows) and time scales from microseconds (film rupture) to minutes (foam rheology).

Steady drainage in emulsions: corrections for surface Plateau borders and a model for high aqueous volume fraction

We compare extensive experimental results for the gravity-driven steady drainage of oil-in-water emulsions with two theoretical predictions, both based on the assumption of Poiseuille flow. The first is from standard foam drainage theory, applicable at low aqueous volume fractions, for which a correction is derived to account for the effects of the confinement of the emulsion. The second arises from considering the permeability of a model porous medium consisting of solid sphere packings, applicable at higher aqueous volume fractions. We find quantitative agreement between experiment and the foam drainage theory at low aqueous volume fractions. At higher aqueous volume fractions, the reduced flow rate calculated from the permeability theory approaches the master curve of the experimental data. Our experimental data demonstrates the analogy between the problem of electrical flow and liquid flow through foams and emulsions.

Foaming properties of a peptide designed to form stimuli-responsive interfacial films

We have designed an amphipathic peptide, AM1, that can self-assemble at the air-water interface to form an interfacial ensemble capable of switching between a mechanically strong cohesive film state and a mobile detergent state in response to changes in the solution conditions. The mechanical properties of the AM1 ensemble in the cohesive film state are qualitatively equivalent to the protein β-LG, while in the mobile detergent state they are equivalent to the low molecular weight surfactant, SDS. In this work the foaming properties of AM1 are compared to those of β-LG and SDS at the same weight concentration and it is found that AM1 adsorbs rapidly to the interface, initially forming a dense foam like that formed by SDS and superior to β-LG. In addition, under solution conditions where interfacially adsorbed AM1 forms a cohesive film state the foam stability is high, comparable to β-LG. However when the interfacially adsorbed AM1 forms a foam under detergent-state conditions, the foam stability is poor. We have achieved control of foam stability through the design of a peptide that exhibits stimuli-responsive changes in the extent of intermolecular interactions between peptide molecules adsorbed at the air-water interface. These results illustrate the exciting potential of peptide surfactants to form a new class of stimuli-responsive foaming agents.

Remarks on the shear viscosity of surfaces stabilised with soluble surfactants

A survey is made of previously reported values of the surface shear viscosity of sodium dodecyl sulphate solution which reveals inconsistencies. The origin of these inconsistencies is thought to be due to the fact that, because SDS is a soluble surfactant, the surface deformation rate is governed by a three-dimensional sublayer adjacent to the surface and is therefore inherently experiment-dependent. Because of this, only an apparent surface shear viscosity that is specific to a particular experiment can be measured. However, for an insoluble surfactant, an intrinsic two-dimensional surface viscosity can be clearly defined. Some methods of measuring an apparent surface shear viscosity assume that the surface shear viscosity is the only surface property that determines the drainage rate from foam or individual Plateau borders but there is experimental evidence to show that other surface properties may be significant.

Interfacial interactions and colloid retention under steady flows in a capillary channel

Colloidal interfacial interactions in a capillary channel under different chemical and flow conditions were studied using confocal microscopy. Fluorescent latex microspheres (1.1 microm) were employed as model colloids and the effects of ionic strength and flow conditions on colloidal retention at air-water interface (AWI) and contact line were examined in static and dynamic (flow) experiments. Colloids were preferentially attached to and accumulated at AWI, but their transport with bulk solution was non-negligible. Changing solution ionic strength in the range 1-100 mM had a marginal effect on colloidal accumulation, indicating forces other than electrostatic are involved. Flow through the open channel resembled Poiseuille flow with AWI acting as a non-stress-free boundary, which resulted in near stagnation of AWI and consequently promoted colloid accumulation. Retention on contact line was likely dominated by film-straining and was more significant in flow relative to static experiments due to hydrodynamic driving force. Modeling and dimensionless analysis of the flow behavior in the capillary channel clearly indicate the important role of apparent surface viscosity and surface tension in colloidal interfacial retention at the pore scale, providing insight that could improve understanding of colloid fate and transport in natural unsaturated porous media.

Fourier-transform rheology of polymer Langmuir monolayers: analysis of the non-linear and plastic behaviors

The linear regime (LR) of viscoelastic behavior has been found to be limited to rather small values of strain, well below the strains found in many technological processes. A Fourier-transform method is described for analyzing the surface rheology data obtained in insoluble Langmuir polymer monolayers beyond the LR. In the concentrate regime, the monolayers show a transition from elastic to plastic behavior, which is characterized by high irreversibility. A simple 2-D rubber model is presented that describes the behavior of the monolayers in the non-linear region not too far from the end of the LR.

Physical chemistry in foam drainage and coarsening

This review covers recent advances in the study of foam drainage and coarsening, focusing especially on the effective role of the foam chemical components on those aging processes. The determination of the relevant parameters controlling foam drainage and coarsening today remains a major issue: are the physical parameters (like bubble size and liquid fraction) sufficient to define a foam and to predict its evolution, or do the chemical components also matter? And if these foam components are important, one has to determine by which mechanisms, and which microscopic parameters involved in these mechanisms are eventually crucial. I report here recent experimental results, shedding light on these issues. It allows us to summarize how the surfactant, the liquid bulk properties, and the gas modify or not the drainage and coarsening features. The coupling between drainage and coarsening is also discussed, as well as the role of the experimental conditions (sample height, shape or foam uniformity).