Gas-liquid Foam Research Articles

Foaming and emulsifying properties of saponin glycosides: a natural non-ionic surfactant

Most synthetic surfactants are harmful to humans and can cause health problems after prolonged exposure or consumption. This study explores in extensive detail the use of a natural surfactant saponin, extracted from fruit of S. mukorossi, to synthesise oil in water nanoemulsions and gas–liquid foams. The effects of saponin concentration (0.1–2 wt%), pH conditions (2–10) and salt concentration (100–500 mM) on stability and average droplet diameter of sunflower oil-in-water emulsion were investigated. Emulsions were stable at pH 4–10 and salt concentration 100–400 mM for up to 30 days. Critical micellar concentration of 0.5 wt% and the average hydrodynamic diameter of saponin micelle of 4.25 nm are observed. A gas inducing impeller-based device was used to generate foam from saponin solution having different gas hold up values (εG: 0.31, 0.45 and 0.62). Average bubble size of foam and its stability (drainage rate and half-life) are measured. 100% of saponin solution gets incorporated into foam in every trial. Foam drainage rate increased and half-life decreased with increase in gas holdup. It is shown that saponin can be an effective natural and non-toxic alternative to synthetic surfactants for various industrial applications.

Simulation of radiative transfer in gas-liquid foams

The results of the radiation light transfer in the gas-liquid foams are presented. To study the probing light transfer in gas-liquid foam-like media at different stages of aging, a Monte Carlo numerical simulation method is proposed. To take into account the re-reflections at the phase interfaces, the approach based on the Fresnel formula is applied. Kelvin cells structures are considered as a model medium, imitating gas bubbles in a liquid matrix during aging (coarsening), which represents the transition of a foam-like medium from “wet” to “dry”. Such transition is caused by the slow liquid flow along the gas cells walls and Plateau-Gibbs channels due to the influence of the gravitational force in an isolated system. During the evolution process, the volume fraction of the liquid phase decreases and the average size of the gas cells of the foam-like medium increases. The three-dimensional cellular structure at different evolution stages is represented as a system of close-packed ordered spheres or tetradecahedrons of various geometric sizes depending on the aging process duration. The Monte Carlo modeling of the radiation light transfer in scattering two-phase foam-like media, taking into account the redistribution at the interfaces of two phases by use of Fresnel formulas at different coursing stages, is presented. The transmittance and backscattering coefficients of a probing light are obtained using the Monte Carlo numerical simulations and Fresnel formulas for the gas-liquid foams. The obtained results are well correlated with the spectrometric measurements of the foamed liquid produced by Gillete (Gillete shaving cream). The influence of the anisotropy factor on the optical length of the probing light was carried out. The development of the theoretical approaches to the diagnostics of the two-phase foam-like materials makes it possible to synthesize foamed materials with established rheological and structural properties to increase and improve the efficiency of evaluating of the functional characteristics for such materials.

Gas-Liquid Foam Dynamics: From Structural Elements to Continuum Descriptions

Gas-liquid foams are important in applications ranging from oil recovery and mineral flotation to food science and microfluidics. Beyond their practical use, they represent an intriguing prototype of a soft material with a complex, viscoelastic rheological response. Crucially, foams allow detailed access to fluid-dynamical processes on the mesoscale of bubbles underlying the large-scale material behavior. This review emphasizes the importance of the geometry and interaction of mesoscale structural elements for the description of the dynamics of entire foams. Using examples including bulk flow of foam under steady shear, interfacial instabilities, and foam fracture through bubble rupture, this article highlights the wide variety of available theoretical descriptions, ranging from network modeling approaches coupling structural element equations of motion to full continuum models with elastoviscoplastic constitutive relations. Foams offer the opportunity to develop rigorous links between such disparate descriptions, providing a blueprint for physical modeling of dynamical multiscale systems with complex structure.

A novel gas inducing rotor-stator impeller for gas-liquid foam generation

A novel gas inducing rotor-stator impeller was used to produce foam from viscous non-Newtonian shear thinning liquid and non-ionic surfactant Tween 80. Foam produced under various experimental conditions, was characterised based on the bubble size and foam half-life. Higher impeller speed and solution viscosity, resulted in smaller size bubbles and more stable foam. Higher gas hold-up is associated with reduction in bubble size and, longer mixing times resulted in increase in bubble size because of coalescence. Empirical models were developed to predict the foam bubble size and its half-life and a good correlation between the measured data and the predicted data was obtained (R2 > 0.94). Foam bubble size was found to be a function of Reynolds number, mixing time number and Froude number. Foam half-life was found to be dependent on the same dimensionless number along with the Weber number. It is shown that, gas inducing impeller, can produce foam with similar properties at ambient conditions and with better process control, compared to foam produced by conventional agitation-based devices.

Local description of foam flow, deformation and pressure drop in narrow constricted channels

We study in extensive detail the flow of dry and wet gas-liquid foams through a narrow two-dimensional channel comprising a complex constriction. The effects of constriction profile and size of aperture, foam flowrate, liquid holdup and liquid viscosity are experimentally investigated and a sophisticated image analysis procedure is developed to extract local fields of foam flow and deformation. Foam dynamics are scrutinised and interpreted in terms of local velocity distribution, elastic strain (texture tensor), plastic deformation (T1 events) and pressure drop. Important foam transformations occur in the vicinity of the constriction. For the first time, the relationship between pressure drop, flowrate and size of constriction aperture is investigated.

A general patterning approach by manipulating the evolution of two-dimensional liquid foams

The evolution of gas-liquid foams has been an attractive topic for more than half a century. However, it remains a challenge to manipulate the evolution of foams, which restricts the development of porous materials with excellent mechanical, thermal, catalytic, electrical or acoustic properties. Here we report a strategy to manipulate the evolution of two-dimensional (2D) liquid foams with a micropatterned surface. We demonstrate that 2D liquid foams can evolve beyond Ostwald ripening (large bubbles always consuming smaller ones). By varying the arrangement of pillars on the surface, we have prepared various patterns of foams in which the size, shape and position of the bubbles can be precisely controlled. Furthermore, these patterned bubbles can serve as a template for the assembly of functional materials, such as nanoparticles and conductive polymers, into desired 2D networks with nanoscale resolution. This methodology provides new insights in controlling curvature-driven evolution and opens a general route for the assembly of functional materials.

Effect of flotation froth properties on froth rheology

Froth flotation is a widely used process of particle separation exploiting differences in surface properties. The froth performance in a flotation cell is expected to be affected by the froth rheology, as it affects the froth residence time that determines the probability of recovery of valuable minerals in the froth phase. Flotation froths have a similar structure to the gas–liquid foams whose rheology has been widely studied. However, to date, very little work has been done in the rheology of flotation froths owing to their instability and the presence of solid particles (on bubble surfaces and in the plateau borders) that are believed to influence froth rheology and complicate any investigation. In this paper, the effects of froth properties on froth rheology were studied by examining the results of 33 flotation tests performed under various conditions that resulted in changes in the froth properties and, consequently, the froth rheology. The experiments were performed in a 20L continuous flotation cell. It was found that the bubble size and the fraction of lamellae covered by solids defined the froth rheology, while the presence of particles in the plateau borders contributed very little to the froth rheology. A model structure was developed by taking into account froth properties to predict froth viscosity.

Gas absorption and reaction in a wet pneumatic foam

Results for the rate of absorption of carbon dioxide gas into a pneumatic (i.e. continuously rising) foam formed from a sodium carbonate–bicarbonate buffer solution stabilised by a polyglycol frother are presented. Previous studies upon the use of gas–liquid foam for gas absorption operations have demonstrated limited potential for the technique, mainly because the liquid films rapidly become saturated with the absorbing gas due to the relatively low liquid fraction within the foam. However, by adding washwater to the free surface of the foam we created a wetter and more stable foam that (1) exhibits a high liquid fraction and therefore avoids film saturation, and (2) creates greater liquid advection past the gas-liquid surfaces, thereby enhancing the mass transfer coefficient. The interfacial area achieved within the foam was in the range 2100–3200m2m−3 (i.e. much higher than in conventional gas-liquid contactors and the liquid-side mass transfer coefficient was the same magnitude as that achieved in packed beds (i.e. 4–8×10–5ms−1). Indeed, by modelling the foam as a packed-bed of solid spheres (with no adjustable constants), the mass-transfer coefficient in a bed of closely packed solid spheres was found to be in satisfactory agreement with the mass-transfer measured in the wet gas-liquid foam. The hydrodynamic state and the slip velocity between gas and liquid phases is required for the design of wet foam absorption columns, and this can be estimated using a theory of pneumatic foam (Stevenson, 2007).

Flooding in a Vertically Rising Gas–Liquid Foam

The maximum liquid fraction that stable pneumatic foam can support is studied in this paper in order to illuminate change of flow regime from turbulent foam flow to the “emulsion regime” in which the distinct interface between the foam layer and the bubbly liquid is lost; this regime transition is nominated as “flooding”. With further increases in gas flow rate, large (or “gross”) bubbles can appear and rise in the column, the existence of which depends on the absolute pressure at the column bottom. Experiments are carried out using foam stabilized by sodium dodecyl sulfate, and the liquid fraction of both the foam layer and the bubbly liquid are measured using a pressure gradient method. The maximum liquid fraction is found to be at a critical point when the interface between the foam and the bubbly liquid layer disappears. The predicted maximum liquid fraction in the foam is given by ε* = (n – 1)/(n + 1), where n is an adjustable constant that depends upon the characteristics of the gas–liquid interface, and the predictions have been experimentally verified.

Self-similar coalescence of clean foams

AbstractWe consider the stability of a planar gas–liquid foam with low liquid fraction, in the absence of surfactants and stabilizing particles. We adopt a network modelling approach introduced by Stewart & Davis (J. Rheol., vol. 56, 2012, p. 543), treating the gas bubbles as polygons, the accumulation of liquid at the bubble vertices (Plateau borders) as dynamic nodes and the liquid bridges separating the bubbles as uniformly thinning free films; these films can rupture due to van der Waals intermolecular attractions. The system is initialized as a periodic array of equally pressurized bubbles, with the initial film thicknesses sampled from a normal distribution. After an initial transient, the first film rupture initiates a phase of dynamic rearrangement where the bubbles rapidly coalesce, evolving toward a new quasi-equilibrium. We present Monte Carlo simulations of this coalescence process, examining the time intervals over which large-scale rearrangement occurs as a function of the model parameters. In addition, we show that when this time interval is rescaled appropriately, the evolution of the normalized number of bubbles is approximately self-similar.

Bio-based macroporous polymer nanocomposites made by mechanical frothing of acrylated epoxidised soybean oil

Mechanical frothing is one of the most commonly used methods to create gas-liquid foams. Until recently, the polymerisation of mechanically frothed gas-liquid foams was limited to the synthesis of quasi two-dimensional polymer structures, such as films. In this study we show that three-dimensional bio-based polymer foams can be created by microwave curing of gas-soybean oil foams created by mechanical frothing using lauryl peroxide as the radical initiator. It was found that the introduction of air during the mechanical frothing was necessary to create the three-dimensional polymer foams. Using bacterial cellulose nanofibrils (BC) simultaneously as a foam stabiliser has potential because it obstructs the flow of liquid from the lamella region in these gas-soybean oil foams while simultaneously acting as nano-filler in the polymer foam. It was found that the stability of the gas-soybean oil foam templates and the mechanical properties of the polymer nanocomposite foams are enhanced upon the addition of BC in to the foams.

Measurement of bubble size distribution in a gas–liquid foam using pulsed-field gradient nuclear magnetic resonance

Pulsed-field gradient nuclear magnetic resonance, previously used for measuring droplet size distributions in emulsions, has been used to measure bubble size distributions in a non-overflowing pneumatic gas-liquid foam that has been created by sparging propane into an aqueous solution of 1.5g/l (5.20mM) SDS. The bubble size distributions measured were reproducible and approximated a Weibull distribution. However, the bubble size distributions did not materially change with position at which they were measured within the froth. An analysis of foam coarsening due to Ostwald ripening in a non-overflowing foam indicates that, for the experimental conditions employed, one would not expect this to be a significant effect. It is therefore apparent that the eventual collapse of the foam is due to bubble bursting (or surface coalescence) rather than Ostwald ripening. This surface coalescence occurs because of evaporation from the free surface of the foam. An analytical solution for the liquid fraction profile for a certain class of non-overflowing pneumatic foam is given, and a mean bubble size that is appropriate for drainage calculations is suggested.

A viscous–inertial model of foam drainage

The common factor that links various current methods of estimating drainage rate through a gas–liquid foam is that all losses of pressure due to flow are assumed to be entirely viscous. However, by drawing analogy with liquid flow through a packed bed, it is apparent that, for foams that are relatively wet or have relatively high Galileo number, there is a significant inertial loss. This is further demonstrated by determining, using computational fluid dynamics, the pressure losses at a constant expansion with fluid flow boundaries. A foam drainage equation that accounts for inertial pressure losses is proposed by adapting the functional form of the Ergun equation for pressure loss due to flow in a packed bed. This is tested against forced drainage data for foam stabilised by SDS with a mono-dispersed bubble size distribution from the literature. It is shown that the model accurately predicts the results with the use of only one adjustable constant, which is, in fact, the number of inertial velocity heads lost due to flow through a slice of foam of one bubble radius in thickness.

The thinning of lamellae in surfactant-free foams with non-Newtonian liquid phase

Thinning rates of liquid lamellae in surfactant-free non-Newtonian gas–liquid foams, appropriate for ceramic or polymer melts and also in metals near the melting point, are derived in two dimensions by matched asymptotic analysis valid at small capillary number. The liquid viscosity is modelled (i) as a power-law function of the shear rate and (ii) by the Ellis law. Equations governing gas–liquid interface dynamics and variations in liquid viscosity are derived within the lamellar, transition and plateau border regions of a corner of the liquid surrounding a gas bubble. The results show that the viscosity varies primarily in the very short transition region lying between the lamellar and the Plateau border regions where the shear rates can become very large. In contrast to a foam with Newtonian liquid, the matching condition which determines the rate of lamellar thinning is non-local. In all cases considered, calculated lamellar thinning rates exhibit an initial transient thinning regime, followed by a t−2 power-law thinning regime, similar to the behaviour seen in foams with Newtonian liquid phase. In semi-arid foam, in which the liquid fraction is O(1) in the small capillary number, results explicitly show that for both the power-law and Ellis-law model of viscosity, the thinning of lamella in non-Newtonian and Newtonian foams is governed by the same equation, from which scaling laws can be deduced. This result is consistent with recently published experimental results on forced foam drainage. However, in an arid foam, which has much smaller volume fraction of liquid resulting in an increase in the Plateau border radius of curvature as lamellar thinning progresses, the scaling law depends on the material and the thinning rate is not independent of the liquid viscosity model parameters. Calculations of thinning rates, viscosities, pressures, interface shapes and shear rates in the transition region are presented using data for real liquids from the literature. Although for shear-thinning fluids the power-law viscosity becomes infinite at the boundaries of the internal transition region where the shear rate is zero, the interface shape, the pressure and the internal shear rates calculated by both rheological models are indistinguishable.

Squeeze-flow and vane rheometry of a gas–liquid foam

The rheology and slip of a dry shaving foam are investigated using squeeze-flow and rotating-vane methods. Constant-force squeeze flow between planar surfaces is used to study the effect of surface roughness on slip and to obtain the yield stress. Non-slip vane measurements are used to obtain the linear shear viscosity and elasticity at small strains, and the yield stress and strain at large strains. Data are compared with the small-strain Maxwell and Kelvin–Voigt linear-viscoelastic models. An apparent dependence of the yield stress and elasticity on the rotational speed of the vane is shown to result from time-dependent rheological parameters as the foam ages. The effect of viscosity in the pre-yield region may give an erroneous identification of yield.

On the forced drainage of foam

The method of so-called ‘forced drainage’ has been extensively used to characterise the steady one-dimensional drainage of gas–liquid foams. In this paper, it is shown that the liquid drainage rate is approximately independent of surface dilational viscosity and surface tension (insofar as it creates capillary suction). However, liquid drainage rate is dependent on surface shear viscosity and Marangoni stresses. For transient foam drainage, an independence upon surface dilational viscosity is not demonstrated. Previously reported forced drainage data have been described in a dimensionally consistent form so that different studies can be directly compared with one another, and this has revealed two curious anomalies in the data, some of which may be due to previously reported, but generally overlooked, multi-dimensional effects in forced drainage experiments.

Foam Self-Clarification Phenomenon: An Experimental Investigation

This paper is focused on the capabilities of gas-liquidfoamstoattenuateacousticwaves.Itispostu- lated that the sound attenuation phenomenon in foams is largely governed by the hydrodynamic resistance of the Plateau-Gibbs channels (PGC) to the flow of liquid through them. It is shown that the addition of solid par- ticles to gas-liquid foams has opposite effects depend- ing on the concentration of the added solid particles. As long as the concentration of the added solid par- ticles is smaller than a certain critical value the sound attenuation coefficient increases and as a result in the sound velocity decreases. However, if the concentration oftheaddedsolidparticlesbecomeslargerthanthiscrit- ical value the attenuation coefficient decreases and the sound velocity increases. When the concentration of the solid particles reaches some critical value, the particles block the Plateau-Gibbs channels and stop the filtra- tion. As a result the attenuation coefficient of the sound wave decreases while the sound velocity, in such three-

The growth, drainage and bursting of foams

The growth of a foam which is simultaneously draining and bursting is analysed mathematically using the foam drainage equation. The bursting is implemented via a novel upper boundary condition: foam films are assumed to possess a well defined tensile strength. Since capillary suction forces on films depend on the cross-sectional area of surrounding Plateau borders, this is equivalent to assuming a limiting Plateau border area at the top of the foam. The case of a gas–liquid foam being blown up into a vertical column is studied in detail, the results being of relevance to froth stability column measurements. Two regimes of behaviour are possible. The first is where the upward velocity of air relative to the column exceeds the downward gravitational velocity of liquid relative to gas at the top of the foam. Such a foam is predicted to grow indefinitely, although the growth rate at late times is markedly less than that at early times. The second regime is where the upward velocity of the air relative to the column is less than the downward gravitational velocity of liquid at the top: capillary suction always retains a role in determining the net liquid motion in such cases, and the foam grows to a finite steady height. The two respective regimes can also be considered in terms of the froth stability, with the first corresponding to stable foams, and the second to unstable ones. At a fixed air flow velocity, decreasing the foam stability and thereby the film tensile strength, increases the critical Plateau border area for film bursting, and thereby the downward gravitational velocity: this can shift a system from the first regime to the second. We focus on the second regime, and in particular on the case where the foam is nearly stable, in the sense that the final froth height is large (compared to a well defined capillary length scale inherent to the foam itself). The steady state height of the foam and the steady state profile of liquid content versus vertical position are computed. A perturbation eigenvalue analysis is used to determine the exponential rate of approach to steady state. The approach to steady state is predicted to be extremely slow in the case of nearly stable froths, for which the rate of approach eigenvalue becomes vanishingly small.

A new law of thinning in foam dynamics

A new law for the thinning of surfactant-free lamellae (applicable to metallic and ceramic foams with mobile interfaces) in a cross-section of an arid gas-liquid foam is derived using matched asymptotic analysis. Two limiting cases are identified at small capillary number: the well-known semi-arid foam having unit-order liquid fraction and the arid foam in which it is small. The lamellar thinning rates in both cases exhibit t -2 power-law behaviour at long times even though the foam liquid area fractions have different orders of magnitude in capillary number. At early times, arid foam thinning is slowed because the curvature of the capillary quasi-static interfacial region must decrease in order to accommodate the flow from the films. Therefore, the thinning of lamellae feeding into a given Plateau border is coupled and the dynamics is distinct from that of the semi-arid foam. Approximations of rupture times in arid and semi-arid foams are found by calculating the times for lamellae to thin to a pre-specified thickness. For given initial lamellar thicknesses, and for arid and semi-arid foams that have identical initial lamellar liquid areas, the arid foam ruptures more quickly than the semi-arid foam. On the other hand the rupture of lamellae is significantly delayed in arid foam compared to semi-arid foam if the initial lamellar thickness and capillary number are the same.

Sound waves in foams

A new (film) model is proposed for the propagation of sound waves in gas–liquid foams. The model explains the effect of sound retardation in foams by foam films inertness. It qualitatively agrees with the experimental data on the velocity of sound propagation in foams of various structures. The high absorption of sound in foams is explained by hydrodynamic losses in foam films. The experimental dependence of the sound absorption coefficient on the foam expansion is explained. The calculated expansion corresponding to the absorption maximum is close to the experimental value.