A correlation of foam stability with surface shear viscosity and area per molecule in mixed surfactant systems

Friction Dynamics of Foams under Nonlinear Motion

Foams are viscoelastic soft materials with complex mechanical properties. Here, we evaluated the friction dynamics of foams between acrylic plates using a sinusoidal motion friction evaluation system and we found some interesting characteristics under accelerated conditions. On a typical solid surface, a symmetrical friction profile, in which static and kinetic frictions are observed, is obtained under reciprocating nonlinear motion. Meanwhile, significant lubricant effects and velocity-dependent friction profiles without static friction were observed in foams. The friction force in foams increased in proportion to the power of velocity, with a power index of <1. These characteristic and dynamic phenomena in foams were observed in this study. They had been caused by the formation of a thick lubricant film and various dissipative modes including surfactant diffusion, viscous dissipation, and wall slip of bubbles. Moreover, the addition of a thickener increased the friction force and the delay time of friction response and improved the foam durability against normal force and shear. These findings are useful for understanding dynamic phenomena in soft materials.

Impact of Fluorocarbon Gaseous Environments on the Permeability of Foam Films to Air

A foam film, free to move and stabilized with tetradecyltrimethylammonium bromide or sodium dodecylsulfate surfactants, is deposited inside of a cylindrical tube. It separates the tube into two distinct gaseous compartments. The first compartment is filled with air, while the second one contains a mixture of air and perfluorohexane vapor (C₆F₁₄), which is a barely water-soluble fluorinated compound. This foam film thus acts as a liquid semipermeable membrane for gases equivalent to the solid semipermeable membranes conventionally used in fluid separation processes. To infer the rate of air transfer through the membrane, we measure the displacement of the mobile foam film. From this, we deduce the instantaneous permeability of the membrane. In contrast to the permeability of solid membranes, which inexorably decreases over time because they become clogged, an anticlogging effect is observed with a permeability that systematically increases over time. Because the thickness of the film is constant over time, we attribute this to the possibility of adsorbing or desorbing fluorinated gas molecules on the liquid membrane. Indeed, because the partial pressure of the fluorinated gas is high at the beginning of the experiment, the density of the adsorbed molecules is also high, which leads to a low permeability to air transfer. On the contrary, at the end of the experiment, the partial pressure in fluorinated gas and thus the density of the adsorbed molecules are low. This leads to a higher permeability and a less clogged membrane.

Bleomycin Polidocanol Foam (BPF) Stability - In Vitro Evidence for the Effectiveness of a Novel Sclerosant for Venous Malformations

OBJECTIVE

This study investigated the in vitro stability of a novel sclerosant, bleomycin polidocanol foam (BPF), for venous malformation (VM) sclerotherapy.

METHODS

The study was designed with control groups treated with polidocanol (0.5%, 1%, and 3%) only. The experimental groups included 21 BPFs, which was made by dissolving bleomycin at seven different concentrations (0.1%-1.5%) in polidocanol (0.5%, 1%, and 3%). The Tessari method was used to prepare sclerosant foam with a liquid:gas ratio of 1:4 at room temperature in vitro. The foam stability was measured for each group. The decay process, one component of foam stability, was recorded with a camera. Foam decay process experiments were performed 10 times per group. The stability indices included drainage rate, drainage time, half life, and microscopic measurement of the foams (mean bubble diameter, minimum and maximum bubble diameters, wall thickness, and bubble diameter distribution).

RESULTS

Compared with the control groups, the half lives of BPFs mainly increased significantly with the addition of bleomycin (p < .001). BPF with 3% polidocanol and 0.1% bleomycin recorded the highest half life (246 ± 1.6 sec), and this group also achieved the smallest bubble diameter and wall thickness (69.9 μm and 5.80 μm) among the experimental groups. For the same polidocanol concentration, the bubble diameter and wall thickness increased when bleomycin was added.

CONCLUSION

Bleomycin concentrations account for different BPF stability. BPF stability mainly increased significantly with the addition of a small amount of bleomycin but this advantage was no longer apparent with increasing bleomycin dose.

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.

Ultra Low Density and Highly Crosslinked Biocompatible Shape Memory Polyurethane Foams

We report the development of highly chemically crosslinked, ultra low density (~0.015 g/cc) polyurethane shape memory foams synthesized from symmetrical, low molecular weight and branched hydroxyl monomers. Sharp single glass transitions (Tg) customizable in the functional range of 45-70 °C were achieved. Thermomechanical testing confirmed shape memory behavior with 97-98% shape recovery over repeated cycles, a glassy storage modulus of 200-300 kPa and recovery stresses of 5-15 kPa. Shape holding tests under constrained storage above the Tg showed stable shape memory. A high volume expansion of up to 70 times was seen on actuation of these foams from a fully compressed state. Low in-vitro cell activation induced by the foam compared to controls demonstrates low acute bio-reactivity. We believe these porous polymeric scaffolds constitute an important class of novel smart biomaterials with multiple potential applications.

Relaxation processes of PGPR at the water/oil interface inferred by oscillatory or transient viscoelasticity measurements

The rheological properties of PolyGlycerol PolyRicinoleate (PGPR) at the oil/water interface were studied using a drop-shaped tensiometer. Small deformation oscillations of the drop area allow the measurement of the interfacial viscoelasticity spectrum, that is, the elastic and viscous moduli as a function of frequency. Another way to obtain such a spectrum is to perform a transient relaxation measurement from which the relaxation modulus as a function of time is deduced and interpreted. Several models containing one or more relaxation times were considered, and their resulting spectra were compared to the oscillatory ones. Similar results suggest that one could in principle use oscillatory or transient relaxations indifferently. However, the transient relaxation technique proved to be more adapted for the determination of the relaxation times. At low PGPR concentrations in oil, the behavior is controlled by long relaxation times, whereas short ones take over when approaching and exceeding the saturation interfacial concentration. This was understood as a shift from a diffusion-dominated regime to a rearrangements-dominated regime.

Effects of surfactant and electrolyte concentrations on bubble formation and stabilization

As interest in the application of microbubbles grows, it is becoming increasingly important to understand the factors affecting their formation and properties in order to effectively generate microbubbles. This paper investigates the effect of surfactant concentration and electrolyte addition on the size distribution and stability of microbubbles. The anionic surfactant sodium dodecyl sulfate (SDS) was used as the surfactant. Minimum bubble diameter and maximum stability were achieved at surfactant concentrations above the CMC. The effect of the electrolyte addition was studied by adding sodium chloride (NaCl) at an SDS concentration below the critical micelle concentration (CMC). Addition of NaCl decreased bubble size and improved bubble preparation to a certain extent. The addition of salt at low concentrations did not affect the surface tension; however, the surface tension was reduced as salt concentration was increased and reached a constant value for NaCl concentrations above 0.25%. The presence of NaCl resulted in a significant decrease in zeta-potential, implying a reduction in the surface charge of SDS micelles. This result suggests that the presence of NaCl may improve the generation and stability of bubbles by enhancing the structures of the adsorption monolayer and interfacial film.

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.

Liquid drainage through aqueous foam: study of the flow on the bubble scale

Macroscopic properties of foams are highly dependent on the liquid volume fraction, which has motivated many studies on foam drainage in the last decade. Theoretical developments and recent experimental results have suggested that two macroscopic drainage regimes could be expected, in relation with flow transitions occurring at the microscopic level, essentially in the Plateau border channels. We have constructed a setup, the Plateau border apparatus, to study the hydrodynamics of a single Plateau border channel, focusing on the surface properties of the foaming solution. Experimental results have shown that the actual theoretical models only partially predict the dissipation of liquid flow through a Plateau border channel. The major discrepancies can be explained considering additional dissipation processes related to the properties of the interface, and to the liquid flows induced in adjoining films as the liquid flows in the channel. Evidence of the hydrodynamic coupling between the channel and the adjoining films is given in the paper.

Foam drainage on the microscale

The drainage of liquid through a foam involves flow in channels, also called Plateau borders, which generally are long and slender. We model this flow by assuming the flow is unidirectional, the shear is transverse to the flow direction, and the liquid/gas interfaces are mobile and characterized by a Newtonian surface viscosity, which does not depend on the shear rate. Numerical finite difference simulations are performed, and analytical approximations for the velocity fields inside the channels and the films that separate the bubbles are given. We compare the liquid flow rates through interior channels, exterior channels (i.e., channels contacting container walls) and films. We find that when the number of exterior channels is comparable to the number of interior channels, i.e., narrow container geometries, the exterior channels can significantly affect the dynamics of the drainage process. Even for highly mobile interfaces, the films do not significantly contribute to the drainage process, unless the amount of liquid in the films is within a factor of ten of the amount of liquid in the channels.

Foam drainage on the microscale II. Imaging flow through single Plateau borders

The liquid in foam forms an interconnected network, which is composed of Plateau borders, nodes, and films. One of the dominant pathways for foam drainage is flow through Plateau borders, and we use confocal microscopy to obtain experimental results for the flow fields inside individual Plateau borders. For three types of surfactants detailed comparisons are made with a model based upon the influence of surface viscosity at free boundaries between the gas in the bubbles and the liquid in the Plateau borders. The model describes the flows well, and we find good agreement between the surface viscosity predicted by this model and representative values found in the literature. We also give a qualitative description of the flow in the nodes.

Effect of counterions on surface and foaming properties of dodecyl sulfate

The influence of counterions of surfactant on interfacial properties is studied by measuring foamability, foam stability, equilibrium and dynamic surface tension, and surface viscosity. The surfactant chosen is anionic dodecyl sulfate with various counterions, Li(+), Na(+), Cs(+), and Mg(++). Surface tension measurements show a decrease in the following order: LiDS > NaDS > CsDS > Mg(DS)(2). Foamability done using shaking method shows similar order as surface tension, i.e., LiDS > NaDS > CsDS > Mg(DS)(2). This has been explained in terms of the differences in micellar stability and diffusion of monomers. This is further confirmed by our dynamic surface tension results, which show the same order as equilibrium surface tension (i.e., LiDS > NaDS > CsDS > Mg(DS)(2)) at low bubble frequencies but the order is LiDS > NaDS = Mg(DS)(2) > CsDS at high bubble frequencies. Foam stability measurements were done at concentrations below and above cmc to elucidate the role of micelles. It was found that there is no significant change in foam stability when counterions are changed for surfactant concentration values below the cmc, but at concentration above cmc the foam stability of CsDS and Mg(DS)(2) are much greater than LiDS and NaDS indicating presence of stable micelles are essential to high foam stabilities. Surface viscosity measurements correlated well with the foam stability trends and gave the following order LiDS < NaDS < CsDS << Mg(DS)(2), indicating that the molecules of CsDS and Mg(DS)(2) are tightly packed at the air/water interface.

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

Foam drainage varies with surfactant. We present direct measurements of the flow velocity profiles across single Plateau borders, which make up the interconnected channel-like network for liquid flow. For protein foams the interface is rigid, whereas small-surfactant foams show significant interfacial mobility. The results agree with a model that takes into account the shearing of the liquid-gas interface transverse to the flow direction. A significant consequence is that bubble size and liquid volume fraction in a foam affect the relative importance of surface rheology on the drainage behavior.

Liquid Drainage in Single Plateau Borders of Foam

This paper reports on an investigation of the influence of the interfacial shear viscosity on the liquid drainage in single Plateau borders of foam. The simplified Navier-Stokes equation governing the liquid flow is solved for the liquid velocity by the numerical computational method. The numerical results show significant influence of the interfacial shear viscosity on the liquid velocity in the Plateau border. Comparison of the numerical results for the average velocity over the cross-section area of the Plateau border to the available analytical solution shows that the available analytical solution underestimates the average velocity. New, simple yet accurate correlations for the dependence of the average velocity on the radius of the cross section of the Plateau border, the pressure gradient, and the interfacial shear viscosity are obtained using the asymptotic analysis and the numerical data.

An Insoluble Surfactant Model for a Vertical Draining Free Film

A mathematical model is constructed to study the evolution of a vertically oriented thin liquid film draining under gravity when there is an insoluble surfactant with finite surface viscosity on its free surface. Lubrication theory for this free film results in three coupled nonlinear partial differential equations describing the free surface shape, the surface velocity, and the surfactant transport at leading order. We will show that in the limit of large surface viscosity, the evolution of the free surface is that obtained for the tangentially immobile case. For mobile films with small surface viscosity, transition from a mobile to an essentially immobile film is observed for large Marangoni effects. It is verified that increasing surface viscosity and the Marangoni effect retard drainage, thereby enhancing film stability. The theoretical results are compared with experiment; the purpose of both is to act as a model problem to evaluate the effectiveness of surfactants for potential use in foam-fabrication processes. Copyright 2000 Academic Press.

Gravitational Drainage of a Tangentially-Immobile Thick Film

We use lubrication theory to derive an evolution equation for the free surface of a draining thick free film with a zero tangential velocity component along the free surface. The films are "thick" because effects that are important in very thin films, such as intermolecular forces, do not play a role in the situation of interest. The evolution equation results from the balance of surface tension, gravity, and dynamic viscosity. Subregions of the film appear and they involve balancing these effects pairwise. Computations are performed on the full evolution equation and for various boundary conditions corresponding to different parts of the film. Matched asymptotics are developed that predict the behavior as the film enters the bath and these agree very well with the computed results. Comparison with experiment is favorable for the rate of decrease of the film thickness. Copyright 1999 Academic Press.