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We address the question of the equivalent role of the pressure and temperature on the mechanical properties of highly filled elastomers. It is well-known that in polymer matrixes the equivalence of temperature and pressure results from free volume variations. Our measurements performed on phenylated polydimethylsiloxane (PDMS) chains filled with silica particles show that a temperature–pressure superposition property is still observed in both linear and nonlinear regimes in these systems. However, the temperature–pressure equivalence involves coefficients that are 2 orders of magnitude larger than those in nonreinforced matrixes. We suggest that the mechanical response of the filled elastomers is controlled by the shape of the rigid network made by fillers that are connected by rigid polymer bridges. In this frame, we provide quantitative evidence that the macroscopic behavior of reinforced elastomers is controlled by the variation in the degree of the confinement of polymer chains between particle surfaces.

This work aims at studying the origin of spontaneous emulsification occurring at the oil/water interface. This phenomenon was observed for the five crude oils tested as well as at the interface of an asphaltene-toluene mixture and water. The kinetics of appearance of water microdroplets was slowed down for increasing salt concentrations, and the microdroplet formation ceases when the chemical potential of water they contain is equal to that of the water in the bulk solution. Nucleation events occur at the oil–water interface and at the solid surface/liquid interface: some water microdroplets are stuck together close to the oil/water interface, whereas others grow in oil and sediment or nucleate at the oil/solid surface. This suggests the following mechanism: water molecules diffuse from the water reservoir into the oil phase and then create droplets. These droplets are simultaneously fed by hydrosoluble “osmogeneous” species increasing the osmotic pressure, inducing an osmotic pumping of water molecules into microdroplets.

In this paper, we report on the competition between evaporation and hydrodynamics for advancing drops of polymer solutions. We thus study advancing drops which are allowed to evaporate. Drying drives the accumulation of polymer at the contact line, whereas the advancing motion tends to homogenize the drop. At high velocity, we experimentally verify classical hydrodynamics predictions. At intermediate velocities, drying dominates and the contact line becomes more viscous than the bulk droplet. In the limiting case of very low velocities, the contact line can be partially pinned on the substrate because of the formation of a glassy defect at the contact line.

We study the effect of salt content on the rheological properties of wormlike micelles formed from hexadecyltrimethylammonium bromide (CTAB) in presence of potassium bromide (KBr) and of cetylpyridinium chlorate (CPClO3) in presence of sodium chlorate (ClO3Na). Upon increasing the salt concentration, at fixed surfactant concentration, we observe for both systems a maximum of the zero-shear viscosity η0. For salt concentrations less than that corresponding to the maximum of [MATH], the variation of η0with the surfactant concentration C can be described by a reptation model. Beyond the maximum, the scaling laws obtained for η0(C) are characterized by values of the exponent much smaller than those predicted by the existing theoretical models. The results are qualitatively interpreted by a description based on a structural evolution upon increasing salt content from a system of entangled linear micelles to a multiconnected network. Measurements of the plateau modulus of CTAB solutions, as a function of KBr, give results supporting the above hypothesis.

We have studied the rheological properties of an assembly of swollen gel beads in a lack of solvent. The system is an amorphous assembly of packed soft spheres in a given volume. We have studied the plastic behavior of the system, and interpreted it in terms of bead rearrangements within the assembly.

We have computed in 3-dimensional foams the influence of interfacial rheology on the macroscopic foam viscoelasticity. We show that the two interfacial moduli play different roles. The dilational viscosity is not at all additive to the macroscopic modulus, while both the interfacial shear viscosity and the internal phase modulus are nearly additive. The dilational viscosity is shunted by a grid mode at high frequency. The calculations are done in a selfconsistent effective medium approximation.

Transient electric birefringence experiments were performed on a aqueous dispersions of salt-free charged giant micelles of a recently introduced gemini surfactant (referred as 12-2-12). In the dilute regime, the system consists of small rod-like micelles, while at higher concentrations (semi-dilute region) the micelles are semi-flexible and entangled. For concentrations far from c*, either dilute or semi-dilute regime, the birefringence has the same sign suggesting that the micelles align with the electric field in the same direction in both dilute and entangled regime. For concentrations close to c*, an anomalous signal with an opposite sign is observed. For the entire surfactant concentration range studied here, we show that the static signals of the electric birefringence can be split into three different components with very different characteristic times, each component being associated to specific orientational mechanism: 1) a parallel alignment to the applied electric field after free rotation in the dilute regime, or confined rotation of small segments in the semi-dilute regime, 2) an alignment perpendicular to the field that appears near c*, and interpreted as the result of pre-smectic ordering as recently proposed by Cates, 3) a parallel alignment after disentanglement occurring in the semi-dilute regime.

In this article, we deal with the propagation of ultrasonic waves in monodisperse concentrated oil–in–water emulsions. Using the approximation of Isakovich, we propose two different models, a mirror model and a core–shell model, aiming to describe the temperature field in the dense medium and to supply a correct expression of the ultrasonic wave vector. The comparison between experimental data and theoretical models shows that the core–shell model leads to a very accurate description of the ultrasonic attenuation, in a wide range of frequencies and concentrations, in the case where the thermo–elastic effect, due to the scattering of thermal waves by the particles, is the dominant loss mechanism.

We study the effects of the three- and fourfold crosslinks on wormlike micelles. This model is worked using a correspondence between magnetic models and equilibrium polymers. We obtain the phase diagrams and the critical behavior of the concentration of extremities and of crosslinks. These crosslinks generate a demixing for both three- and fourfold connections. We show that, due to the difference of parity between the three- and fourfold connections, the behavior is quite different for each case.

We study the linear response of “living” systems, which break and recombine randomly. The mean field approximation gives an analytical solution for the time evolution of the response. As an example, the viscosity of living polymers is computed. New scaling laws for viscosity are found in the case of very short lifetimes. The laws do not depend in an obvious way on the exponent of the chain relaxation time with respect to its length. The critical behaviour is confirmed by an exact calculation in the limit of very short lifetimes.

We investigate the thermodynamic properties of a dilute solution of living polymers confined between two solid repulsive walls. We consider both the case of rigid and of flexible macromolecules. When the confined system is taken to be at equilibrium with an external reservoir the living polymers behave similarly to a polydisperse solution of unbreakable chains. However for closed gaps the behaviour is quite different from a classical polymer solution because the worm-like micelles can adapt their intrinsic polydispersity in order to release the confinement constraint. In particular, for rigid living polymers this leads to a divergence of the average chain-length in the limit of strong confinement, as well as to a non-monotonic behaviour of the pressure acting on the walls.

Abstract: In this paper, we report on the competition between evaporation and hydrodynamics for advancing drops of polymer solutions. We thus study advancing drops which are allowed to evaporate. Drying drives the accumulation of polymer at the contact line, whereas the advancing motion tends to homogenize the drop. At high velocity, we experimentally verify classical hydrodynamics predictions. At intermediate velocities, drying dominates and the contact line becomes more viscous than the bulk droplet. In the limiting case of very low velocities, the contact line can be partially pinned on the substrate because of the formation of a glassy defect at the contact line.

In this paper, we report on the competition between evaporation and hydrodynamics for advancing drops of polymer solutions. We thus study advancing drops which are allowed to evaporate. Drying drives the accumulation of polymer at the contact line, whereas the advancing motion tends to homogenize the drop. At high velocity, we experimentally verify classical hydrodynamics predictions. At intermediate velocities, drying dominates and the contact line becomes more viscous than the bulk droplet. In the limiting case of very low velocities, the contact line can be partially pinned on the substrate because of the formation of a glassy defect at the contact line.

This article presents the first experimental study of an advancing contact line for a colloidal suspension. A competition between the hydrodynamic flow due to the drop velocity and the drying is exhibited:  drying accounts for particle agglomeration that pins the contact line whereas the liquid flow dilutes the agglomerated particles and allows the contact line to advance continuously. The dilution dominates at low concentration and high velocity, but at high concentration and low velocity, the contact line can be pinned by the particle agglomeration, which leads to a stick−slip motion of the contact line. The calculation of the critical speed splitting both regimes gives an order of magnitude comparable to that of experiments. Moreover, a model of agglomeration gives an estimation of both the size of the wrinkles formed during stick−slip and the force exerted by the wrinkle on the contact line.

This paper reports on the synthesis and characterization of a series of amphiphilic polymers which exhibit spectacular reversible shear-induced gelation in water. The systems consist of hydrophobically modified copolymers based on N,N-dimethylacrylamide and acrylic acid. The influence of molecular parameters such as the molecular weight and composition of the terpolymers on the rheological properties has been established. In the linear regime, the polymers behave like classical associating polymers: the sharp increase of the viscosity with concentration is the result of the transformation of intra- into intermolecular associations. Moreover, the dynamics of the physical gels obtained at high concentrations is tremendously slowed by the presence of alkyl side chains. Under shear the solutions display a Newtonian plateau at low shear rate followed by a shear-induced gelation. The latter is characterized in terms of a critical shear rate which sharply decreases as the polymer volume fraction increases. Thus, it appears that the shear-induced gelation may be controlled by adjusting the polymer molecular features. The networks built under shear closely behave like highly concentrated solutions at rest, suggesting a possible parallel between increasing shear and increasing concentration. This latter fact is reminiscent of the situation encountered with wormlike micelles. Shear flow could induce a transition between individual aggregates and a transient network as an increase in concentration does, thanks to the sharp modification of the electrostatic screening at the overlap concentration.

We analyze the influence of a glass transition gradient near the particle surfaces on the nonlinear mechanical behavior of reinforced elastomers. We studied systems consisting of grafted silica particles dispersed in a cross-linked poly(ethyl acrylate) matrix. Both particle/matrix interfaces and dispersion states of the particles have been precisely characterized. On the basis of previous studies on the same systems evidencing a glass transition gradient around the particles, we show that the precocious nonlinear mechanical behavior of these filled systems is related to the strain-softening of the glassy polymer shell surrounding the particles surfaces. Comparing strain-softening and melting of the glassy polymer shell, we observe that the strain-softening is enhanced by strong local stress amplifications.

We analyze the influence of a glass transition gradient near the particle surfaces on the nonlinear mechanical behavior of reinforced elastomers. We studied systems consisting of grafted silica particles dispersed in a cross-linked poly(ethyl acrylate) matrix. Both particle/matrix interfaces and dispersion states of the particles have been precisely characterized. On the basis of previous studies on the same systems evidencing a glass transition gradient around the particles, we show that the precocious nonlinear mechanical behavior of these filled systems is related to the strain-softening of the glassy polymer shell surrounding the particles surfaces. Comparing strain-softening and melting of the glassy polymer shell, we observe that the strain-softening is enhanced by strong local stress amplifications.

Cement pastes are used in various conditions of fluidity. This leads to the necessary introduction of polymers in order to control the rheology and to stabilize the slurry against segregation. This work describes the influence of hydrophobically modified acrylate polyelectrolytes on the rheological behavior of a tricalcium silicate paste at a constant solid volume ratio φ = 0.32.
© 2003 Society of Chemical Industry

Filled elastomers exhibit a complex dependence of their viscoelastic modulus as a function of both temperature and frequency. Otherwise, recent observations on thin polymer films have shown that their glass transition temperature depends on their thickness. On the basis of these recent results and on a recent model, we propose that the mechanical behavior of the filled elastomer is strongly influenced by a gradient of the glass transition temperature in the vicinity of the particles. This allows us to suggest a specific temperature−frequency superposition law for filled rubbers. This law seems to apply very successfully on two systems with different dispersion qualities, revealing the existence of a glass transition temperature gradient in the vicinity of the particles.

The swelling properties of model filled elastomers consisting of crosslinked polyethylacrylate chains mixed with grafted silica nanoparticles are analysed using a continuous media mechanics approach. We show that the swelling restriction observed in these filled elastomers cannot be simply explained through the increase of the topological constraints density resulting from the interactions at the interface between the particles and the polymer chains. Strong interactions at the particle/matrix interface lead to geometrical constraints which control and restrict the swelling of the polymer network. We propose a model giving an analytical relation between the swelling properties of the filled elastomer, one of the elastomer without particles, and the volume fraction of solid particles. This model describes relatively well the experimental data obtained for varying topological constraint density at the particle surface.

An associating amphiphilic polyelectrolyte was used as an emulsifier of the n-dodecane−water system. First, the dynamic rheological behavior of polydisperse concentrated direct emulsions with dispersed phase volume fractions up to 0.91 was investigated. Both the Princen and Kiss (J. Colloid Interface Sci. 1983, 91, 160; 1986, 112, 427) and Mason et al. (Phys. Rev. Lett. 1995, 75, 2051) approaches on emulsion elasticity give an adequate fitting of the rheological behavior leading to the conclusion that the emulsion elasticity mainly arises from droplet compression. Second, the shear-induced formation of ordered monodisperse emulsions was studied. More specifically, the conditions under which ordered structures are obtained by shearing polydisperse premixed emulsions within the two-parallel circular glass plates of a homemade shear apparatus were reported. The light scattering patterns, first-order ring, first-order diffuse ring with 6 bright spots, and second-order diffuse ring with 12 bright spots successively observed while shearing the emulsion samples reveal the progressive formation of ordered layers of hexagonal close-packed planes of oil droplets as the shear rate increases. The observation of the diffraction patterns also shows that the level of droplet organization decreases after cessation of flow thus suggesting that the droplets occupy well-defined spatial positions within a sample under shear as compared to a sample after cessation of flow. The radius of the droplets forming the colloidal crystal was systematically measured under shear as a function of the radial distance from the center of the glass slides, the gap spacing, the polymer concentration, and the oil content to follow the development of long-range ordering within the emulsions. The formation of the droplet crystal was found to depend only on the shear rate. Smaller droplets are obtained with increasing both the shear rate and the polymer concentration, and only a slight change in the droplet size could be detected upon varying the volume fraction of the dispersed phase. The results are in qualitative agreement with a coalescence−fractionation mechanism of the droplets under shear.

Abstract:: It has been shown over the last few years that the dynamics close to the glass transition is strongly heterogeneous, both by measuring the diffusion coefficient of tagged particles or by NMR studies. Recent experiments have also demonstrated that the glass transition temperature of thin polymer films can be shifted as compared to the same polymer in the bulk. We propose here first a thermodynamical model for van der Waals liquids, which accounts for experimental results regarding the bulk modulus of polymer melts and the evolution of the density with temperature. This model allows us to describe the density fluctuations in such van der Waals liquids. Then, by considering the thermally induced density fluctuations in the bulk, we propose that the 3D glass transition is controlled by the percolation of small domains of slow dynamics, which allows to explain the heterogeneous dynamics close to T g. We show then that these domains percolate at a lower temperature in the quasi-2D case of thin suspended polymer films and we calculate the corresponding glass transition temperature reduction, in quantitative agreement with experimental results of Jones and co-workers. In the case of strongly adsorbed films, we show that the strong adsorption amounts to enhance the slow domains percolation. This effect leads to 1) a broadening of the glass transition and 2) an increase of T g in quantitative agreement with experimental results. For both strongly and weakly adsorbed films, the shift in T g is given by a power law, the exponent being the inverse of that of the correlation length of 3D percolation.

Abstract:: We introduce a rheological model to describe the low-frequency mechanical properties of systems near a fluid/paste transition. We propose a Landau-like expansion for the vicinity of this transition, treating the stress relaxation rate as an order parameter. This leads to a formally simple model that allows us to describe the interplay between aging and non-linearities in the mechanical response of the system. We focus here on systems prepared by fluidification under a strong shear, on which mechanical measurments are performed (oscillatory rheology, stress relaxation, response to a steady shear rate), after a waiting time during which the system evolves on its own.

We have studied the viscoelastic reponse of a concentrated emulsion. We have detected a slow relaxation in uniaxal flow that does not exist in simple shear flow. We demonstrate that the relaxation originates in the permeation of the water through the network of oil droplets, using a two-fluid model. In addition, we have measured directly the high-frequency behavior up to 10 kHz. It exhibits an anomalous relaxation behavior, in accordance with previous light scattering observations.

The effect of associative polymers on the structure and rheological behavior of colloidal suspensions is discussed. Adding associative polymer is known to increase the viscosity of the suspensions. At high shear rates the increase is close to what could be expected on the basis of the hydrodynamic effects of the added polymer. At low shear rates the viscosity increases much more. Small-angle light scattering (SALS) during flow is used here to investigate the underlying structural mechanisms. The SALS patterns indicate that the associative polymer changes the particulate structure: characteristic butterfly patterns appear even at relatively low particle volume fractions. They are not present in the suspensions without associative polymer. The patterns indicate that fluctuations in particle concentration are more pronounced in the flow direction than in the vorticity direction and that anisotropic particulate structures with an orientation along the vorticity direction develop. The evolution of their characteristic length scale during flow has been followed over time. Changing the hydrophilic part of the polymer from polyacrylamide to polyacrylic acid induces stronger associative interactions. In the suspensions this results in a reduction of the relative viscosity rather than an increase. The difference in degree of associativity between the polymers also has an effect on the SALS patterns in the suspensions both at rest and during flow. The rheology as well as the SALS suggest the presence of a strong polymer network in the second system. The competition between adsorption of the associative polymer on the particles with the intermolecular associations between the polymer chains seems to be responsible for the observed differences.

We study the effects of the three- and fourfold crosslinks on wormlike micelles. This model is worked using a correspondence between magnetic models and equilibrium polymers. We obtain the phase diagrams and the critical behavior of the concentration of extremities and of crosslinks. These crosslinks generate a demixing for both three- and fourfold connections. We show that, due to the difference of parity between the three- and fourfold connections, the behavior is quite different for each case.

An emulsion is a dispersion of one fluid in another nonmiscible fluid, stabilized by a surfactant. Such a mixture is not at thermodynamic equilibrium, and some energy is needed to create the unfavorable interface. This energy is provided by a mechanical stirring. Due to the preparation process, the surfactant is shared among the oil/water interface (droplets surface) and the continuous phase. In this paper, we estimate the emulsification rate in relation with both the dispersed phase volume fraction and the mechanical stirring. For low volume fraction, this rate is very low showing that the mechanical stirring is inadequate, whereas it becomes more efficient (but still insufficient) for higher volume fractions. Using small-angle neutron scattering under shear and rheological measurements, we follow the surfactant distribution when applying a steady flow: small shear flows help the aging whereas high shear flows improve the fragmenting and hence the emulsification.

Many self-diffusion measurements have been performed on giant micelles by either fluorescence recovery after photobleaching (FRAP) or Fourier transform pulsed-gradient spin echo (FT PGSE) and reported by many authors. As a general feature, one observes a minimum of the diffusion coefficient as a function of surfactant concentration, which is not clearly understood and has often been attributed to the existence of connections. In this paper, we explain that this minimum may result from the competition between two mechanisms:  the diffusion of the micelle itself and the surfactant molecule diffusion on the micelle.

In this article, we deal with the propagation of ultrasonic waves in monodisperse concentrated oil?in?water emulsions. Using the approximation of Isakovich, we propose two different models, a mirror model and a core?shell model, aiming to describe the temperature field in the dense medium and to supply a correct expression of the ultrasonic wave vector. The comparison between experimental data and theoretical models shows that the core?shell model leads to a very accurate description of the ultrasonic attenuation, in a wide range of frequencies and concentrations, in the case where the thermo?elastic effect, due to the scattering of thermal waves by the particles, is the dominant loss mechanism.

Transient electric birefringence experiments were performed on a aqueous dispersions of salt-free charged giant micelles of a recently introduced gemini surfactant (referred as 12-2-12). In the dilute regime, the system consists of small rod-like micelles, while at higher concentrations (semi-dilute region) the micelles are semi-flexible and entangled. For concentrations far from c*, either dilute or semi-dilute regime, the birefringence has the same sign suggesting that the micelles align with the electric field in the same direction in both dilute and entangled regime. For concentrations close to c*, an anomalous signal with an opposite sign is observed. For the entire surfactant concentration range studied here, we show that the static signals of the electric birefringence can be split into three different components with very different characteristic times, each component being associated to specific orientational mechanism: 1) a parallel alignment to the applied electric field after free rotation in the dilute regime, or confined rotation of small segments in the semi-dilute regime, 2)?an alignment perpendicular to the field that appears near c*, and interpreted as the result of pre-smectic ordering as recently proposed by Cates, 3) a parallel alignment after disentanglement occurring in the semi-dilute regime.

We have studied the rheological properties of an assembly of swollen gel beads in a lack of solvent. The system is an amorphous assembly of packed soft spheres in a given volume. We have studied the plastic behavior of the system, and interpreted it in terms of bead rearrangements within the assembly. Nous avons ?tudi? les propri?t?s rh?ologiques d'un assemblage de billes de gel gonfl?es en d?faut de solvant. Le syst?me est donc une assembl?e amorphe de sph?res molles ?cras?es ? volume total constant. Nous avons ?tudi? divers aspects du comportement plastique et nous l'avons interpr?t? en termes de r?organisations de billes dans l'assemblage.

We have computed in 3-dimensional foams the influence of interfacial rheology on the macroscopic foam viscoelasticity. We show that the two interfacial moduli play different roles. The dilational viscosity is not at all additive to the macroscopic modulus, while both the interfacial shear viscosity and the internal phase modulus are nearly additive. The dilational viscosity is shunted by a grid mode at high frequency. The calculations are done in a selfconsistent effective medium approximation.

We investigate the thermodynamic properties of a dilute solution of living polymers confined between two solid repulsive walls. We consider both the case of rigid and of flexible macromolecules. When the confined system is taken to be at equilibrium with an external reservoir the living polymers behave similarly to a polydisperse solution of unbreakable chains. However for closed gaps the behaviour is quite different from a classical polymer solution because the worm-like micelles can adapt their intrinsic polydispersity in order to release the confinement constraint. In particular, for rigid living polymers this leads to a divergence of the average chain-length in the limit of strong confinement, as well as to a non-monotonic behaviour of the pressure acting on the walls.

We study the linear response of ?living? systems, which break and recombine randomly. The mean field approximation gives an analytical solution for the time evolution of the response. As an example, the viscosity of living polymers is computed. New scaling laws for viscosity are found in the case of very short lifetimes. The laws do not depend in an obvious way on the exponent of the chain relaxation time with respect to its length. The critical behaviour is confirmed by an exact calculation in the limit of very short lifetimes. Nous ?tudions la r?ponse lin?aire de syst?mes vivants, c'est-?-dire s'associant et se coupant ? l'?quilibre thermodynamique. Une approximation de champ moyen donne une solution analytique pour l'?volution temporelle de la r?ponse. Nous en d?duisons une expression analytique de la viscosit? des polym?res vivants. Nous trouvons de nouvelles lois d'?chelles pour cette viscosit? dans la limite des temps de vie courts devant les temps de relaxations propres. Ces lois d?pendent de fa?on non ?vidente de l'exposant du temps de relaxation par rapport ? la longueur de la cha?ne. Nous v?rifions par un calcul exact l'aspect critique de la r?ponse dans cette limite.

The direct observation of a shear-induced structure in dilute/semidilute giant micellar solutions in water is reported. At rest, the micelles are randomly oriented and the zero-shear viscosity is concentration dependent and varies between 10^-3 and 10^-2 Pa s. Under shear, the solutions have no measurable anisotropy as long as the applied shear rate is less than a critical value determined from optical birefringence and electric conductivity measurements. Above this “critical shear rate”, the viscosity increases, and the solution becomes strongly anisotropic. However, it was found that the “critical shear rate” depends strongly on the gap distance of the Couette cell, thus excluding the possibility of a phase transition at the critical shear rate. Cryo-transmission electron microscopy is used to show shear-induced aggregation of wormlike micelles. It is concluded that shear induces a phase separation between a surfactant rich and a surfactant poor phase at shear rates much lower than the critical value. Initially, the surfactant rich phase forms a network-like superstructure with domain sizes increasing with shear. A strong anisotropy due to the deformation of the network is observed when the domain size reaches the order of the gap size of the Couette. This phenomenon was observed for a large range of concentrations from far below up to several times above the overlap concentration φ*.

Linear viscoelastic behavior of micellar solutions of cetyltrimethylammonium hydroxynaphthalene carboxylate is studied at temperatures above the vesicle−micelle transition under salt-free conditions and also in the presence of salt (sodium bromide and potassium acetate). The salt-free solutions, beyond a crossover volume fraction φ*, obey the micellar growth model of Mackintosh et al. The low values of φ* indicate that the effective charge on the micelles is very low. The decrease of the terminal relaxation time upon increasing surfactant concentration for volume fractions larger than ~6φ* is attributed to the formation of intermicellar connections, favored by the diminution of the electrostatic contributions. A similar effect is found by the addition of a salt that does not bind with the surfactant. The phase diagrams shown in this study suggest that the branched micelles constitute the intermediate structures between linear micelles and bilayers.

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.