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In this paper, we investigate the thinning dynamics of evaporating surfactant-stabilised surface bubbles by considering the role of physical-chemistry of solutions used in a liquid bath. We study the impact of the surfactant concentration below and above the cmc (critical micelle concentration) and the role of ambient humidity. First, in a humidity-saturated atmosphere, we show that if the initial thickness depends on the surfactant concentration and is limited by the surface elasticity, the drainage dynamics are very well described from the capillary and gravity contributions. These dynamics are independent of the surfactant concentration. Second, our study reveals that the physical-chemistry impacts the thinning dynamics through evaporation. We include in the model the additional contribution due to evaporation, which shows a good description of the experimental data below the cmc. Above the cmc, although this model is unsatisfactory at short times, the dynamics at long times is correctly rendered and we establish that the increase of the surfactant concentration decreases the impact of evaporation. Finally, the addition of a hygroscopic compound, glycerol, can be also rationalized by our model. We demonstrate that glycerol decreases the bubble thinning rate at ambient humidity, thus increasing their stability.

Large bubbles are primarily used in physics laboratories to study 2D turbulence, surface wavers, and fundamental properties of soap systems. Outside of physics, blowing large bubbles is also a part of many performances and shows. Both the scientific and artistic communities usually want to get reasonably stable foam films. The purpose of this article is to propose the main physical ingredients needed for an effective recipe to make stable films and bubbles. We propose controlled experiments to measure both the ease of generating a bubble, and its stability in different stabilizing solutions, which we choose by adding the ingredients contained in a bubble artist's recipe one by one. Our main findings are that (i) the surfactant concentration must not be too high, (ii) the solution must contain long flexible polymer chains to allow for easy bubble generation and (iii) the addition of glycerol provides improved bubble stability by preventing evaporation. Finally, we propose an efficient recipe, which takes into account these considerations.

Recent advances have demonstrated that evaporation can play a significant role on soap film stability, which is a key concern in many industrial areas but also for children playing with bubbles. Thus, evaporation leads to a film thinning but also to a film cooling, which has been overlooked for soapy objects. Here, we study the temperature variation of an evaporating soap film for different values of relative humidity and glycerol concentrations. We evidence that the temperature of soap films can decrease after their creation up to 8$~^\text{o}$C. We propose a model describing the temperature drop of soap films after their formation that is in quantitative agreement with our experiments. We emphasize that this cooling effect is significant and must be carefully considered in future studies on the dynamics of soap films., Comment: 6 pages

International audience; Production, drainage, and stability of foam films, i.e., films in contact with their menisci, are fascinating problems that remain still unsolved. In this article, we propose to explore the regime of large velocities and large film sizes. This is not accessible in experiments classically conducted in the literature and allows us to study the regime of large extension and large extension rates. With our setup, we make soap films up to 2 m high by pulling a horizontal fishing line driven by belts out of a soapy solution at velocities ranging from 20 cm/s to 250 cm/s. We characterize the thickness profile of the central part of the film that behaves like a rubber band under tension. We show that its thickness profile is well described by a static model in which a homogeneous elastic film is stretched by its own weight. This leads to an exponential thickness profile with a characteristic length given by a competition between gravity and surface elasticity. The prefactor is fixed by the shape and area of the film, governed by the fishing line motion but also by a continuous extraction of foam film from the lateral menisci, thicker than the central part, and that progressively invades the film from its lateral boundaries. The model we propose captures the subtle interplay between gravity, film elasticity, and film extraction and leads to predictions in good agreement with our experimental data.

Quoi de commun entre un marionnettiste et une doctorante en physique, un couple de clowns et une physico-chimiste des matériaux, une vidéaste et un hydrodynamicien ? Une passion commune pour les films, bulles et mousses de savon, et une volonté tenace de construire l’interface artscience pour que les deux parties en retirent des idées fructueuses. Un atelier « Spectacle vivant et sciences » a réuni des artistes et des chercheurs en octobre 2017, et à nouveau en novembre 2019 au Laboratoire de Physique des Solides, à l’Université Paris-Saclay. Accrochez-vous, ça va buller...

Artists, using empirical knowledge, manage to generate and play with giant soap films and bubbles. Until now, scientific studies of soap films generated at a controlled velocity and without any feeding from the top, studied films of a few square centimeters. The present work aims to present a new setup to generate and characterize giant soap films (2 m $$\times $$ 0.7 m). Our setup is enclosed in a humidity-controlled box of 2.2 m high, 1 m long, and 0.75 m large. Soap films are entrained by a fishing line withdrawn out of a bubbling solution at various velocities. We measure the maximum height of the generated soap films, as well as their lifetime, thanks to automatic detection. This is allowed by light-sensitive resistors collecting the light reflected on the soap films and ensures robust statistical measurements. In the meantime, thickness measurements are performed with a UV–VIS-spectrometer, allowing us to map the soap film’s thickness over time.

Although street artists have the know-how to blow bubbles over one meter in length, the bubble width is typically determined by the size of the hoop, or wand they use. In this article we explore a regime in which, by blowing gently downwards, we generate bubbles with radii up to ten times larger than the wand. We observe the big bubbles at lowest air speeds, analogous to the dripping mode observed in droplet formation. We also explore the impact of the surfactant chosen to stabilize the bubbles. We are able to create bubbles of comparable size using either Fairy liquid, a commercially available detergent often used by street artists, or sodium dodecyl sulfate (SDS) solutions. The bubbles obtained from Fairy liquid detach from the wand and are stable for several seconds, however those from SDS tend to burst just before detachment.

Surface bubbles are present in many industrial processes and in nature, as well as in carbonated beverages. They have motivated many theoretical, numerical and experimental works. This paper presents the current knowledge on the physics of surface bubbles lifetime and shows the diversity of mechanisms at play that depend on the properties of the bath, the interfaces and the ambient air. In particular, we explore the role of drainage and evaporation on film thinning. We highlight the existence of two different scenarios depending on whether the cap film ruptures at large or small thickness compared to the thickness at which van der Waals interaction come in to play., SCOPUS: re.j, info:eu-repo/semantics/published

The addition of sweeteners in fizzy beverages not only affects the sugar content but also the bubbles stability. In this article, we propose a model experiment, in which the lifetime of hundreds of single bubbles is measured, to assess the stability of bubbles in solutions containing either sucrose or sweeteners. We show that the bubbles are indeed more stable in presence of sweeteners, which are surface active molecules and adsorb at the interface. Additionally, we test an antifoam at different concentrations and show that our experiment allows to identify the best concentration to reproduce the stability obtained in sucrose when we replace this latter by a sweetener.

Complex liquids flow is known to be drastically affected by the roughness condition at the interfaces. We combined stresses measurements and observations of the flow during the motion of different rough surfaces in dry liquid foams. We visually show that three distinct friction regimes exists: slippage, stick-slip motion, and anchored soap films. Our stress measurements are validated for slippage and anchored regimes based on existing models, and we propose a leverage rule to describe the stresses during the stick-slip regime. We find that the occurrence of the stick-slip or anchored regimes is controlled by the roughness factor, defined as the ratio between the size of the surface asperities and the radius of curvature of the Plateau borders., 12 pages, Supplemental information included

While coalescence is ultimately the most drastic destabilization process in foams, its underlying processes are still unclear. To better understand them, we track individual coalescence events in two-dimensional foams at controlled capillary pressure. We obtain statistical information revealing the influence of the different parameters which have been previously proposed to explain coalescence. Our main conclusion is that coalescence probability is simply proportional to the area of the thin film separating two bubbles, suggesting that coalescence is mostly stochastic.

The interaction of surfactants in solution with hydrophobic materials is at the root of the process of detergency. Lipid containing micelles are formed in solution, and through water rinsing, these structures are washed out from the surface. The presence of air in the solution and the formation of foam add complexity to the system due to the increased proportion of water interfaces in contact with the surface. The latter situation is more difficult to understand. In this work, we propose, as a first step, to explore the role of the interfaces in cleaning silicon wafers previously coated with a model lipid mixture representing human sebum has been investigated. It turns out that the presence of interfaces enhances the cleaning efficiency. The effect of altering the surface properties of the silicon wafer was also investigated and it was found that changing the contact angle of the wafer brought a very strong effect on the cleaning efficiency.

The coarsening of quasi-2D wet foams is well described theoretically by the model of Schimming and Durian, that takes into account the diffusion through the Plateau borders and the vertices in a rigorous manner. In this article, we describe an experimental study of coarsening in which the foam film permeability is measured in such quasi-2D wet foams. We first performed a full characterization of the structure of the studied foams. Then we measured the coarsening rates. It appears that, in these foams, the film thicknesses are still too small for the Plateau borders and the vertices to contribute, but the surface Plateau borders lead to a smaller coarsening rate compared to dry foams. This rate increases with capillary pressure and follows well the prediction of the model. We demonstrate the importance of working in controlled pressure conditions during permeability measurements. Indeed, permeability depends on film thickness itself depending on capillary pressure.

Although soap films are prone to evaporate due to their large surface to volume ratio, the effect of evaporation on macroscopic film features has often been disregarded in the literature. In this work, we experimentally investigate the influence of environmental humidity on soap film stability. An original experiment allows to measure both the maximum length of a film pulled at constant velocity and its thinning dynamics in a controlled atmosphere for various values of the relative humidity [Formula: see text]. At first order, the environmental humidity seems to have almost no impact on most of the film thinning dynamics. However, we find that the film length at rupture increases continuously with [Formula: see text]. To rationalize our observations, we propose that film bursting occurs when the thinning due to evaporation becomes comparable to the thinning due to liquid drainage. This rupture criterion turns out to be in reasonable agreement with an estimation of the evaporation rate in our experiment.

The gas permeability of the thin liquid films which separate neighbouring bubbles in a liquid foam plays a key role in foam coarsening. Different approaches have been developed in the past to measure the film permeability, relying on measurements either on individual films/bubbles or on bulk foams. While the first approach may not be sufficiently representative of a real foam film, the latter is hardly feasible due to a lack of quantitative description of the coarsening of bulk foams with non-negligible liquid content. Here we show that a good compromise between these approaches can be achieved by investigating the coarsening of quasi-two-dimensional foams. More precisely, we propose a particularly simple approach in which we follow the evolution of the number of bubbles of an initially monodisperse foam. In this case, a large number of bubbles disappear simultaneously, leading to a “catastrophic” event, which can be identified easily and accurately related to the film permeability. We demonstrate the potential of this technique by using aqueous foams stabilised by sodium dodecyl sulfate having different liquid fractions and containing different gases. The experiments are compared to Surface Evolver simulations.

In this paper, we investigate how the drainage and rupture of surfactant-stabilised bubbles floating at the surface of a liquid pool depend on the concentration of surface-active molecules in water. Drainage measurements at the apex of bubbles indicate that the flow profile is increasingly plug-like as the surfactant concentration is decreased from several times the critical micellar concentration (cmc) to just below the cmc. High-speed observations of bubble bursting reveal that the position at which a hole nucleates in the bubble cap also depends on the surfactant concentration. On average, the rupture is initiated close to the bubble foot for low concentrations (< cmc) while its locus moves towards the top of the bubble cap as the concentration increases above the cmc. In order to explain this transition, we propose that marginal regeneration may be responsible for bubble rupture at low concentrations but that bursting at the apex for higher concentrations is driven by gravitational drainage.

In this paper, we derive a lubrication model to describe the non-stationary free liquid film that is created when a vertical frame is pulled out of a liquid reservoir at a given velocity. We here focus on the case of a pure liquid, corresponding to a stress-free boundary condition at the liquid/air interfaces of the film, and thus employ an essentially extensional description of the flow. Taking into account van der Waals interactions between the interfaces, we observe that film rupture is well defined in time as well as in space, which allows us to compute the critical thickness and the film height at the moment of rupture. The theoretical predictions of the model turn out to be in quantitative agreement with experimental measurements of the break-up height of silicone oil films in a wide range of pulling velocities and supporting fibre diameters.

The purpose of this article is to compare experiments carried out with single vertical foam films and with foams. We focus on the generation of films and foams and measure (i) the quantity of water entrained and (ii) the stability of the systems. The surfactants we used are C12E6, b-C12G2 and their 1 : 1 mixture because those systems are very well characterised in the literature and are known to stabilise foams with very different properties. We show that the quantity of water uptake in foams and single vertical films scales in the same way with the velocity of generation. However, the different surfactant solutions have different foamabilities, whereas the films they stabilise have exactly the same thickness. Moreover, the foamability of a C12E6 solution is much lower than that of a b-C12G2 solution or of a solution of the 1 : 1 mixture. This is due to the rapid rupture of the C12E6 foam films during foam generation. Surprisingly, the isolated films have exactly the same lifetime for all the surfactant solutions. We conclude that, though drawing a correlation between films and foams is tempting, the results obtained do not allow correlating of film and foam stability during the generation process. The only difference we observed between the single films stabilised by the different solutions is the stability of their respective black films. We thus suggest that the stability of black films during foam generation plays an important role which should be explored further in future work.

Obtaining stable liquid foams is an important issue in view of their numerous applications. In some of these, the liquid foam in itself is of interest, in others, the liquid foam acts as a precursor for the generation of solid foam. In this short review, we will make a survey of the existing results in the area. This will include foams stabilised by surfactants, proteins and particles. The origin of the stability is related to the slowing down of coarsening, drainage or coalescence, and eventually to their arrest. The three effects are frequently coupled and in many cases, they act simultaneously and enhance one another. Drainage can be arrested if the liquid of the foam either gels or solidifies. Coalescence is slowed down by gelified foam films, and it can be arrested if the films become very thick and/or rigid. These mechanisms are thus qualitatively easy to identify, but they are less easy to model in order to obtain quantitative predictions. The slowing down of coarsening requests either very thick or small films, and its arrest was observed in cases where the surface compression modulus was large. The detail of the mechanisms at play remains unclear.

We have studied foams stabilised by surfactant-decorated nanoparticles adsorbed at the bubble surfaces. We show that the controlled compression of a single bubble allows one to understand the coarsening behavior of these foams. When bubbles are compressed, the particles become tightly packed in the surface layer. They lose their mobility, and the interface becomes solid-like when the jammed state is reached. Further compression leads to interfacial buckling characterised by crumpled surfaces. We find that the surface concentration of particles at which the jamming and the buckling transitions occur are independent of the surfactant concentration. This is a surprising feature. It suggests that the surfactants are mandatory to help the particles adsorb at the interface and that they change the equilibrium surface concentration of the decorated particles. But they do not affect the surface properties once the particles are adsorbed. We measured the compression elastic modulus of the surface in the jammed state and found it to be compatible with the Gibbs condition for which the spontaneous dissolution of bubbles is arrested. Due to this effect, the coarsening process of a foam composed of many close-packed bubbles occurs in two steps. In the first step, coarsening is slow and coalescence of the bigger bubbles is observed. In the second step, a number of very small bubbles remains, which exhibit crumpled surfaces and are stable over long times. This suggests that foam coarsening is arrested once the smallest bubbles become fully covered after the initial shrinking step.

A solid withdrawn from a liquid bath entrains a film. In this review, after recalling the predictions and results for pure Newtonian liquids coated on simple solids, we analyze the deviations to this ideal case exploring successively three potential sources of complexity: the liquid-air interface, the bulk rheological properties of the liquid and the mechanical or chemical properties of the solid. For these different complexities, we show that significant effects on the film thickness are observed experimentally and we summarize the theoretical analysis presented in the literature, which attempt to rationalize these measurements.

The properties of liquid dispersions, such as foams or emulsions, depend strongly on the volume fraction $\phi$ of the continuous phase. Concentrating on the example of foams, we show experimentally and theoretically that $\phi$ may be related to the fraction $\phi_s$ of the surface at a wall which is wetted by the continuous phase - given an expression for the interfacial energy or osmotic pressure of the bulk system. Since the surface fraction $\phi_s$ can be readly determined from optical measurement and since there are good general approximations available for interfacial energy and osmotic pressure we thus arrive at an advantageous method of estimating $\phi$. The same relationship between $\phi$ and $\phi_s$ is also expected to provide a good approximation of the fraction of the bubble or drop surface which is wetted by the continuous phase. This is a parameter of great importance for the rheology and ageing of liquid dispersions., Comment: main article: 5 pages, 3 figures supplementary material: 3 pages, 4figures

International audience; Foam drainage dynamics is known to be strongly affected by the nature of the surfactants stabilising the liquid/gas interface. In the present work, we consider a 2D microfoam stabilized by both soluble (sodium dodecylsulfate) and poorly soluble (dodecanol) surfactants. The drainage dynamics is driven by a thermocapillary Marangoni stress at the liquid/gas interface [V. Miralles et al., Phys. Rev. Lett., 2014, 112, 238302] and the presence of dodecanol at the interface induces interface stress acting against the applied thermocapillary stress, which slows down the drainage dynamics. We define a damping parameter that we measure as a function of the geometrical characteristics of the foam. We compare it with predictions based on the interface rheological properties of the solution.

a b s t r a c t We review here how millifluidic techniques can be used to construct versatile Labs-on-a-Chip, in which on-chip bubble generation and flow chemistry are efficiently combined to generate porous solids with highly monodisperse and ordered pore structures. With this approach, a liquid foam with well-controlled structural and chemical properties is first generated, which is then solidified in situ. We discuss here the interplay of various important processing parameters. In particular, we demonstrate that solidification time and foam life time need to be matched wisely in order to control precisely the properties of the final porous material. We also discuss a simple and inexpensive route to the manufacture of chemically and pressure-resistant, millifluidic Labs-on-a-Chip via the machining and hot-pressing of Cyclic Olefin Copolymers. We illustrate our review using the example of the generation of two types of hydrogel foams (synthetic acrylamide-based and biopolymer chitosan-based) and of particle foams, which may be used as green bodies for the manufacture of porous ceramics. The newly designed materials can be used to investigate fundamental questions of the structure-property relationship of porous solid. © 2012 Elsevier B.V. All rights reserved.

Much interest is being given these days to the development of techniques, which render liquid foams not only super-stable, but also responsive to non-contact stimulation. Both of these goals can be met simultaneously by the application of magnetic particles as stabilising agents, which not only suppress the traditional ageing mechanisms of foams (such as coalescence and coarsening), but also provide means of non-contact manipulation and characterisation by the application of magnetic fields. In this spirit we provide here detailed investigations into the use of commercially available MAGSILICA ® H8 nanoparticles, which consist of single-domain iron oxides surrounded by a silica shell, for the generation of super-stable bubbles and foams, and study their response to various types of magnetic fields. More specifically, we investigate the foaming behaviour of hydrophobic magnetic particles in water/ethanol mixtures for which we confirm that best foaming behaviour is achieved for intermediate wettabilities of the particles. We characterise the magnetisation of these foams and use magnetic field gradients to not only validate the super-stability of bubbles, but also to demonstrate the ease of their magnetic manipulation. Both kinds of measurements provide us with more detailed information on particle-detachment forces and the presence of a thick shell of aggregated particles at the gas/liquid interface. Last but not least, we demonstrate how such foams may conveniently be heated and kept at well-controlled temperatures through the application of oscillating magnetic fields.

We use a thin film pressure balance to probe the rheological properties of thin liquid films. These films are made from mixed aqueous solutions of surfactants and polyelectrolytes. They drain under applied pressure in a noncontinuous way due to a stratification process of the polyelectrolytes network. The stratification kinetics was studied for films stabilized by different surfactants. Using a theoretical model, it is possible to examine the effect of both the surfactant and the film thickness on the local dissipation. On one hand, it was observed that dissipation depends on the polyelectrolyte concentration only, regardless whether the surfactant is neutral or bears electric charges opposite to those of the polyelectrolyte. On the other hand, it was found that dissipation is stronger in thinner films.

We have investigated the rheological behaviour of silica nanoparticle layers at the air-water interface. Both compressed and deposited layers have been studied in Langmuir troughs and with a bicone rheometer. The compressed layers are more homogeneous and rigid, and the elastic response to continuous, step and oscillatory compression are similar, provided the compression is fast enough and relaxation is prevented. The deposited layers are less rigid and more viscoelastic. Their shear moduli deduced from the oscillatory uniaxial compression are much smaller than those deduced from pure shear deformation suggesting that the effective shear rate is smaller than expected in the compression measurements.

Rising foams created by continuously blowing gas into a surfactant solution are widely used in many technical processes, such as flotation. The prediction of the liquid fraction profile in such flowing foams is of particular importance since this parameter controls the stability and the rheology of the final product. Using drift flux analysis and recently developed semi-empirical expressions for foam permeability and osmotic pressure, we build a model predicting the liquid fraction profile as a function of height. The theoretical profiles are very different if the interfaces are considered as mobile or rigid, but all of our experimental profiles are described by the model with mobile interfaces. Even the systems with dodecanol are well known to behave as rigid in forced drainage experiments. This is because in rising foams the liquid fraction profile is fixed by the flux at the bottom of the foam. Here the foam is wet with higher permeability and the interfaces are not in equilibrium. These results demonstrate once again that it is not only the surfactant system that controls the mobility of the interface, but also the hydrodynamic problem under consideration. For example liquid flow through the foam during generation or in forced drainage is intrinsically different.

International audience; Reflets de la Physique n° 46 34 Enseigner la physique à l'université Colloque régional, 6-7 juillet 2015 – Paris Un colloque sur l'enseignement de la physique à l'université : une gageure ! Les 6 et 7 juillet 2015 s'est tenu à Paris le colloque « EPU2015 : Enseigner la physique à l'université ». Ce colloque avait pour objectif de réunir les acteurs de l'enseignement à l'université et de promouvoir une réflexion centrée sur l'enseignement d'une discipline spécifique : la physique. Cette caractéristique apparaît nécessaire : l'identité disciplinaire est une composante forte de la manière dont se façonne le métier d'enseignant à l'université dans un contexte particulier où les enseignants sont, pour la plupart, des chercheurs. La récente refonte des programmes de physique dans l'enseignement secondaire a suscité d'importantes adaptations des maquettes de licence, et de nombreuses expériences pédagogiques ont été testées dans les universités ces dernières années. Dans ce contexte, où chaque université tire les premiers bilans de ces modifications, le colloque « Enseigner la physique à l'université » se proposait de mettre en commun ces expériences nouvellement acquises dans les différents départements de physique des universités franciliennes. L'un des partis pris de cette conférence était également d'associer des enseignants issus de deux communautés : la recherche en physique et celle sur l'enseignement de la physique. Au-delà des défiances qui ont pu marquer certaines de leurs relations, physiciens et didacticiens se sont donc retrouvés autour d'échanges aussi complémentaires que constructifs. Ces deux journées ont constitué une expérience novatrice pour la plupart des participants (et des organisateurs), qui ont eu pour la première fois l'occasion d'échanger sur leurs pratiques pédagogiques et leurs questionnements en dehors des frontières de leur université. Pour cette première expérience, où la popularité que rencontrerait cet événement auprès de la communauté visée était imprévisible, nous avions souhaité lui donner un caractère régional. Le colloque EPU 2015 a donc été annoncé essentiellement dans les universités d'Île-de-France, mais certains collègues de province s'étaient déplacés pour y participer. Au total, le nombre d'inscrits s'élevait à 115 ; 60 à 80 participants étaient présents tout au long des deux journées, et 40 communications orales ont été présentées. Le programme des communications a été établi en alternant retours d'expériences et analyses sur le fond de certaines pratiques d'enseignement. Les deux journées se sont déroulées dans une atmosphère de partage et de découverte. Elles ont fait ressortir une culture commune des enseignants de physique à l'université, et un souhait réel de poursuivre les échanges qui ont eu lieu autour des pratiques pédagogiques. Sur le fond, les journées EPU 2015 ont été l'occasion de découvrir un large spectre d'expériences et des pratiques variées : par les thèmes choisis (le rôle de l'expérimentation, de la problématisation, la question de la transition lycée/université, celle de la motivation des étudiants, etc.), mais aussi par les espaces questionnés (TP, cours en amphi, TD, etc.). Le plus frappant, c'est que chacun paraissait porté par une volonté partagée de faire de l'enseignement un espace de rapprochement des étudiants avec la physique. Enseigner : rapprocher les étudiants et la physique Rapprocher les étudiants et la physique, c'est rapprocher les étudiants et l'enseignant lui-même, vecteur d'enthousiasme pour cette physique à laquelle il consacre sa vie de chercheur. Cela peut passer par l'installation d'une forme de complicité, de « conni-vence » avec l'enseignant, que semblent favoriser le travail en petits groupes au sein des TD (Lidgi-Guigui), ou les « manips de cours », dont les résultats surprenants captent l'intérêt, et dont l'enjeu (succès ou échec) renforce la complicité enseignant-étudiant (Boissé). Rapprocher les étudiants et la physique, c'est recréer de l'appétence pour une discipline délaissée par des lycéens que les sciences faisaient pourtant rêver lorsqu'ils étaient enfants (Fuchs). Rapprocher les étudiants et la physique, c'est présenter une physique plus « authentique », plus « proche de la science telle qu'elle se fait », celle qui s'incarne, par exemple, dans les nanosatellites (a) (Rambaux, Hallouin) ; une physique indisso-ciable dans sa pratique de sa dimension expérimentale (Latour), une physique stimulante dont on va trouver trace dans le sport (Elias, voir encadré p. 36) et dans les jouets (Baumberger). « On a les étudiants que l'on a » Loin des poncifs souvent rebattus, les journées EPU 2015 ont laissé peu de place aux lamentations. Certes les étudiants ont un passé lycéen moins mathématique, plus culturel, moins formel (Fontaine), certes ils ne connaissent plus la formule de l'aire d'un Le premier colloque « Enseigner la physique à l'université », qui s'est tenu à Paris début juillet 2015, a regroupé une centaine d'enseignants-chercheurs, franciliens pour la plupart. À travers une quarantaine de présentations orales (*) , un enrichissant partage d'expériences et de pratiques a été fait dans un cadre dépassant les frontières de chaque université. Loin d'être une corvée, l'enseignement s'est ainsi vu flatté comme un domaine de réflexions, d'innovations, où la réussite des étudiants, formés pour mais aussi par la physique, est au coeur des préoccupations d'enseignants enthousiastes qui mettent naturellement leurs qualités de physiciens au service de leurs enseignements. (*) Les noms cités entre parenthèses sont ceux des intervenants, dont les présentations peuvent être trouvées sur le site internet du colloque.

An optical method for probing contact angle distribution along contact lines of any shape using a laser sheet is proposed. This method is applied to a dry patch formed inside a film flowing along an inclined plane, both liquid and solid being transparent. Falling normally to the plane, a laser sheet cuts the contact line and is moved along this line. Distortions of the sheet trace observed on a screen put below the plane allow us to extract the contact angle distribution and the local line inclination along the line. Our results show that the contact angle around a dry patch is nearly constant and equal to the static advancing angle, at least when the evolution of its shape is followed for increasing flow rates. This supports a model of dry patch shape recently proposed by Podgorski and co-workers. Preliminary results obtained for decreasing flow are also qualitatively observed.

International audience; Foams made of gas bubbles dispersed in a liquid have limited stability and disappear rapidly unless surface active species are used. Foams can be a nuisance or very much sought after, however the control over the foaming and stability is still hampered because of the limited understanding of foam properties. On Earth, liquid rapidly drains out of the foam because of gravity, the liquid films formed between bubbles thin and break. In microgravity conditions, gravity drainage is suppressed and stability is expected to be greatly enhanced. We describe investigations of foams that are very unstable on Earth, including foams made with liquids containing antifoaming agents. Experiments performed in the International Space Station (ISS) show that foam generation can still be limited, however once created these foams are very stable.

The purpose of this article is to compare experiments carried out with single vertical foam films and with foams. We focus on the generation of films and foams and measure (i) the quantity of water entrained and (ii) the stability of the systems. The surfactants we used are C12E6, β-C12G2 and their 1 : 1 mixture because those systems are very well characterised in the literature and are known to stabilise foams with very different properties. We show that the quantity of water uptake in foams and single vertical films scales in the same way with the velocity of generation. However, the different surfactant solutions have different foamabilities, whereas the films they stabilise have exactly the same thickness. Moreover, the foamability of a C12E6 solution is much lower than that of a β-C12G2 solution or of a solution of the 1 : 1 mixture. This is due to the rapid rupture of the C12E6 foam films during foam generation. Surprisingly, the isolated films have exactly the same lifetime for all the surfactant solutions. We conclude that, though drawing a correlation between films and foams is tempting, the results obtained do not allow correlating of film and foam stability during the generation process. The only difference we observed between the single films stabilised by the different solutions is the stability of their respective black films. We thus suggest that the stability of black films during foam generation plays an important role which should be explored further in future work.

Recent advances in the coalescence in liquid foams are reviewed, with a special focus on the multiscale structure of foams. Studies concerning the stability of isolated foam films, on the one hand, and the coalescence process in macroscopic foams, on the other hand, are not always in good agreement. This discrepancy reveals that two routes can induce coalescence in a foam. The first route is thermodynamic and shows that coalescence is governed by a stochastic rupture of foam films. The second route relies on a mechanically induced rupture of the films, due to the spontaneous evolution of foams. From a literature review, the evaluation of the different timescales involved in these mechanisms allows defining the limiting parameters of foam coalescence.

International audience; Many physical properties of concentrated dispersions of immiscible fluids are captured by the concept of an osmotic pressure, which measures how much energy is required to deform the bubbles or drops upon compaction. This pressure has a strong impact on the flow and drainage behavior of dispersions. Nevertheless, theoretical models describing its variation with the volume fraction phi of the continuous phase are so far available only in the limits of low or high phi and experimental data are scarce. We report an experimental study of osmotic pressure in foams and emulsions, showing how the effects of phi, disorder, grain size, polydispersity and interfacial tension can all be captured by a single law which satisfies previously established theoretical constraints. Building on this result, we propose the first equation which accurately describes the variation of the volume fraction with the height of a fluid dispersion under gravity.

An antibubble is a spherical air film that is immersed in a surfactant mixture and drains under the action of hydrostatic pressure. A dynamical model of this film is proposed that accounts for the surface shear viscosity effects in the case of purely viscous interfaces, which applies for surfactants whose adsorption rate is much larger than advection rate and at a concentration much above the critical micelle concentration. Our model shows that the lifetime of the antibubbles in this case increases with surface shear viscosity, denoted ", whose value is measured independently, all in agreement with experimental measurements. We also found that the critical thickness, hc, at film rupture due to van der Waals interactions slightly depends on the surface shear viscosity, namely hc / " 1=6 . DOI: 10.1103/PhysRevLett.109.264502

International audience; We present a large range of experimental data concerning the influence of surfactants on the well-known Landau-Levich–Derjaguin experiment where a liquid film is generated by pulling a plate out of a bath. The thickness h of the film was measured as a function of the pulling velocity V for different kinds of surfactants (C12E6, which is a nonionic surfactant, and DeTAB and DTAB, which are ionic) and at various concentrations near and above the critical micellar concentration (cmc). We report the thickening factor α = h/hLLD, where hLLD is the film thickness obtained without a surfactant effect, i.e., as for a pure fluid but with the same viscosity and surface tension as the surfactant solution, over a wide range of capillary numbers (Ca = ηV/γ, with η being the surfactant solution viscosity and γ its surface tension) and identify three regimes: (i) at small Ca α is large due to confinement and surface elasticity (or Marangoni) effects, (ii) for increasing Ca there is an intermediate regime where α decreases as Ca increases, and (iii) at larger (but still small) Ca α is slightly higher than unity due to surface viscosity effects. In the case of nonionic surfactants, the second regime begins at a fixed Ca, independent of the surfactant concentration, while for ionic surfactants the transition depends on the concentration, which we suggest is probably due to the existence of an electrostatic barrier to surface adsorption. Control of the physical chemistry at the interface allowed us to elucidate the nature of the three regimes in terms of surface rheological properties.

International audience; In this study we have investigated mixtures of oil droplets and gas bubbles and show that the oil can have two very different roles, either suppressing foaming or stabilising the foam. We have foamed emulsions made from two different oils (rapeseed and dodecane). For both oils the requirement for the creation of foamulsions is the presence of surfactant above a certain critical threshold, independent of the concentration of oil present. Although the foamability is comparable, the stability of the foamed emulsions is very different for the two oils studied. Varying a few simple parameters gives access to a wide range of behaviours, indeed three different stability regimes are observed: a regime with rapid collapse (within a few minutes), a regime where the oil has no impact, and a regime of high stability. This last regime occurs at high oil fraction in the emulsion, and the strong slowing down of ageing processes is due to the confinement of packed oil droplets between bubbles. We thus show that a simple system consisting of surfactant, water, oil and gas is very versatile and can be controlled by choosing the appropriate physical chemical parameters.

Classical Frankel's law describes the formation of soap films and their evolution upon pulling, a model situation of film dynamics in foams (formation, rheology, and destabilization). With the purpose of relating film pulling to foam dynamics, we have built a new setup able to give an instantaneous measurement of film thickness, thus allowing us to determine film thickness profile during pulling. We found that only the lower part of the film is of uniform thickness and follows Frankel's law, provided the entrainment velocity is small. We show that this is due to confinement effects: there is not enough surfactant in the bulk to fully cover the newly created surfaces which results in immobile film surfaces. At large velocities, surfaces become mobile and then Frankel's law breaks down, leading to a faster drainage and thus to a nonstationary thickness at the bottom of the film. These findings should help in understanding the large dispersion of previous experimental data reported during the last 40 years and clarifying the pulling phenomenon of thin liquid films.

International audience; We have studied the properties (surface pressure, compression and shear moduli, texture) of silica nanoparticle layers at the air-water interface. Particle hydrophobicity or equivalently the contact angle between particles, air and water, is the main factor that influences surface organisation and surface elastic moduli. The surface layers are denser for particles of higher hydrophobicity. The compression and shear moduli, as well as the yield and melt strains, present a maximum for contact angles around 90°. The dependence of mechanical properties on particle hydrophobicity is closely related to the foamability and stability of the foams made from dispersions.

International audience; Foams are dispersions of bubbles in liquids, often water. They are frequently stabilized by surfactant or polymer, but like Pickering emulsions, they can be also stabilized solely by particles. If the particles have a moderate hydrophobicity, the foams can be extremely stable (lifetimes of the order of years). Due to technical preparation difficulties, very few studies can be found in the literature to date. We will discuss the origin of these difficulties. We will also describe experiments using partially hydrophobic fumed silica particles, in which the foam properties were correlated with the properties of spread and adsorbed layers of these particles at the air-water interface. These combined experiments allowed us to define the conditions necessary to stop bubble disproportionation.

International audience; Using mixtures of the anionic myristic acid (C13COOH) and the cationic cetyl trimethylammonium chloride (C(16)TA(+)Cl(-)) in aqueous solutions at a 2 : 1 ratio, we show that the outstanding stability of foams generated from sufficiently concentrated "catanionic" surfactant mixtures can be explained by a synergy effect between two fundamentally different mechanisms. Applying a multi-scale approach, in which we link static and dynamic properties of the bulk solutions, isolated gas/liquid interfaces, thin liquid films and foams, we identify these two mechanisms to be as follows: firstly, cationic mixtures create tightly packed surfactant layers at gas/liquid interfaces, which are strongly viscoelastic and also confer high disjoining pressures when two interfaces are approaching each other to form a thin liquid film. Foams created with such kind of interfaces tend to be extremely stable against coalescence (film rupture) and coarsening (gas exchange). However, typical time scales to cover the interfaces are much longer than typical foaming times. This is why a second mechanism plays a key role, which is due to the presence of micron-sized catanionic vesicles in the foaming solution. The bilayers of these vesicles are in a gel-like state, therefore leading to nearly indestructible objects which act like elastic micro-spheres. At sufficiently high concentrations, these vesicles jam in the presence of the confinement between bubbles, slowing down the drainage of liquid during the initial foaming process and therefore providing time for the interfaces to be covered. Furthermore, the tightly packed vesicles strongly reduce bubble coalescence and gas transfer between bubbles.

The role of surface rheology in fundamental fluid dynamical systems, such as liquid coating flows and soap film formation, is poorly understood. We investigate the role of surface viscosity in the classical film-coating problem. We propose a theoretical model that predicts film thickening based on a purely surface-viscous theory. The theory is supported by a set of new experimental data that demonstrates slight thickening even at very high surfactant concentrations for which Marangoni effects are irrelevant. The model and experiments represent a new regime that has not been identified before., SCOPUS: ar.j, info:eu-repo/semantics/published

International audience; We have performed a quantitative study of the coarsening of foams stabilised by partially hydrophobic silica nanoparticles. We have used a variety of techniques: optical and electron microscopy, microfluidics, and multiple light scattering. Using earlier studies of planar particle monolayers, we have been able to correlate the interfacial properties and the macroscopic temporal evolution of the foam. This has shed light on the origin of the absence of coarsening of particle-stabilised foams. Such particle-stabilised foams appear to be the only known foam system where coarsening is inhibited by surface elasticity.

In plants, osmotically driven flows are believed to be responsible for translocation of sugar in the pipe-like phloem cell network, spanning the entire length of the plant. In this paper, we present an experimental and theoretical study of transient osmotically driven flows through pipes with semipermeable walls. We extend the experimental work of Eschrich, Evert and Young \cite[]{Eschrich:1972} by providing a more accurate version of their experiment allowing for better comparison with theory. In the experiments we measure the dynamics and structure of a "sugar front", i.e. the transport and decay of a sudden loading of sugar in a pipe which is closed in both ends. We include measurements of pressure inside the membrane tube allowing us to compare the experiments directly with theory and, in particular, to confirm quantitatively the exponential decay of the front in a closed tube.In a novel setup we are able to measure the entire concentration profile as the sugar front moves. In contrast to previous studies we find very good agreement between experiment and theory. In the limit of low axial resistance (valid in our experiments as well as in many cases in plants) we show that the equations can be solved exactly by the method of characteristics yielding, in general, an implicit solution. Further we show that under more general conditions the equations of motion can be rewritten as a single integro-differential equation, which can be readily solved numerically. The applicability of our results to plants is discussed and it is shown that it is probable that the pressure-flow hypothesis can account for short distance transport of sugar in plants., Comment: 34 pages, Submitted to Journal of Fluid Mechanics on May 28, 2008

International audience; 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.