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International audience

This experimental thesis on the physics of liquid foams highlights the coupling between the different organization length scales in foams. In the first part of this work we report some results about the physical chemistry of aqueous foams by testing new formulations. We show how a surfactant foam doped with laponite presents an unusual behaviour, and interfacial studies about a thermo sensitive polymer, poly(N-isopropylacrylamide), and a photosensitive surfactant (AzoTAB). In a second part, we talk about a new experimental protocol in oscillatory rheology applied to aqueous foams and discuss about acoustical properties of this material., Dans cette thèse expérimentale sur la physique des mousses liquides, plusieurs thématiques sont abordées ayant pour point commun la mise en évidence du couplage entre les différentes échelles d'organisation de la mousse. La première partie traite plus spécifiquement de physico chimie par la modification de la formulation des solutions utilisées. Nous avons ainsi réalisé des mousses dopées à la laponite, présentant des propriétés inhabituelles. Nous exposons par ailleurs les résultats d'études interfaciales d'un polymère thermosensible, le poly(N-isopropylacrylamide) et d'un tensioactif photosensible (AzoTAB). Dans la seconde partie de cette thèse, nous discutons d'un nouveau protocole de rhéologie appliqué aux mousses ainsi que de propriétés acoustiques de ce matériau.

International audience; We present an experimental investigation of the acoustic properties of liquid foams. Velocity and attenuation of sound were measured in different foam samples, with a systematic study of the role of parameters such as the frequency, the liquid volume fraction, the bubble size, the nature of the gas, and the nature of the surfactants. Three experimental setups were used to cover a frequency range spanning three orders of magnitude (500 Hz to 600 kHz): an impedance tube [Pierre, 2013a], a pair of narrow-band 40 kHz transducers [Ben Salem, 2013], and a pair of broadband transducers [Pierre, 2013b]. Two main regimes of propagation were identified, with a limit at approximately fR=5 kHz.mm, where f is the frequency and R the average bubble size. At low frequencies (or for small bubbles), the velocity was found to follow the so called mixture law, i.e. depending only on the liquid volume fraction, with no clear influence neither on the bubble size, nor on the type of surfactant. A noticeable exception were Gillette foam samples, in which the velocity was higher than expected. The attenuation, on the other hand, was found to depend mainly on the nature of the gas and the bubble size. Three sources of attenuation were identified: thermal losses due to heat exchange during bubble oscillations, local viscous losses, and viscous losses along the wall of the tube (Kirchhoff losses). The latter term could be related to a macroscopic viscosity of the foam, whose order of magnitude and dependence on R were in good agreement with previous measurements at lower frequencies [Costa, 2013].In the second regime, at high frequencies (or for large bubbles), the propagation of acoustic waves was dispersive, with a peak of attenuation and a velocity reaching values substantially higher than in the previous regime. The frequency of the maximum of attenuation was close to the Minnaert resonance of the individual bubbles of the foams, suggesting that foams might behave acoustically like (very concentrated) bubbly liquids.

International audience; Liquids that produce stable foams on Earth also produce stable foams in microgravity environments as it could be expected (1). It is less obvious to guess what will happen with liquids that produce small amounts of foam or no foam at all on Earth. Experiments in parabolic flights and in the International Space Station showed that even water can produce significant amount of foams, although only if the liquid volume fraction is higher than about 35%. Ethanol-water mixtures also produce foams in microgravity, the amount being maximum in a range of ethanol volume fraction of 10-20% as on Earth (but the foam volume in microgravity is much larger). Still more unexpected, aqueous surfactant solutions containing antifoam particles (hydrophobic silica, nonionic solutions above the cloud point) produce appreciable amounts of foams in microgravity. All the foams produced in microgravity remain stable over long periods of time. We will explain how we have been able to rationalize these observations.

European Conference on Foams and Applications

Liquid dispersions like foams or emulsions, as well as gels, are potentially good templates for conceiving stimuli-responsive materials ; in particular, this is due to the hierarchical organization at different lengthscales and to the fact that tiny effects at mesoscopic scales can have strong macroscopic impact. So, doping soft materials with responsive molecules is a possible route to create macroscopic responsive materials. However, one has to understand how the responsivity at the scale of the molecule can be efficiently transposed to supramolecular and macroscopic structures. In that respect, we have investigated - starting from a model thermo-responsive polymer - if and how responsive liquid interfaces, foams and gels can be made. By responsive, we mean drastic and reversible changes induced by changing temperature only over a small range of degrees. Here we report results based on the use of poly N-isopropylacrylamide - PNIPAM - a thermo-sensitive polymer, which is known to have various reversible configurations in bulk depending on the temperature. We first present interfacial results showing how dynamical properties (both the dilational and shear interfacial rheology) can be widely and reversibly tuned by small changes of temperature, and how the mixing with surfactants modify these results. Based on these results, we present our attempts to make thermo-sensitive 3D foams, containing PNIPAM associated or not with surfactants : we then discuss if and how the foam properties can be strongly and reversibly tuned by the external temperature. Lastly, we present results on pnipam-based gels ; together with responsive properties, we will show that macroscopic deformations (bending, twisting) can be induced if the thermal stimuli is not spatially homogeneous. All together, these studies have allowed us to better understand how complex the route from a stimuli-responsive molecule to a stimuli-responsive materials can be.

International audience; Using simply light at different wavelengths, we show how an oil droplet floating on an aqueous solution can be trapped and rapidly moved along any desired shapes. The technique is based on the presence of a surfactant (adsorbed at the oil-water interface) which configuration and polarity change with the light wavelength. A partial illumination of the droplet, with either visible or UV light, is first used to create wavelength-dependent interfacial tension gradients, meaning that the gradient direction depends on the wavelength of the illumination. Such \textit{chromocapillary} gradients are then able to induce interfacial flows, finally resulting in reversible droplet motions in directions depending on the light wavelength. By combining ultraviolet and visible light, we then made a chromocapillary trap to capture a droplet on the liquid surface. The trapped droplet can be dragged across the surface at 300 microns per second by moving the trap around. We discuss the potential use of chromocapillary effects in microfluidic devices and in light-responsive materials.

International audience; We report a set of new experimental results on the acoustic properties of aqueous foams. Many issues are still pending on foam acoustics; it is still not well understood how sound propagates and is attenuated inside foam, and how the acoustic properties depend on the bubble size and liquid fraction. Experimentally, problems often arise from the coupling between such acoustic measurements and the foam aging, which irreversibly modifies both the bubble size and the liquid fraction (also creating vertical gradients of these parameters). First, we present new measurements of sound velocity and sound attenuation (in the ultrasonic frequency range) on various types of controlled foams, and in particular on foams doped with clay particles in order to obtain a coarsening, but not draining, foam. This allows us to study accurately the dependence of the sound features with the bubble and sample sizes and the liquid fraction. In particular, we observe a non-monotonic behavior for the attenuation as foam ages, strikingly reminiscent of results found for bubbly liquids. Secondly, we have studied how a sound wave (emitted by an external speaker) induces bubble displacement. We show that it is possible to detect - by a light scattering technique (Diffusive Wave Spectroscopy) - the tiny acoustic-induced motion of the bubbles. Comparisons between theory and experiments allow us to elucidate the non-trivial acoustic displacement profile inside the foam; in particular, we find that the acoustic wave creates a localized shear in the vicinity of the solid walls holding the foam, in agreement with novel predictions on foam mechanical behaviour at high frequency.

International audience

International audience; We report a new set of rheological data on well controlled aqueous foams. We investigate and analyze how the linear viscoelastic regime, the foam yielding and the non-linear regimes above yielding actually depends on the interfacial properties, bubble size, liquid fraction and foam age. Results are compared to previous works on foams and emulsions, and to models. The viscoelastic linear properties and yield stress are strongly dependent on the liquid fraction, and for a low molecular weight surfactant, providing "fluid-like" interfaces, a universal behavior is recovered. However, discrepancies are observed for protein foams, and are discussed in relation to the interface and thin film properties. We also discuss the features of the non linear regimes above the yield stress, which cannot be fully explained by recent models. As the foam ages, the evolution of the viscoelastic properties can be interpreted in terms of foam drainage and coarsening; nevertheless, some of the aging effects remain unexplained. We also present the results of a new mode of oscillatory experiments, at constant shear rate the macroscopic results obtained with this new protocol turn out to be strikingly well correlated to microscopic measurements at the bubble scale. We then show that a same solid-liquid transition is obtained either by applying a deformation, or by the foam coarsening; we propose that the transition is controlled by a Deborah number De, which can be seen either as a frequency ratio or a deformation ratio. For De < 1, the foam is fluid-like and the bubbles are unjammed (and the opposite is true when De > 1).

poster

International audience

Aqueous foams are usually made with solutions of low molecular weight surfactants. The properties of such surfactant foams are well characterized, and foams are now widely used in industry as well as everyday life. Today, in order to create new applications, it is important to develop new types of foams or emulsions having original behavior, especially in their aging. It is then also tempting to use foam as templates for creating responsive materials. Here we present our first attempts of making new type of foams by adding original chemical components in the foam formulation. We first present results on foams containing laponite particles. Solutions of laponite are an example of colloidal glasses. By adjusting the laponite concentration, we can tune the timescale of the laponite glass transition. In that respect, within the liquid network of the foam, this glass transition can be obtained at any desired time during the foam aging. We then show how this effect has an impact on the drainage, coarsening and rheological properties of the foam. We also report results with foams containing thermosensitive polymers, and show how the foam behavior can be tuned by the external temperature. Lastly, we present some results with foam stabilized by different types of surfactant di-mer (gemini-like molecules) and tri-mer. For all our experiments, we have monitored the foaming and the 3D foam stability by electrical, optical and acoustic methods. Different types of foams are made for all the systems : at various bubble size and liquid fraction, in order to determine for which conditions the effects are optimized.