Reine-Marie Guillermic

Surfactant foams doped with laponite: unusual behaviors induced by aging and confinement

We report results on foams stabilized by surfactant (sodium dodecyl sulfate) and containing clay particles (laponite). We have studied how these foams age with time (drainage and coarsening) and their rheological properties. Due to the doping with laponite, which provides an additional time evolution of the foaming fluid itself, unusual behaviors are observed: especially, drainage arrest and re-start and enhanced elasticity are observed as a function of time. These results can be interpreted in terms of both confinement of the laponite inside the foam liquid channels, and competition between the laponite aging and the one of the foam (controlled by its own physical parameters). By playing with these foam parameters and those of the bulk solution containing laponite, we can control the time evolution and these non-monotonous features. Qualitatively, it is found that time, laponite concentration and confinement have all the same effect, enhancing the jamming of the interstitial fluid inside the foam.

Oscillatory rheology of aqueous foams: surfactant, liquid fraction, experimental protocol and aging effects

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).

Dynamics of poly-nipam chains in competition with surfactants at liquid interfaces: from thermoresponsive interfacial rheology to foams

We report results on the interfacial viscoelasticity and foaming of solutions of a thermoresponsive polymer (poly-n-isopropylacrylamide) with and without added surfactants, and as a function of temperature. With pure polymer solution, a clear transition is evidenced: both interfacial shear and dilational rheology shift from a fluid-like to a solid-like behavior at a well-defined temperature. The high temperature regime shows that the layer shares many features with soft glassy systems. At all temperatures, the foaming is low and the foam produced is unstable. Adding a surfactant not only helps to foam and to stabilize the foam, but also removes the thermal responsivity of the interfacial viscoelasticity. Under the conditions used here, we observe that the surfactant concentration threshold for altering the high temperature interfacial viscoelasticity is low, and is of the order of 1% of the surfactant critical micelle concentration. It turns out to be very different from critical values for the polymer–surfactant association found previously by structural studies (in bulk and at interface), and also below the threshold value required to stabilize the foam.

Propagation of ultrasound in aqueous foams: bubble size dependence and resonance effects

We report experimental results on the propagation of ultrasonic waves (at frequencies in the range of 40 kHz) in aqueous foams. Monitoring the acoustics of the foams as they age, i.e. as the mean bubble radius increases by coarsening, we recover at short times some trends that are already known: decrease of the speed of sound and increase of attenuation. At long times, we have identified, for the first time, robust non-monotonic behaviors of the speed of sound and attenuation, associated with a critical bubble size, which decreases at increasing frequency. The experimental features appear to be surprisingly reminiscent of the Minnaert resonance known for a single isolated bubble in a fluid. Transposing the Minnaert theoretical framework to the limit of a dense packing of bubbles gives some qualitative agreement with the data, but still cannot explain quantitatively the measured properties.

Acoustic characterisation of liquid foams with an impedance tube

Acoustic measurements provide convenient non-invasive means for the characterisation of materials. We show here for the first time how a commercial impedance tube can be used to provide accurate measurements of the velocity and attenuation of acoustic waves in liquid foams, as well as their effective “acoustic” density, over the 0.5-6kHz frequency range. We demonstrate this using two types of liquid foams: a commercial shaving foam and “home-made” foams with well-controlled physico-chemical and structural properties. The sound velocity in the latter foams is found to be independent of the bubble size distribution and is very well described by Wood’s law. This implies that the impedance technique may be a convenient way to measure in situ the density of liquid foams. Important questions remain concerning the acoustic attenuation, which is found to be influenced in a currently unpredictible manner by the physico-chemical composition and the bubble size distribution of the characterised foams. We confirm differences in sound velocities in the two types of foams (having the same structural properties) which suggests that the physico-chemical composition of liquid foams has a non-negligible effect on their acoustic properties.Graphical abstract

Shaping complex fluids—How foams stand up for themselves

Being able to model at what point a yield stress material starts to flow under its own weight is of great importance for many practical applications. However, describing the deformation of yield stress fluids under gravity is anything but a simple exercise due to the feedback between the shape of the deposited material and the locally acting stresses. In this article, we concentrate on a specific aspect of this problem: What is the maximum height of a pile of a yield stress fluid which can be obtained under gravity? For this purpose we use the example of liquid foams in which the yield stress is strongly coupled to the bubble size and the liquid fraction. We show that a good agreement between models and experiments is obtained over a wide parameter range in two limiting cases: When the yield stress is either higher or much lower than the normal stresses encountered in the material.

A PDMS-based broadband acoustic impedance matched material for underwater applications

HighlightsA composite material that is impedance matched to water is designed and fabricated.Impedance matching is achieved by dispersing subwavelength particles in soft solids.An impedance‐matched material containing TiO2 particles in PDMS is demonstrated.The design is aided by two models to predict the appropriate particle concentration.The impedance matching is broadband, enabling a wide range of applications. ABSTRACT Having a material that is matched in acoustic impedance with the surrounding medium is a considerable asset for many underwater acoustic applications. In this work, impedance matching is achieved by dispersing small, deeply subwavelength sized particles in a soft matrix, and the appropriate concentration is determined with the help of Coherent Potential Approximation and Waterman & Truell models. We show experimentally the validity of the models using mixtures of Polydimethylsiloxane (PDMS) and TiO2 particles. The optimized composite material has the same longitudinal acoustic impedance as water and therefore the acoustic reflection coefficient is essentially zero over a wide range of frequencies (0.5–6 MHz). PDMS‐based materials can be cured in a mold to achieve desired sample shape, which makes them very easy to handle and to use. Various applications can be envisioned, such the use of impedance‐matched PDMS in the design and fabrication of acoustically transparent cells for samples, perfectly matched layers for ultrasonic experiments, or superabsorbing metamaterials for water‐borne acoustic waves.

Investigating the effects of formulation and processing conditions on the mechanical properties of wheat flour noodle dough

Characterization of the mechanical properties of soft food materials is crucial in the food industry, both for process design and for quality enhancement purposes. The goal of this project is to develop a non-contact on-line quality control technique for use in processing of sheeted products such as Asian noodles. In the case of the Asian noodle industry, composition and work input during the sheeting process are important parameters that influence the mechanical properties of the dough, and as a consequence, final product quality. An accurate and fast determination of noodle properties and how they are influenced by formulation and processing parameters will certainly improve quality control during production. To address this need, we are conducting a full study of noodle dough with ultrasonic techniques. A non-contact ultrasonic technique in transmission is used to assess the mechanical properties of noodle dough sheets and has been evaluated in pilot-plant trials, showing its feasibility for real-time ...

Shaving foam: A complex system for acoustic wave propagation

While liquid foams have applications in an increasing number of industrial areas (food, cosmetic or petroleum industry), it remains difficult to non-invasively probe their structure and/or composition. Since the propagation of acoustic waves is very sensitive to parameters such that the liquid fraction, the bubble size distribution, or even the nature of the liquid phase, acoustic spectroscopy could be a very powerful tool to determine the structure and/or composition of liquid foams. In this context, we present an investigation of the acoustic properties of a useful and common foam, often considered as a model system: shaving foam. Phase velocity and attenuation of acoustic waves in a commercial shaving foam (Gillette) were measured over a broad frequency range (0.5 to 600 kHz), using four different experimental setups: an impedance tube (0.5-6 kHz), an acousto-optic setup based on Diffusive Wave Spectroscopy (0.4-10 kHz), and two transmission setups with narrow-band (40 kHz) and broad-band (60-600 kHz) transducers. We present the results and discuss the advantages and shortcomings of each setup in terms of a potential spectroscopy technique.