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Marangoni flow is among the most intriguing effects in complex fluids and interfacial science. We report here on a fluorescent surfactant that enables to monitor Marangoni flows under quasi-steady conditions, without the need of invasive tracers. The Marangoni zone is clearly visible, and its dynamics can be quantitatively probed both at the air-water interface and within the bulk. In particular, we show that the Marangoni zone exhibits unexpected dependencies with the container size and water depth with the pyrene-tailed surfactant. Additionally, recirculation flows are evidenced by fluorescence near the bottom of the container. This fluorescent probe may find other useful applications in deciphering the complexity of the ubiquitous Marangoni effect.

International audience; The physical properties of an ensemble of tightly packed particles like bubbles, drops or solid grains are controlled by their interactions. For the case of bubbles and drops it has recently been shown theoretically and computationally that their interactions cannot generally be represented by pair-wise additive potentials, as is commonly done for simulations of soft grain packings. This has important consequences for the mechanical properties of foams and emulsions, especially for strongly deformed bubbles or droplets well above the jamming point. Here we provide the first experimental confirmation of this prediction by quantifying the interactions between bubbles in simple model foams consisting of trains of equal-volume bubbles confined in square capillaries. The obtained interaction laws agree quantitatively with Surface Evolver simulations and are well described by an analytically derived expression based on the recently developed non-pairwise interaction model of Höhler et al. [Soft Matter, 2017, 13(7), 1371], based on Morse-Witten theory. While all experiments are done at Bond numbers sufficiently low for the hydrostatic pressure variation across one bubble to be negligible, we provide the full analysis taking into account gravity in the appendix for the interested reader. Even though the article focuses on foams, all results directly apply to the case of emulsions.

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.

International audience; It is prerequisite to characterize nanoparticles in dispersions in particular with respect to their size and size distribution before they can be used in toxicological testing. This requires robust methods with high reproducibility. The aim of this study was to apply interlaboratory comparisons using Dynamic Light Scattering (DLS), in order to evaluate the reproducibility of nanoparticle size determinations. DLS is well established in most nanotoxicological laboratories. Due to the fact that this technique appears easy to use it has become a standard technique nowadays. Still, reproducibility and in particular differences between different commercial instruments have not been addressed systematically before. Our aim was to develop a robust standard operating procedure (SOP). Therefore, at the beginning, experiments were performed with dispersions of rather monodisperse and spherical particles in water. Significant differences were observed between the results obtained with different commercial instruments for DLS, especially when fitting the data using mathematical inversion methods. As a consequence of the different settings applied when fitting the data, significant errors could be made especially when working with dilute dispersions of very small particles. Here we have identified options how to overcome possible issues. However, It should be noted, that in practice, nanoparticles may have significant polydispersities and/or be non-spherical. The potential of the DLS technique in this situation should be noted that in nanoparticles may have significant polydispersities and/or be non-spherical. The potential of the DLS technique in this situation is evaluated. A combination of two different techniques (DLS and electron microscopy for instance) is recommended to fully interpret the results.

The temperature dependence of the hydrodynamic boundary condition between a PDMS melt and two different non-attractive surfaces made of either an OTS (octadecyltrichlorosilane) self-assembled monolayer (SAM) or a grafted layer of short PDMS chains has been characterized. A slip length proportional to the fluid viscosity is observed on both surfaces. The slip temperature dependence is deeply influenced by the surfaces. The viscous stress exerted by the polymer liquid on the surface is observed to follow exactly the same temperature dependences as the friction stress of a cross-linked elastomer sliding on the same surfaces. Far above the glass transition temperature, these observations are rationalized in the framework of a molecular model based on activation energies: increase or decrease of the slip length with increasing temperatures can be observed depending on how the activation energy of the bulk viscosity compares to that of the interfacial Navier's friction coefficient., 6 pages, 5 figures

Nanoparticle in vitro toxicity studies often report contradictory results with one main reason being insufficient material characterization. In particular the characterization of nanoparticles in biological media remains challenging. Our aim was to provide robust protocols for two of the most commonly applied techniques for particle sizing, i.e. dynamic light scattering (DLS) and differential centrifugal sedimentation (DCS) that should be readily applicable also for users not specialized in nanoparticle physico-chemical characterization. A large number of participants (40, although not all participated in all rounds) were recruited for a series of inter-laboratory comparison (ILC) studies covering many different instrument types, commercial and custom-built, as another possible source of variation. ILCs were organized in a consecutive manner starting with dispersions in water employing well-characterized near-spherical silica nanoparticles (nominal 19 nm and 100 nm diameter) and two types of functionalized spherical polystyrene nanoparticles (nominal 50 nm diameter). At first each laboratory used their in-house established procedures. In particular for the 19 nm silica particles, the reproducibility of the methods was unacceptably high (reported results were between 10 nm and 50 nm). When comparing the results of the first ILC round it was observed that the DCS methods performed significantly worse than the DLS methods, thus emphasizing the need for standard operating procedures (SOPs). SOPs have been developed by four expert laboratories but were tested for robustness by a larger number of independent users in a second ILC (11 for DLS and 4 for DCS). In a similar approach another SOP for complex biological fluids, i.e. cell culture medium containing serum was developed, again confirmed via an ILC with 8 participating laboratories. Our study confirms that well-established and fit-for-purpose SOPs are indispensable for obtaining reliable and comparable particle size data. Our results also show that these SOPs must be optimized with respect to the intended measurement system (e.g. particle size technique, type of dispersant) and that they must be sufficiently detailed (e.g. avoiding ambiguity regarding measurand definition, etc.). SOPs may be developed by a small number of expert laboratories but for their widespread applicability they need to be verified by a larger number of laboratories., This work has been supported by the EU FP7 Capacities project QualityNano (grant no. INFRA-2010-262163).

Wrinkled morphology is a distinctive phenotype observed in mature biofilms produced by a great number of bacteria. Here we study the formation of macroscopic structures (wrinkles and folds) observed during the maturation of Bacillus subtilis pellicles in relation to their mechanical response. We show how the mechanical buckling instability can explain their formation. By performing simple tests, we highlight the role of confining geometry and growth in determining the symmetry of wrinkles. We also experimentally demonstrate that the pellicles are soft elastic materials for small deformations induced by a tensile device. The wrinkled structures are then described by using the equations of elastic plates, which include the growth process as a simple parameter representing biomass production. This growth controls buckling instability, which triggers the formation of wrinkles. We also describe how the structure of ripples is modified when capillary effects are dominant. Finally, the experiments performed on a mutant strain indicate that the presence of an extracellular matrix is required to maintain a connective and elastic pellicle.

The characterisation of the dynamic properties of viscoelastic monolayers of surfactants at the gasliquid interface is very important in the analysis and prediction of foam stability. With most of the relevant dynamic processes being rapid (thermal fluctuation, film coalescence etc.) it is important to probe interfacial dynamics at high deformation rates. Today, only few techniques allow this, one of them being the characterisation of the propagation of electrocapillary waves on the liquid surface. Traditionally, this technique has been applied in a continuous mode (i.e. at constant frequency) in order to ensure reliable accuracy. Here we explore the possibility to analyse the propagation of an excited pulse in order to access the interfacial properties in one single Fourier treatment over a wide range of frequencies. The main advantage of this approach is that the measurement times and the required liquid volumes can be reduced significantly. This occurs at the cost of precision in the measurement, due partly to the presence of a pronounced resonance of the liquid surface. The pulsed approach may therefore be used to prescan the surface response before a more in-depth scan at constant frequency; or to follow the changes of the interfacial properties during surfactant adsorption., 17 pages,9 figures

International audience; Confocal microscopy has become a powerful tool to obtain high-resolution, three-dimensional images after reconstruction of a point-by-point scan of the local fluorescence properties of a sample. For technical reasons, commercial confocal microscopes perform this scan in a manner that the x-y-plane of the scan is perpendicular to gravity and can be several square centimetres large (defined by the movement of displacement stages). The depth- or z-scan, on the other hand, is parallel to gravity and typically limited to 10–1000 micrometres depth. This is a constraint that limits the number of experimental scenarios in which confocal microscopy can be employed, since many experimental systems ask for images in which the main extension of the scan has an angle with gravity which is not 90 degrees. Using the example of different experimental systems (particle dispersions, emulsions, foams, foamed emulsions) that sediment or cream under gravity, we show here how the optical path of any confocal microscope can be inverted to obtain any imaging angle with respect to gravity without losing all pre-programmed scanning and reconstruction facilities.

The adsorption of nonionic surfactants of the alkyl-phenol-poly(ethylene oxide) family and of acrylic latex particles on several anhydrous (but hydrating) or fully hydrated mineral phases of Portland cement was studied. No or negligible adsorption of the surfactant was observed. This was assigned to the ionized character of the surface silanol groups in calcium-silicate-hydrates and to the strongly ionic character of the OH groups in calcium hydroxide and in the calcium-sulfoaluminate-hydrates, which prevents the formation of surface-ethoxy hydrogen bonds. In contrast, provided they are properly stabilized by the surfactant, the latex particles form a loose monolayer on the surface of hydrating tricalcium silicate particles. The attractive interaction between the positive mineral surface and the negative latex surface appears to be the driving force for adsorption. In line with this, adsorption is reduced by sulfate anions, which adsorb specifically onto the silicate surface. Compared to tricalcium silicate, portlandite and gypsum interact only marginally with the latex particles. Our results show that the stability of the nonionic surfactant/latex/cement systems is essentially controlled by the latex colloidal stability and the latex-cement interactions, the surfactant having little direct interaction, if any, with the mineral surfaces.