Aqueous foams in microgravity, measuring bubble sizes
Marina Pasquet, Nicolo Galvani, Olivier Pitois, Sylvie Cohen-Addad, Reinhard Höhler, Anthony T. Chieco, Sam Dillavou, Jesse M. Hanlan, Douglas J. Durian, Emmanu… 展开作者
Marina Pasquet, Nicolo Galvani, Olivier Pitois, Sylvie Cohen-Addad, Reinhard Höhler, Anthony T. Chieco, Sam Dillavou, Jesse M. Hanlan, Douglas J. Durian, Emmanuelle Rio, Anniina Salonen, Dominique Langevin
The paper describes a study of wet foams in microgravity whose bubble size distribution evolves due to diffusive gas exchange. We focus on the comparison between the size of bubbles determined from images of the foam surface and the size of bubbles in the bulk foam, determined from Diffuse Transmission Spectroscopy (DTS). Extracting the bubble size distribution from images of a foam surface is difficult so we have used three different procedures : manual analysis, automatic analysis with a customized Python script and machine learning analysis. Once various pitfalls were identified and taken into account, all the three procedures yield identical results within error bars. DTS only allows the determination of an average bubble radius which is proportional to the photon transport mean free path $\ell^*$. The relation between the measured diffuse transmitted light intensity and {$\ell^*$} previously derived for slab-shaped samples of infinite lateral extent does not apply to the cuboid geometry of the cells used in the microgravity experiment. A new more general expression of the diffuse intensity transmitted with specific optical boundary conditions has been derived and applied to determine the average bubble radius. The temporal evolution of the average bubble radii deduced from DTS and of the same average radii of the bubbles measured at the sample surface are in very good agreement throughout the coarsening. Finally, ground experiments were performed to compare bubble size distributions in a bulk wet foam and at its surface at times so short that diffusive gas exchange is insignificant. They were found to be similar, confirming that bubbles seen at the surface are representative of the bulk foam bubbles.
We report the experimental study of aqueous foam wetting in microgravity. The liquid fraction $\ell$ along the bubble edges is measured and is found to be a relevant dynamical parameter during the capillary process. The penetration of the liquid in the foam, the foam inflation, and the rigidity loss are shown all to obey strict diffusion processes.
We used coherent light scattering in a multi-speckle detection scheme to investigate the mesoscale dynamics in aqueous foam. Time-resolved correlation of the scattered speckle intensities reveals the details of foam dynamics during aging. We introduce Temporal Contrast Analysis, a novel statistical tool that can be effective in characterizing structural rearrangements. Using Temporal Contrast Analysis we were able to detect two distinct dynamical components present during foam aging: spontaneous and intermittent, avalanche-like events and continuous, flow-like rearrangements in the foam struct…
A finite simple shear deformation of an elastic solid induces unequal normal stresses. This nonlinear phenomenon, known as the Poynting effect, is governed by a universal relation between shear strain and first normal stresses difference, valid for non-dissipative elastic materials. We provide the first experimental evidence that an analog of the Poynting effect exists in aqueous foams where besides the elastic stress, there are significant viscous or plastic stresses. These results are interpreted in the framework of a constitutive model, derived from a physical description of foam rheology.
Like emulsions, pastes and many other forms of soft condensed matter, aqueous foams present slow mechanical relaxations when subjected to a stress too small to induce any plastic flow. To identify the physical origin of this viscoelastic behaviour, we have simulated how dry disordered coarsening 2D foams respond to a small applied stress. We show that the mechanism of long time relaxation is driven by coarsening induced rearrangements of small bubble clusters. These findings are in full agreement with a scaling law previously derived from experimental creep data for 3D foams. Moreover, we find…
In this paper, I study the coarsening dynamics of two-dimensional dry foam sandwiched by deformable membranes. The time-varying deformation of the confining membranes gives rise to a significant alteration in the evolution of polygonal cells of bubbles when compared to the case of rigid membranes. This alteration is attributed to the correlation between the rate of inter-cell gas transfer and temporal fluctuation in surface curvature within a cell domain. The existing material constants are referred to understand the utility of the correlation effect toward the artificial control of the coarse…
We measure the liquid content, the bubble speeds, and the distribution of bubble sizes, in a vertical column of aqueous foam maintained in steady-state by continuous bubbling of gas into a surfactant solution. Nearly round bubbles accumulate at the solution/foam interface, and subsequently rise with constant speed. Upon moving up the column, they become larger due to gas diffusion and more polyhedral due to drainage. The size distribution is monodisperse near the bottom and polydisperse near the top, but there is an unexpected range of intermediate heights where it is bidisperse with small bub…
Investigation into the physics of foaming has traditionally been focused on aqueous systems. Non-aqueous foams, by contrast, are not well understood, but have been the subject of a recent surge in interest motivated by the need to manage foaming across industrial applications. In this review, we provide a comprehensive discussion of the current state-of-the-art methods for characterizing non-aqueous foams, with a critical evaluation of the advantages and limitations of each. Subsequently we present a concise overview of the current understanding of the mechanisms and methods used for stabilizi…
We utilize total-internal reflection to isolate the two-dimensional `surface foam' formed at the planar boundary of a three-dimensional sample. The resulting images of surface Plateau borders are consistent with Plateau's laws for a truly two-dimensional foam. Samples are allowed to coarsen into a self-similar scaling state where statistical distributions are independent of time, except for an overall scale factor. There we find that statistical measures of side number distributions, size-topology correlations, and bubble shapes, are all very similar to those for two-dimensional foams. However…
The interaction between oil and foam has been the subject of various studies. Indeed, oil can be an efficient defoaming agent, which can be highly valuable in various industrial applications where undesired foaming may occur, as seen in jet-dyeing processes or waste water treatment plant. However, oil and foam can also constructively interact as observed in detergency, fire-fighting, food and petroleum industries, where oil can be in the foam structure or put into contact with the foam without observing a catastrophic break-up of the foam. Under specific physico-chemistry conditions, the oil p…
We detail the statistical distribution of bubble rearrangements in a sheared two-dimensional foam. Such rearrangements, known as T1 events, are vital to mechanisms resulting in flow through microscopic mechanical yielding. We find that at a constant rate of shear, the rate of occurrence of T1 events shows only small fluctuations. This rate is however seen to vary significantly with a variation in the initial configuration of bubbles constituting the foam. In addition, we detail the spatial and orientational distribution of T1 events and relate them to the distribution of stresses in the bulk o…
Responsive foams and interfaces, are interesting for active materials that respond to external stimuli. We have used the photochromic reaction of a spiropyran surfactant to render interfacial, rising bubble as well as foaming properties active to light stimuli. In order to address the air-water interface, we have applied sum-frequency generation (SFG) which has provided qualitative information on the surface excess and the interfacial charging state as a function of light irradiation and pH. Under blue light, the surfactant forms a closed ring spiro form (SP), whereas under dark conditions the…
The velocity of a two-dimensional aqueous foam has been measured as it flows through two parallel channels, at a constant overall volumetric flow rate. The flux distribution between the two channels is studied as a function of the ratio of their widths. A peculiar dependence of the velocity ratio on the width ratio is observed when the foam structure in the narrower channel is either single staircase or bamboo. In particular, discontinuities in the velocity ratios are observed at the transitions between double and single staircase and between single staircase and bamboo. A theoretical model ac…
When using appropriate surfactants, oil and aqueous foam can be intimately mixed without the foam being destroyed. In this Letter, we show that a foam, initially free of oil, can draw an oil drop under the action of capillary forces and stretch it through the aqueous network. We focus on the suction of oil by a single horizontal foam channel, known as a Plateau border. In such confined channels, imbibition dynamics are governed by a balance between capillarity and viscosity. Yet, the scaling law for our system differs from that of classical imbibition in porous media such as aqueous foam. This…
Using 2D numerical simulations as well as analytical modelling, we show how slow viscoelastic dynamics of aqueous foam are linked to coarsening induced intermittent bubble rearrangements. The macroscopic strain rate is expressed in terms of mesoscopic dynamics where local rearrangements are represented by strain fields induced by force dipoles. We relate these dipoles quantitatively to local geometrical features of the corresponding bubble rearrangements. We also explain how applying a small macroscopic stress biases the outcome of coarsening induced rearrangements.
In this paper we study the main surface characteristics which control the foamability of solutions of various surfactants. Experiments with anionic, cationic and nonionic surfactants with different head groups and chain lengths are performed in a wide concentration range, from 0.001 mM to 100 mM. The electrolyte concentration is also varied from 0 up to 100 mM. For all surfactants studied, three regions in the dependence of the foamability, VA, on the logarithm of surfactant concentration, lgCS, are observed. In Region 1, VA is very low and depends weakly on CS. In Region 2, VA increases steep…
We use speckle-visibility spectroscopy to measure the time-dependence of bubble rearrangement events that are driven by coarsening in an aqueous foam. This technique gives the time-trace for the average scattering site speed within a prescribed volume of the sample. Results are analyzed in terms of distributions of event times, event speeds, and event displacements. The distribution of rest times between successive events is also measured; comparison with diffusing-wave spectroscopy results shows that the spatial structure of a typical event consists of a core of only a few bubbles which under…
Aqueous foams are an important model system that displays coarsening dynamics. Coarsening in dispersions and foams is well understood in the dilute and dry limits, where the gas fraction tends to zero and one, respectively. However, foams are known to undergo a jamming transition from a fluid-like to a solid-like state at an intermediate gas fraction,$ϕ_c$. Much less is known about coarsening dynamics in wet foams near jamming, and the link to mechanical response, if any, remains poorly understood. Here, we probe coarsening and mechanical response using numerical simulations of a variant of th…
Elasticity of soft materials can be greatly influenced by the presence of air bubbles. Such a capillary effect is expected for a wide range of materials, from polymer gels to concentrated emulsions and colloidal suspensions. Whereas experimental results and theory exist for describing the elasto-capillary behavior of bubbly materials (i.e. with moderate gas volume fractions), foamy systems still require a dedicated study in order to increase our understanding of elasticity in aerated materials over the full range of gas volume fractions. Here we elaborate well-controlled foams with concentrate…
Yield stress of aerated cement paste is studied. Samples are prepared by mixing aqueous foam with cement paste, which allows controlling bubble size, gas volume fraction and yield stress of the cement paste. Two distinct behaviors are observed depending on the surfactant used to prepare the precursor aqueous foam: (i) For a surfactant with low adsorption ability with respect to cement grains, bubbles tend to decrease the yield stress of the paste with magnitude governed by the Bingham capillary number, which accounts for bubble deformability. (ii) For a surfactant with high adsorption ability,…
We study the rheological behavior of mixtures of foams and pastes, which can be described as suspensions of bubbles in yield stress fluids. Model systems are designed by mixing monodisperse aqueous foams and concentrated emulsions. The elastic modulus of the suspensions decreases with the bubble volume fraction. This decrease is all the sharper as the elastic capillary number (defined as the ratio of the paste elastic modulus to the bubble capillary pressure) is high, which accounts for the softening of the bubbles as compared to the paste. By contrast, the yield stress of most studied materia…
The diffusion-driven growth of a dense cloud of bubbles immersed in a gas-supersaturated liquid is a problem that finds applications in several modern technologies such as solvent-exchange micro-reactors, nanotechnology or the manufacturing of foamy materials. However, under Earth's gravity conditions, these dynamics can only be observed for a very limited time if the cloud is not attached to a surface, due to the action of buoyancy, i.e. of gravity effects. Here, we present experimental observations of the time evolution of dense bubble clouds growing in CO$_2$-supersaturated water in microgr…
The interaction of amino acids (glycine, L-valine, L-isoleucine and L-leucine) with alumina surface was studied to induce partial hydrophobization and production of ultrastable particle-stabilised foams. The evaluation of these amino acids was carried out by mechano-quantum simulations followed by experimental tests (foamability, zeta potential, contact angle and foam lifetime measurements). The experimental results agreed with the trends pointed out by the simulations. The selected amino acids interact with alumina particles in aqueous media and in a broader pH range, leading to hydrophobizat…
We studied a system formed by a mixture of a thermoresponsive negatively charged graft copolymer (Alg-g-PNIPAAm) with a brush-type structure, and an oppositely charged surfactant (DTAB), in bulk and at the air-solution interface. We performed experiments of surface tension, electrophoretic mobility, dynamic and static light scattering and atomic force microscopy in order to characterize the complexes formed as a function of DTAB concentration and temperature. We found that these polymer-surfactant complexes are able to respond by changing their sizes, both in bulk and at the air-solution inter…
