Slow viscoelastic relaxation and aging in aqueous foam

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 that the temporal statistics of coarsening induced bubble rearrangements are described by a Poisson process.Comment: 7 pages, 3 figure

Fluidization and transport of vibrated granular matter : a review of landmark and recent contributions

We present a short retrospective review of the existing literature about the dynamics of (dry) granular matter under the effect of vibrations. The main objective is the development of an integrated resource where vital information about past findings and recent discoveries is provided in a single treatment. Special attention is paid to those works where successful synthetic routes to as-yet unknown phenomena were identified. Such landmark results are analyzed, while smoothly blending them with a history of the field and introducing possible categorizations of the prevalent dynamics. Although no classification is perfect, and it is hard to distillate general properties out of specific observations or realizations, two possible ways to interpret the existing results are defined according to the type of forcing or the emerging (ensuing) regime of motion. In particular, first results concerning the case where vibrations and gravity are concurrent (vertical shaking) are examined, then the companion situation with vibrations perpendicular to gravity (horizontal shaking) is described. Universality classes are introduced as follows: 1) Regimes where sand self-organizes leading to highly regular geometrical "pulsating" patterns (thin layer case); 2) Regimes where the material undergoes "fluidization" and develops an internal multicellular convective state (tick layers case); 3) Regimes where the free interface separating the sand from the overlying gas changes inclination or develops a kind a patterned configuration consisting of stable valleys and mountains or travelling waves; 4) Regimes where segregation is produced, i.e. particles of a given size tend to be separated from the other grains. Where possible, an analogy or parallelism is drawn with respect to the companion field of fluid-dynamics for which the assumption of "continuum" can be applied

Convective states and patterning behavior in Lunar Regolith under the effect of vertical vibrations

In the field of space exploration, it is essential to deal with solid “particles” for various applications, for example transporting lunar and Martian soil (typically regolith), for mining, to study geological aspects and establish habitats on the Moon or Mars. However, methods to handle these materials remain untested because they are made of abrasive and reactive components which by their nature make them hard to handle mechanically. In the present project, novel strategies to manipulate such granular systems based on the application of “vibrations” are explored. Special attention is devoted to the unique states which are produced when such materials are subjected to concurrent vibrations and gravity (vertical shaking). By means of a concerted approach based on experimental work in synergy with relevant computational tools for systems where the assumption of continuum is not applicable, we show that circumstances exist where lunar regolith can behave as a kind of “fluid” and produce interesting patterning behaviors. The problem is parametrically investigated by allowing the frequency and spatial amplitude of the imposed vibrations to span relatively wide intervals for different depths of the considered layer of material (simulant). The results are critically discussed and placed in a proper theoretical context through comparisons with earlier experimental findings where the analysis was limited to monodisperse collection of particles (spheres with fixed density and diameter as opposed to the irregular shape and varying size of lunar regolith)

Effect of vertical and horizontal vibrations, vessel size and layer height on the fluidization of lunar regolith

In the coming years with such a major focus being applied to the further exploration and eventual habitation of the Moon and Mars, it has never been more important to further the understanding of how to manipulate the resources abundantly available on these planets. Along these lines, this project aims to develop novel ways of handling the regolith that makes up the lunar soil by use of “vibrations”. The final objective is the definition and validation of a new approach able to overcome well-known drawbacks related to these materials due to their extremely abrasive and reactive nature, which makes their manipulation and transport particularly difficult, especially in the low gravity lunar environment. Through the application of vibrations with various orientations to small samples of lunar regolith simulant, the solid granular material can be forced to display a fluidlike convective behavior, forming structures such as self-sustaining heaps. The effect that varying the height of the bed of lunar regolith simulant has on the mode and rate of convection exhibited by the bed is investigated while the size of the vessel containing the bed is also varied to assess the influence of this additional parameter. Some recent experimental findings are critically discussed in relation to the resulting (enhanced) ability to manipulate and transport lunar regolith

Percolating Reaction-Diffusion Waves (PERWAVES) — Sounding Rocket Combustion Experiments

Percolating reaction–diffusion waves in disordered random media are encountered in many branches of modern science, ranging from physics and biology to material science and combustion. Most disordered reaction–diffusion systems, however, have complex morphologies and reaction kinetics that complicate the study of the dynamics. Flames in suspensions of heterogeneously reacting metal-fuel particles is a rare example of a reaction–diffusion wave with a simple structure formed by point-like heat sources having well-defined ignition temperature thresholds and combustion times. Particle sedimentation and natural convection can be suppressed in the free-fall conditions of sounding rocket experiments, enabling the properties of percolating flames in suspensions to be observed, studied, and compared with emerging theoretical models. The current paper describes the design of the European Space Agency PERWAVES microgravity combustion apparatus, built by the Airbus Defense and Space team from Bremen in collaboration with the scientific research teams from McGill University and the Technical University of Eindhoven, and discusses the results of two sounding-rocket flight experiments. The apparatus allows multiple flame experiments in quartz glass tubes filled with uniform suspensions of 25-micron iron particles in oxygen/xenon gas mixtures. The experiments performed during the MAXUS-9 (April 2017) and TEXUS-56 (November 2019) sounding rocket flights have confirmed flame propagation in the discrete mode, which is a pre-requisite for percolating-flame behavior, and have allowed observation of the flame structure in the vicinity of the propagation threshold

Particle Vibration, an instrument to study particle accumulation structures on board the International Space Station

The scientific and technological aspects of the PARTICLE VIBRATION Project (also known as T-PAOLA i.e. "Thermovibrationally-driven Particle self-Assembly and Ordering mechanisms in Low grAvity") are described in detail. The project relies on the combined use of the Selectable Optical Diagnostics Instrument (SODI), a Class-2 device developed by ESA for scientific experiments in the field of fluids on board the International Space Station, and the Microgravity Science Glovebox (MSG), a Class-1 general purpose facility under the responsibility of NASA. The related modular architecture has recently been expanded under the umbrella of new scientific research funded by the UK Space Agency to allow for a novel class of experiments dealing with multiphase (solid-liquid) flows. The final aim of this microgravity project is the identification of new dispersed-phase self-organization phenomena driven by the application of vibrations and the ensuing development of new contactless particle manipulations strategies. In the present paper, emphasis is given to the related space hardware and software, the experiment protocol, the ground tests and procedures and all the adaptations that had to be implemented to overcome a number of technological and physical issues, both general and system-specific

Soft Dynamics simulation: 2. Elastic spheres undergoing a T1 process in a viscous fluid

Robust empirical constitutive laws for granular materials in air or in a viscous fluid have been expressed in terms of timescales based on the dynamics of a single particle. However, some behaviours such as viscosity bifurcation or shear localization, observed also in foams, emulsions, and block copolymer cubic phases, seem to involve other micro-timescales which may be related to the dynamics of local particle reorganizations. In the present work, we consider a T1 process as an example of a rearrangement. Using the Soft dynamics simulation method introduced in the first paper of this series, we describe theoretically and numerically the motion of four elastic spheres in a viscous fluid. Hydrodynamic interactions are described at the level of lubrication (Poiseuille squeezing and Couette shear flow) and the elastic deflection of the particle surface is modeled as Hertzian. The duration of the simulated T1 process can vary substantially as a consequence of minute changes in the initial separations, consistently with predictions. For the first time, a collective behaviour is thus found to depend on another parameter than the typical volume fraction in particles.Comment: 11 pages - 5 figure

Simulation et modélisation multiéchelles de la rhéologie des mousses 2D

We show how macroscopic flow, slow viscoelastic dynamics and rheological memory of 2D liquid foams are related to processes at the film scale and how these are coupled to aging phenomena.The first main topic that we have studied is the coupling between local, mesoscopic and macroscopic length scales. We present a new law relating the stretching of individual films in response to a macroscopic strain. This relationship allows the elasticity of 2D foams to be modelled as function of the microstructure. Moreover, we discuss how macroscopic flow results from bubble rearrangements that can be represented as force dipoles acting within an elastic continuous medium.The second main topic concerns is the coupling between rheology and aging. Using the dipole picture, we establish an analytic expression of the strain due to rearrangements induced by aging. On this basis, we formulate a constitutive rheological model considering intrinsic aging dynamics as well as topological and geometrical constraints.Nous montrons comment l'écoulement, la dynamique viscoélastique lente et la mémoire rhéologique des mousses liquides 2D à l'échelle macroscopique s'expliquent par des processus à l'échelle des films et bulles et comment ils sont couplés au vieillissement.Un premier fil conducteur est le couplage entre les échelles macroscopique, mésoscopique et locale. Nous montrons que l'allongement d'un film individuel sous l'effet d'une déformation macroscopique suit une nouvelle loi qui permet de modéliser l'élasticité des mousses 2D en fonction de la microstructure. Par ailleurs, nous discutons comment un écoulementmacroscopique résulte de réarrangements de bulles agissant comme des dipôles de force au sein d'un milieu continu élastique.Un second fil conducteur est le couplage entre rhéologie et vieillissement. A l'aide de la représentation dipolaire, nous établissons une loi analytique décrivant la déformation induite par les réarrangements dus au vieillissement. Sur cette base, nous formulons un modèle rhéologique constitutif prenant en compte la dynamique intrinsèque due au vieillissement, et les contraintes géométriques et topologiques