Physics of gas marbles : from granular film to assembly – PhyGaMa

The physical description of these new objects at different scales and for different loading regimes aims at linking the microstructure to the global behavior of the material and at identifying the physical mechanisms dominating the interactions between the grains of the shell: capillarity, contacts between grains and viscous dissipation (in the liquid film and at the liquid/gas interfaces). To address these issues, we conduct experiments at different scales of the material where the parameters of the continuous and dispersed phases are controlled to reveal the main physical characteristics of these new objects. At the mesoscopic level, we wish to understand the link between the particle network and the behavior of gas bead membranes subjected to different solicitations: global and local forcing, vibrations and finally classical rheometry. In the quasi-static regime, we study the influence of the capillary pressure on the mechanical behavior of the membrane and on the granular network. In the dynamic regime, the relationship between the contact dissipation between grains and the interfacial or viscous dissipation is discussed. Finally, on the way to the macroscopic scale, our first objective will be to create gas beads with controlled properties, and our second objective will be to correlate the static behavior of a gas bead and a gas bead assembly to the properties of the membrane and/or the shell.

Thanks to the development on a centimetre scale of a film balance conventionally made on a millimetre scale, it has been possible to control the liquid pressure in a granular film (a liquid film charged with spherical particles about a hundred microns in diameter). Manually triggered punctual rupture experiments show that there is a threshold liquid depression above which the film does not burst [Retailleau 2023]. In the case where the granular film bursts in a few milliseconds, the dynamic study highlights the dual liquid and solid character of a granular film during the same event: the opening at short time is described by an equilibrium between surface tension and intrinsic viscous dissipation, while the opening at long time is similar to the fracturing of an elastic membrane. Initial vibrational studies have shown that for liquid pressures close to atmospheric pressure, a granular film can be described as a membrane under tension whose tension is close to twice the surface tension of the liquid-air interfaces.

When submitted to simple shear, granular films behave like attractive granular media and can be described by a generalised friction law in which the confining pressure is the capillary pressure. Sheared in a large gap, they exhibit a viscous flow region and a quasistatic plastic region. We have shown that the rearrangement dynamics in the plastic region are controlled by the equilibrium between capillary stresses and viscous dissipation in the liquid/air interfaces [Lalieu 2023-a]. The behaviour of granular rafts in which the particles cross only one interface is similar, but the transition between the viscous and plastic flow regimes occurs at a shear stress 100 times lower than for films [Lalieu 2023-b].

The extensional rheology of granular films is explored by stretching a catenoid which destabilises beyond a critical height by pinching in a few milliseconds. The pinching dynamics are characterised by an exponential decrease in the miminum radius of the catenoid over time when the viscous dissipation of the film becomes greater than that of the evacuated air. An effective viscosity of the granular film can then be deduced, enabling quantitative modelling of the burst rate of a granular film.

Finally, in order to create structured multi-scale aerated materials, we produce gas beads resulting from the encapsulation of air by a granular film. We show that a compact stack of gas beads can be made using hydrogel films.

Finally, in the perspective of exploring new coupled propagation modes and opening the way to new vibration dampers, the dynamic properties of gas beads and a gas bead stack will be studied by wave propagation and interpreted in terms of the damping properties of the granular films and the gas bead assembly.

In addition, the two techniques developed during this project (film balance and film rheology in a cylindrical Couette) will be combined to carry out rheological measurements of granular films at controlled liquid pressure.

in progress ...

Research in line with the pioneer work of Pickering succeeded recently to produce new objects which can be described as gas pockets in air and are named gas marbles. They are made of gas surrounded by a layer of grains constrained by thin liquid film in gas environment. The exceptional mechanical strength of their granular shell promise them to many applications. Among these, we note here that using gas marbles can be relevant for the generation of materials with hierarchical porosity. Moreover, these new hollow marbles offer a promising route to new vibration dampers. In this proposal, our objective is to study the physics of this new material.
In a ternary diagram (solid-liquid-gas), gas marbles share the domain of high gas, low solid and very low liquid fraction with already well-known three phase materials: wet (unsaturated) granular materials, Pickering foams, granular foams. At the microscale of all these materials, three physical mechanisms inducing (possibly multicore) interactions between grains are at play: capillarity, grains contacts and viscous dissipation (in the liquid film and at the liquid/gas interfaces). The time scales, length scales and patterns differs from one systems to another, as the particles and liquid network differ. The main concern common to all dispersed media made of two or three phases are: How microstructure of the dispersed phases and microscale interactions are related and affect the macroscale behavior of the material? Are all these systems one class of material in a sense that they obey the same constitutive laws but with different state parameters defined by the physic at play at the microscale?
Studying gas marbles would help to give new inputs to these questions and propose a unified view to the subject. To tackle these questions, we will carry out experiments at the different scales of the material where the parameters of the continuous phase and dispersed phase will be controlled in order to reveal the main physical features of these new objects.
At the mesoscale, we want to understand the link between the particle network and the behavior of the gas marble membrane when subject to different solicitations: global and local forcing, vibrations and classical rheometry. In the quasi-static regime, we will study the influence of the capillary pressure on the elasticity of the membrane and the granular network in terms of particle fraction, neighbor’s number, aggregation coefficient. In the dynamic regime, the relevance of grains contact dissipation compare to the interfacial dissipation or liquid viscous dissipation will be address.
Finally, on the route to the macroscale, we have the first objective to create a gas marble with controlled properties, the second one is to correlate the static behavior of one gas marble and a gas marbles assembly to the properties of the membrane and/or shell.
Finally, the dynamical properties of one gas marbles and gas marbles assembly will be studied by vibrational waves propagation and interpreted in regards to the damping properties of the granular films and the network assembly of the gas marbles. The multi-scale and ternary structure of the material might reveal different coupling modes of propagation (waves propagation in air and in the granular skeleton).