Authors: Yousra Timounay1, Olivier Pitois2, Florence Rouyer2
1Physics Department, Syracuse University, United States,
2Université Paris-Est, Laboratoire Navier, UMR 8205 CNRS, ENPC ParisTech, IFSTTAR, France.
Isolating small volumes of gas or liquid in the form of bubbles or droplets is important for a wide range of applications (pharmaceutics, cosmetics…), and they are major elements in the production of emulsions, foams, sprays, etc. Advanced applications require such fluid objects having specific features such as high mobility on solid substrates and significant robustness. The mechanical properties of individual bubbles/drops strongly impact the behavior of the material they are part of.
Controlling the properties of bubbles/drops and developing methods to tune them is therefore important. Surfactants and macromolecules (such as proteins) -- adsorbed at fluid interfaces -- lower intern overpressure (also called Laplace pressure ΔPcap) of bubbles and drops by lowering surface tension. Solid particles can be advantageously used to confer unequaled properties to the resulting "armored" bubbles and drops. Droplets coated by solid particles, known as liquid marbles, exhibit unusual features; they are highly mobile  and their drying rate can be significantly decreased .
The dissolution of "armored" bubbles can be stopped as well . Moreover, droplets and bubbles can be stable in a non-spherical shape when covered by particles . Solid particles adsorbed at the interface can also mechanically strengthen bubbles and drops, so the later can sustain slightly negative pressures, i.e. ΔP ∼ ΔPcap, before collapsing [3,5,6,7]. Note that in these systems, particles are bounded to by a unique fluid-fluid interface.
In our recent article , we have shown that the same concept can be applied to air bubbles within an air environment. These particulate bubbles, that we call gas marbles, are delimited by a monolayer of solid particles confined in a thin liquid film, i.e. bounded by two interfaces. Collapse pressures we measured are ∼10 times greater than the one reported for liquid marbles, i.e. ΔP ∼ 10 ΔPcap. Moreover, gas marbles can sustain a significant overpressure before inflating. We attribute these remarkable results to the cohesion between the grains that form the shell delimiting gas marbles. Indeed, observation of the shell revealed that the liquid distributes in the pores between the particles (see Fig 1). This capillary structure is likely to induce attractive interactions between the particles. Thus, the remarkable strength of gas marbles results from the cohesion in the particulate shell.
Fig. 1 (a) is a side picture of a gas marble with 590 µm particles. (b) is a close-up showing liquid menisci between the particles. (c) and (d) are schemas showing the structure of a gas marble. Adapted from 
Gas marbles can be generated by immersing and withdrawing a rectangular frame into a surfactant solution with a particle raft laying on its interface. The particulate interface initially -- lifted by the frame -- eventually detaches due to its weight and closes over itself creating a gas marble. This technique is similar to how soap bubbles are created; in our case gravity replaces blowing onto the film to deform it. The liquid used to create gas marbles is an aqueous solution of Sodium Dodecyl Sulfate (SDS) with a surface tension equal to 36 mN/m. The particles are monosized polystyrene beads. Three different particle diameters were used: 250, 315 and 590 µm. To modify their wetting properties, the particles have been immersed into a mixture made up of octane solution and dissolved silane. After the silanization, the advancing contact angle between the particles and the SDS solution is equal to 95 ± 10o making them slightly hydrophobic. Pictures and schemes showing the structure of gas marbles are presented in Fig 1. The diameter of gas marbles we study ranges between 5 to 12 mm.
We have characterized the properties of gas marbles by studying their behavior when submitted to pressure variations. In practice, we connect a gas marble to a syringe and a pressure sensor, by changing the volume of the syringe using a syringe pump, we induce either inflation or deflation of the gas marble under study. Increasing the syringe volume results in a decrease of the gas marble inner pressure, and vice versa. Thanks to two cameras placed above and below, we were able to follow the shape of gas marbles during experiments.
We have observed the same qualitative behavior during all experiments. First, the inner relative pressure of gas marbles is equal to zero at equilibrium (no pressure variation imposed). This result shows that -- contrary to soap bubbles -- Laplace’s law does not apply to gas marbles. Such effect results from the surface stress supported by the force network arising from contacts between particles trapped in the monolayer discussed above [9,10,11]. Second, during both inflation and deflation experiments, gas marbles go through a first regime where they do not deform despite the imposed overpressure/under-pressure. Eventually gas marbles deform at a critical relative pressure. A fracture consisting in the stretching of the liquid connecting the particles is observed at the critical overpressure while gas marbles collapse at the critical under-pressure. Critical relative pressures we measured do not seem to depend on the diameter of gas marbles and the size of the particles composing the shell within the size range we explored. Moreover, critical relative pressures have the same amplitude (∼ 500 Pa) for inflation and deflation experiments.
As said before, we attribute these exceptional properties to the particle confinement imposed by the double interface delimiting gas marbles, whereas a single interface is present for armored bubbles and drops. Concerning applications, gas marbles could be used to improve the stability of particle laden foams and emulsions. They could also be used to store and transport small volumes of gas.
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