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We show an increase of the sedimentation velocity as small particles are confined in circular capillaries. In general, confinement slows down sedimentation. But, we show that at low Reynolds numbers and in 1D confinement this is not the case. Particle sedimentation velocity is not homogeneous, which can lead to the formation of structures. These structures are enhanced and stabilized in the presence of walls and in the absence of other dissipative mechanisms. As a consequence, it is possible to achieve sedimentation velocities that even exceed the Stokes velocity. The segregation at critical capillary diameters has been directly observed using a large scale model. These simple experiments offer a new insight into the old problem of sedimentation under confinement. [ABSTRACT FROM AUTHOR]

Effective cross sections of nano-objects are fundamental properties that determine their ability to interact with light. However, measuring them for individual resonators directly and quantitatively remains challenging, particularly because of the very low signals involved. Here, we experimentally measure the thermal emission cross section of metal-insulator-metal nanoresonators using a stealthy hyperuniform distribution based on a hierarchical Poisson-disk algorithm. In such distributions, there are no long-range interactions between antennas, and we show that the light emitted by such metasurfaces behaves as the sum of cross sections of independent nanoantennas, enabling direct retrieval of the single resonator contribution. The emission cross section at resonance is found to be on the order of λ_{0}^{2}/3, a value that is nearly 3 times larger than the theoretical maximal absorption cross section of a single particle, but remains smaller than the maximal extinction cross section. This measurement technique can be generalized to any single resonator cross section, and we also apply it to a lossy dielectric layer.

While coalescence is ultimately the most drastic destabilization process in foams, its underlying processes are still unclear. To better understand them, we track individual coalescence events in two-dimensional foams at controlled capillary pressure. We obtain statistical information revealing the influence of the different parameters which have been previously proposed to explain coalescence. Our main conclusion is that coalescence probability is simply proportional to the area of the thin film separating two bubbles, suggesting that coalescence is mostly stochastic.

We studied the stability of foams containing small bubbles (radius ≲ 50  μm). The foams are made from aqueous surfactant solutions containing various amounts of glycerol. The foams start breaking at their top, when the liquid volume fraction has decreased sufficiently during liquid drainage. Unlike in foams with larger bubbles, the liquid fraction at which the foam destabilizes is surprisingly high. In order to interpret this observation we propose that film rupture occurs during reorganization events (T1) induced by bubble coarsening, which is particularly rapid in the case of small bubbles. New films are therefore formed rapidly and if their thickness is too small, they cannot be sufficiently covered by surfactant and they break. Using literature data for the duration of T1 events and the thickness of the new films, we show that this mechanism is consistent with the behavior of the foams studied.

Quasi-two-dimensional (quasi-2D) foams consist of monolayers of bubbles squeezed between two narrowly spaced plates. These simplified foams have served successfully in the past to shed light on numerous issues in foam physics. Here we consider the electrical conductivity of such model foams. We compare experiments to a model which we propose, and which successfully relates the structural and the conductive properties of the foam over the full range of the investigated liquid content. We show in particular that in the case of quasi-2D foams the liquid in the nodes needs to be taken into account even at low liquid content. We think that these results may provide different approaches for the characterization of foam properties and for the in situ characterization of the liquid content of foams in confining geometries, such as microfluidics.

We report surface shear rheological measurements on monolayers of silica nanoparticles at the air-water interface. We have used the method of strain-rate frequency superposition (SRFS) to characterize the structural relaxation. We show that the rheological properties of the layers have the same universal linear and nonlinear behavior as three-dimensional soft materials. We also discuss the original healing properties of these monolayers.

We have studied the collapse of pseudo-bi-dimensional foams. These foams are made of uniformly sized soap bubbles packed in an hexagonal lattice sitting at the top of a liquid surface. The collapse process follows the sequence: (1) rupture of a first bubble, driven by thermal fluctuations and (2) a cascade of bursting bubbles. We present a simple numerical model which captures the main characteristics of the dynamics of foam collapse. We show that in a certain range of viscosities of the foaming solutions, the size distribution of the avalanches follows power laws as in self-organized criticality processes.

We report the results of fluid transport experiments in aqueous foams under microgravity. Using optical and electrical methods, the capillary motion of the foam fluid and the local liquid fractions are monitored. We show that foams can be continuously wetted up to high liquid fractions ( approximately 0.3), without any bubble motion instabilities. Data are compared to drainage models: For liquid fractions above 0.2, discrepancies are found and identified. These new results on foam hydrodynamics and structure can be useful for other poroelastic materials, such as plants and biological tissues.

We present a quantitative theory and experiments for the expansion dynamics of domains in stratifying foam films. Foam films containing micelles, colloidal particles or polymer-surfactant complexes often form layered structures and thin in a stepwise fashion: circular domains of lower thickness are formed and expand following a R(t) proportional variant t1/2 law. In the present paper the film is modeled by an incompressible three-dimensional fluid with incompressible surfaces. The film tension difference between the film and domains results in the formation of a rim at the domain boundary and a gradient in film thickness and pressure in the surrounding film. The material transport due to this gradient lets the domains grow. We present experiments utilizing the thin balance method to qualitatively confirm the thinning mechanism and to determine material parameters including local film viscosity of a film composed of 4400 ppm acrylamide/acrylamidomethylpropanesulfonate-copolymer and 0.006 mmol/l dodecyltrimethylammonium bromide solution. We found a film viscosity of about 60 times the bulk viscosity, consistent with previous measurement in the same system but using another method.

We report surface rheological measurements on Langmuir films of submicron-sized polymer coils (pancakes). The dynamics of the sol phase is found to be governed by free volume as in the simplest percolation problem. The observed rheology of the percolated phase (gel-like) is compatible with predictions for compressed arrangements of highly deformed soft particles interacting through their contact interfaces.

We have studied free horizontal liquid films made with semidilute polyelectrolyte solutions. A stratification phenomenon is observed during film thinning, with a step size close to the mesh size of the polymer network: dark domains nucleate and expand, the outer polymer layer dewetting a thinner film. The kinetics of dark spot expansion is not simply related to bulk viscosity and becomes faster when the film thickness decreases, suggesting an increase of the chain mobility of the confined polymer chains. These findings are similar to recent ones for other confined liquids and are the first reported so far for freely standing films.