Divided Media and Complex Fluids



Laser-light and Interactions with particles [conference LIP2020]

The next LIP conference will be held on August 22-28th, 2020 in the Institute of Physics, Warsaw, Poland. Main topics: interactions between laser beams and particles, encompassing the following fundamental topics : particle characterization methods, near-field, far-field and time-resolved scattering, plasmonics and other resonances, complex shaped particles and aggregates, multiple scattering and random media, mechanical effects of light, laser beams description (contributions acoustical and quantum beams are also welcomed) and application domains: two-and multiphase-flow characterization, aerosol science and atmospheric environment, plasma and soft matter physics, biomedical optical engineering, remote sensing…

Conveners: D. Jakubczyk, M. Kolwas, F. Onofri, G. Gouesbet

Website: / Flyer (call for abstracts): pdf

Gas-assisted discharge flow of granular media from silos [paper in PRFluids]

We studied experimentally the discharge of a vertical silo filled by spherical glass beads and assisted by injection of air from the top at a constant flow rate, a situation which has practical interest for nuclear safety or air-assisted discharge of hoppers. Using a two-phase continuum model with a frictional rheology to describe particle-particle interactions, we reveal the role played by the air-pressure gradient at the orifice and proposed a simple analytical model to predicts the mass flow rate of a granular media discharged from a silo with injection of gas.

Zhou et al Phys. Rev. Fluids  – Published 18 December 2019

Avalanches of Brownian granular materials [paper in PRL]

Macroscopic granular materials and colloids are usually studied separatly by different communities. But what happens if the particles in a granular flow are so small that their thermal agitation becomes comparable to their weight? In this paper we combine microfluidic experiments and a simple model to study the avalanche behavior of such a ‘Brownian granular medium’. We show that thermal agitation can completely erase the flow threshold of conventional granular media, a first step to bridge the gap between the physics of granular matter and colloids.

Bérut et al Phys. Rev. Lett. – Published 12 December 2019

Rheology of immersed and dry frictional spheres [paper in PRFluids]

Pressure-imposed rheometry is used to examine the influence of surface roughness on the rheology of immersed and dry frictional spheres in the dense regime. The quasistatic value of the effective friction coefficient is not significantly affected by particle roughness while the critical volume fraction at jamming decreases with increasing roughness. These values are found to be similar in immersed and dry conditions. Rescaling the volume fraction by the maximum volume fraction leads to collapses of rheological data on master curves.

Tapia et al Phys. Rev. Fluids  – Published 17 October 2019

Interparticle friction determines hysteresis in granular flow [paper in PRX]

Hysteresis is a major feature of the solid-liquid transition in granular materials but its origin is still debated. To study this phenomenon, we monitor the avalanche dynamics of non-Brownian suspensions in slowly rotating drums.  By using microsilica particles whose interparticle friction coefficient can be turned off, we show that microscopic friction, conversely to inertia, is key to triggering hysteresis in granular suspensions.

Perrin et al Phys. Rev. X  – Published 16 August 2019; see Focus story in Physics APS, news in Physics Today

Memory effects and transient gravitropic response in plants [paper in J Exp Bot]

In this paper, we study the plant gravitropic response to transient inclinations at the organ scale and the associated motion of statoliths at the cellular level. Our results reveal the existence of a memory process in the signalling pathway, independent of statolith dynamics. By combining this memory process with statolith motion, we build a mathematical model that unifies the different laws found in the literature and that predicts the early bending response of shoots to arbitrary gravi-stimulations.

Chauvet et al  J Exp Bot 70,1955–1967 (2019) – Published 27 March 2019

Breakup of a particulate suspension jet [paper in PRFluids]

As viscosity is increased, a liquid capillary jet accelerated by gravity stretches over increasingly large distances before eventually breaking up. This Newtonian behavior is profoundly altered for particulate suspensions. Adding solid particles to a liquid, which increases the effective viscosity, can paradoxically shorten the jet considerably. This apparent contradiction is rationalized by considering finite-size effects occurring at the scale of a few particles. A model is presented which captures the breakup length of suspension jets for a broad range of conditions.

Château et al  Phys. Rev. Fluid 4, 012001(R) – Published 10 January 2019

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