Abstract:

Many natural and industrial processes rely on the displacement of mixed liquid, gas, and granular materials through confining structures. Specifically, such multi-phase flows through porous geometries play an essential role in engineering applications ranging from oil recovery and CO2 sequestration to filtration. Despite their tremendous potential economic and environmental impact, understanding these three-phase flows remains a formidable challenge. First, we study the formation of granular plugs during the slow drainage of a liquid-grain mixture in a tube illustrating the onset of a “bulldozing” instability. We define a new unstable regime leading to the periodic formation of dunes, analogous to the road “washboarding” instability, where a key element is the strong increase of the frictional forces with the angle of attack of the pushing meniscus at the liquid/air interface. Second, we develop an original method to control the frictional interactions at the origin of this bulldozing instability, by using ferromagnetic particles: submitted to a magnetic field, they acquire a magnetic moment, leading to tunable interactions of pairs of magnetic dipoles. We demonstrate the emergence of a radial force along the walls of the container, whose amplitude and direction are determined by the applied magnetic field. This "magnetic Janssen effect" allows us to control the apparent mass of a granular column, paving the way towards the design of granular meta-materials, for which mechanical (un)jamming properties can be remotely controlled or even programmed.