Compression of a soft sphere packing

Abstract

Mechanical properties of packings of deformable spheres of polyelectrolyte gel are studied experimentally. These particles are plunged into a brine. They have the property to swell and shrink when the concentration of salt of the solution is varied. An oedometric compression is performed imposing cycles of deformation at constant speed and constant salinity Cs. Under many different conditions, we study the laws of deformation relating the macroscopic compression force F , to the macroscopic strain ε. We find empirical non linear relations of the type F ∼ ε m . The values of this exponent m are discussed and compared to the results of measurements on a single sphere compressed on a plane as well as to the results of experiments and simulations on dry model granular assemblies. The swelling and deswelling properties of the spheres are used to perform isotropic compression tests. In this situation we determine the relation between the force at equilibrium and the macroscopic strain ε(Cs). The results are compared with those obtained in the oedometric compression tests.

Spanning the scales of granular materials through microscopic force imaging

“…A fit of these data to a power law yields an exponent β =2.2±0.2, which is significantly larger than the microscopic (Hertzian) force law exponent of 1.5 measured for intergrain forces. This non-Hertzian packing mechanics has even been observed in much larger packings 21 , so it is not a finite size effect—the key point is that F depends on ‹f› as well as on additional microscopic properties. This is already partly revealed through the small hysteresis visible in the compression loop of Fig.…”

“…These last two variables are related by Hertz’ law, together with the radius of curvature, r , at contacts. A key observation is that F(Δ) and ‹f›(‹δ›) follow substantially different functional relations 21 .…”

Hysteresis effects studied by numerical simulations: Cyclic loading-unloading of a realistic sand model

“…This hysteretic behavior is commonly observed in laboratory granular materials (Lachhab and Weill, 1999) and naturally occurring sands (Bremer et al, 2001). In these systems, sample elastic properties evolve during cycling until a final pack is obtained for which reproducible measurements can be made.…”

“…This procedure is similar to the laboratory compaction of grainlike materials under odeometric conditions (Lachhab and Weill, 1999). In such experiments, the sample is compressed in one direction by a piston, simultaneously keeping constant vessel dimensions in the lateral directions.…”

Imaging soft sphere packings in a novel triaxial shear setup

“…Boundary pressures are measured with pressure transducers; to image packing structure and evolution we use refractive index matched (RIM) imaging [7]. We explore in particular the potential of hydrogel particles in the study of granular materials [8,9], whose properties make it possible to study the role friction and packing mechanics and allow for the extraction of contact and force networks [5,10]. …”