Résumé
Soft-particle materials such as colloidal pastes, vesicles and microgel suspensions, arecomposed of individual elementary particles, which can undergo large deformations withoutrupture. In this respect, they are different from rigid-particle materials in which the plasticbehavior is essentially dictated by particle rearrangements. Particle shape change underloading in soft materials leads to enhanced space filling and thus specific assembly properties.The compaction, shear behavior and other rheological properties of soft-particle assembliesbeyond the "jamming" limit remain unexplored due to the lack of proper numerical andexperimental tools.The molecular dynamics method is widely used for the simulation of particle assemblies due toits ability to account for particle interactions and complex loading conditions. However, sincethis approach is based on the rigid-body assumption, it cannot be used with large particledeformations. To model the mechanical properties of soft particles as well as their mutualinteractions, a new methodology is proposed. It is based on an implicit formulation of theMaterial Point Method (MPM) for modeling large particle deformations coupled with theContact Dynamics (CD) method for the treatment of frictional and cohesive contacts betweenparticles. In this approach, each particle is discretized into a set of material points. At eachtime step, the information carried by these points is projected onto a background mesh, whereequations of motion are solved by taking into account frictional contacts between particles.This solution is then used to update the information associated with material points. Thisimplicit MPM-CD model is implemented in a manner that the contact variables (velocity,force...) can be computed simultaneously with bulk variables.We used this model to analyze the compaction process of 2D soft-particle packings. Thepacking can reach high solid fractions by particle shape change and still flow plastically. Thecompaction is a nonlinear process in which new contacts are formed between particles andthe contact areas increase. We find that the evolution of the packing fraction is a slowlogarithmic function of the driving stress as a consequence of increasing contact area. We alsoevidence the effect of friction, which favors strong stress chains and thus the elongation ofparticles, leading to a larger packing fraction at a given level of compressive stress ascompared to frictionless particle packing.