Modeling soft granular media
Résumé
Soft-particle materials such as colloidal pastes, vesicles and microgel suspensions, are
composed of individual elementary particles, which can undergo large deformations without
rupture. In this respect, they are different from rigid-particle materials in which the plastic
behavior is essentially dictated by particle rearrangements. Particle shape change under
loading in soft materials leads to enhanced space filling and thus specific assembly properties.
The compaction, shear behavior and other rheological properties of soft-particle assemblies
beyond the "jamming" limit remain unexplored due to the lack of proper numerical and
experimental tools.
The molecular dynamics method is widely used for the simulation of particle assemblies due to
its ability to account for particle interactions and complex loading conditions. However, since
this approach is based on the rigid-body assumption, it cannot be used with large particle
deformations. To model the mechanical properties of soft particles as well as their mutual
interactions, a new methodology is proposed. It is based on an implicit formulation of the
Material Point Method (MPM) for modeling large particle deformations coupled with the
Contact Dynamics (CD) method for the treatment of frictional and cohesive contacts between
particles. In this approach, each particle is discretized into a set of material points. At each
time step, the information carried by these points is projected onto a background mesh, where
equations 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. This
implicit 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. The
packing can reach high solid fractions by particle shape change and still flow plastically. The
compaction is a nonlinear process in which new contacts are formed between particles and
the contact areas increase. We find that the evolution of the packing fraction is a slow
logarithmic function of the driving stress as a consequence of increasing contact area. We also
evidence the effect of friction, which favors strong stress chains and thus the elongation of
particles, leading to a larger packing fraction at a given level of compressive stress as
compared to frictionless particle packing.
Format : Poster
