Silbert Research Group

The Silbert research group studies the complex interplay between structure, dynamics, and driving mechanisms in soft matter material systems that typically exist far from thermodynamic equilibrium. We use a range of theoretical and computational methods to investigate the relation between particle scale properties and macroscopic behavior. Our major themes include the jamming transition in athermal particle packings, complex flows, and mechanical response in granular materials.

NEW - Granular Aspherical and Complex Particles in LAMMPS

Jamming Transition

The Jamming Transition signifies the transition between different mechanical phases. For packings of frictionless, monodisperse spheres interacting through purely repulsive, short-range interactions at zero temperature and applied shear, there is a critical density, or packing fraction, that defines the jamming transition where the particles first osculate. Below this critical density, all particles are out of contact and therefore are non-interacting due to the short-range nature of the potential. Therefore packings below the jamming transition cannot support any perturbations and are considered to exist in the unjammed or fluid-like phase. At packing fractions larger than the critical value, the jammed, or solid-like phase, has finite bulk and shear moduli and is mechanically rigid, requiring finite energy to deform or flow.

Complex Flows

Dense colloidal suspensions, such as paints and inks, typically exhibit non-linear rheological behavior, as do most polymeric liquids, such as toothpaste. Over a range of applied shear rates and stresses, the viscosity of such complex fluids can exhibit shear thinning - a decrease in viscosity with increasing shear - or shear thickening - an increase in viscosity with increasing shear. If this wasn't the case then painting would be much less fun and a lot more messy. And squeezing toothpaste out of a tube would be cumbersome. Granular materials similarly exhibit complex rheological properties. Clogging in pipes and grain silos and rock and snow avalanches are examples where granular flows have huge industrial and environmental impact, usually in a negative sense. Yet on the face of it, the interaction between two grains is relatively well understood. However, when collections of dissipative and frictional particles interact, predicting their flow properties becomes problematic.

Mechanical Response

Investigating the mechanical robustness of a material can be carried out in a number of ways. In granular materials, a common method involves applying localized force perturbations then extracting the resulting stress profile in response to the perturbation. Such methods are sometimes refered to as Green Function technique. The expected results for elastic solids is well known. However, it turns out that granular packings exhibit a range of stress response profiles that span the gamut of possibilities. Ranging from linear, elastic-like behavior to that of anisotropic, fragile matter. The origin of this wide range of stress properties is still a debatable topic, but the major components determining the underlying stress state of a granular systems include the particle friction and the structural configuration of the packing.

Link to publication list

Research Topics

Colloid Rheology

Granular Dynamics

Granular Statics


Lennard Jones

Follow this link to the OpenSIUC project for selected publications and simulation Videos

Research supported by

National Science Foundation