Dynamics of Shaped Colloids

    - NIST/Georgetown

Though there has been significant progress understanding colloidal suspensions of isotropic, spherical particles, relatively little is known about how particle shape or directional interactions influence the suspension dynamics. We use a novel synthesis technique first described in Rossi et al, Soft Matter 2010 to create bulk quantities of mono-disperse cubes, ellipsoids and peanut-shaped particles. Using 3D confocal-rheology, we characterize the dynamics of these shaped particles under shear.

Grains under slow, cyclic shear

- New York University

Using molecular dynamics simulations (LAMMPS), we investigate the response of granular matter to periodic shear. We find that the frictional grains can organize into a handful of well-defined spatiotemporal patterns, including previously observed crystalline and vortex states. Remarkably, we discovered a new disordered Limit Cycle phase in which the system self-organizes so that each independent grain in the system precisely follows the same trajectory from cycle to cycle.

Breakup of a Granular Stream

- University of Chicago

Thin liquid streams commonly break up into droplets due to the surface tension driven Plateau-Rayleigh instability. Remarkably, similar patterns of droplets can be observed in thin streams of sand, even though sand is typically considered to lack surface tension. Using high-speed video imaging in the co-moving frame and directly measuring grain-grain interactions with Atomic Force Microscopy, we find that droplet formation is driven by minute, nanoNewton cohesive forces from a combination of van der Waals interactions and capillary bridges between nanoscale surface asperities. Read more here. 

High-Speed X-ray Radiography of Granular Impact

- University of Chicago

Using high-speed X-ray radiography we track both the motion of the sphere and local changes in the bed packing fraction during the impact of a solid sphere into a fine-grained granular bed. We find that the ambient air pressure  plays a key role in the impact dynamics by resisting packing changes.  In an initially loose bed the ambient air prevents compaction, resulting in a more fluid-like response of the bed that allows the impacting sphere to easily sink into the bed. In vacuum a large region of compacted grains builds ahead of the sphere, rapidly bringing it to rest. Read more here.

MD simulation of grains under cyclic shear

Shear Thickening

    - NIST/Georgetown

Exploring both viscous and normal forces in shear thickening colloidal suspensions, we find a previously hidden transition in the sign of the 1st Normal Stress Difference. This suggests a scenario where shear thickening is driven primarily by frictional contacts, with hydrodynamic forces playing a supporting role at lower concentrations. Our results highlight the critical role of short ranged contact forces in shear thickening suspensions. Ongoing experiments seek to further understand the effects particle size, shape and other surface properties.