Soft Matter Seminar: Droplet Formation in Freely Falling Granular Streams
Friday, May 13, 2011 - 1:15pm - 2:15pm
Thin liquid streams commonly break up into characteristic droplet patterns due to the surface tension driven Plateau-Rayleigh instability. Remarkably, droplet formation can also occur in streams of dry granular materials, even though flows of macroscopic particles are considered to lack surface tension. Using high-speed video imaging in the co-moving frame, measuring grain-grain interactions with Atomic Force Microscopy, and controlling the properties of the grain surfaces, we demonstrate that droplet formation is driven by minute cohesive forces due to a combination of van der Waals interactions and capillary bridges between nanoscale surface asperities.
Molecular Dynamics simulations allow us to further probe the role of inter-particle interactions and map out a spectrum of behaviors, from gas-like jets where all grains drift apart to liquid-like streams which break up into large droplets. These simulations reveal that the droplet formation process is controlled by the competition between cohesive forces and the velocity gradient associated with vertical free fall.
Host: Daniel Blair