Collective dynamics of matter with granularity
Gravish, Nicholas Grey
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Granular materials are abundant in the natural and industrial environment. Typical granular materials are collections of inert, passive particles in which the constituent grains of the material are macroscopic; thus they fill space, are athermal, and interact through only local contact forces. This definition can be broadened to include non-inert particles as well-active particles-in which the grains of an active granular material possess an internal energy source which drives motion. Active granular materials are found in many areas of the biological world, from cattle stampedes and pedestrian traffic flow, to the subterranean world of ant colonies and their collective motion within the nest. We study the rheology and dynamics of inert granular material, and an active granular system of collections of fire-ants, which together we call matter with granularity. In both of these systems we observe bifurcations in the force and flow dynamics which results from confinement effects of the effectively rigid granular materials. In inert granular systems, the onset of flow among particles that are closely packed together causes them to dilate as particles must separate away from each other to accommodate flow. Dilation is a property unique to matter with granularity and other complex fluids in which particles interact locally and occupy space. We explore how dilation influences the inert granular system in situations of local and global forcing: drag of an immersed intruder and avalanche flow respectively. We next study collections of fire ants which also interact with each other locally through contact forces and exclude volume. We study the construction of, and locomotion within subterranean tunnels by groups of fire ants. We find that the traffic dynamics of ants within confined tunnels are significantly affected by tunnel diameter. Reducing tunnel diameter increases the formation of traffic jams due to the inability of ants to pass each other easily. However, we show that jamming within tunnels may have beneficial effects on subterranean locomotion. Individual ants jam there bodies against the walls of vertical tunnels to resist falling. From physics studies of fire ant mobility in confined spaces, we show that subterranean tunnel size has a significant effect on the stability and mobility of ants within these environments.