Research

November 30, 2017 at 7:57 pm

Castillo Models Granular Systems on the Move, Like Crowded Subway Cars

Simulated packing densities with regions of fast (gray) and slow (red) moving particles, with three different squares

Nano-level simulated packing densities with regions of fast (gray) and slow (red) moving particles.

By Raymond Humienny
NQPI writing intern

Fluidized granular systems show some puzzling behaviors, especially when they are about to get stuck.

Ohio University Physics & Astronomy professor and Nanoscale & Quantum Phenomena Institute member Dr. Horacio Castillo is working to understand phenomena happening in dense granular matter. To Castillo, dense granular matter are like crowded subway cars.

“If the car is crowded and people do not get out of the way, you cannot move,” Castillo said. “This is sort of reinforcing because the person who would actually like to move for you cannot move because there is someone else blocking their motion.”

Granular materials are the second largest group of materials manipulated by humans, after water. Practical applications include asphalt, concrete and various powders in the chemical, pharmaceutical and manufacturing industries.

Castillo and his colleagues ran numerical simulations of two-dimensional granular systems. They modeled elastic and inelastic collisions between particles in a fluid state.

As the density of the system in Castillo’s simulation increases, particle collision results in a gradual slowing of system dynamics. At high enough density, the system can reach a state known as dynamical arrest, in which the dynamics are almost completely stopped. A phenomenon called dynamic heterogeneity accompanies this slowdown, generating regions of fast- and slow-moving particles.

In the initial research published in Physical Review Letters, Castillo and his colleagues reported that as the system became denser, the size of slow and fast regions in this two-dimensional system grew dramatically.

In a further study published in Soft Matter, they examined the distributions of region sizes in more detail. They found that the distribution of fast region sizes was unremarkable, but the distribution of slow region sizes showed a power law behavior characteristic of percolation transitions. These transitions appear, for example, in the study of forest fires, rock porosity and fluid diffusion in random media.

Castillo said researchers have conducted follow-up studies to understand the differences between two- and three-dimensional systems. This work has established clearer connections between how dynamical arrest and dynamical heterogeneity occur in fluidized granular systems and how they occur in other soft matter systems.

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