Cascading failure and robustness in metabolic networks.

We investigate the relationship between structure and robustness in the metabolic networks of Escherichia coli, Methanosarcina barkeri, Staphylococcus aureus, and Saccharomyces cerevisiae, using a cascading failure model based on a topological flux balance criterion. We find that, compared to appropriate null models, the metabolic networks are exceptionally robust. Furthermore, by decomposing each network into rigid clusters and branched metabolites, we demonstrate that the enhanced robustness is related to the organization of branched metabolites, as rigid cluster formations in the metabolic networks appear to be consistent with null model behavior. Finally, we show that cascading in the metabolic networks can be described as a percolation process.

Evolving loop structure in gradually tilted two-dimensional granular packings.

Granular packings, especially near the jamming transition, form fragile networks where small perturbations can lead to destabilization and large scale rearrangements. A key stabilizing element in two dimensions is the contact loop, yet surprisingly little is known about contact loop statistics in realistic granular networks. In this paper, we use particle dynamics to study the evolution of contact loop structure in a gradually tilted two-dimensional granular bed. We find that the resulting contact loop distributions (1) are sensitive to material properties, (2) deviate from the expected structure of a randomly wired lattice, and (3) are uniquely dependent on tilting angle. Also, we introduce a quantitative measure of loop stability xi and show that increased tilting results in a gradual destabilization of individual loops. We briefly discuss the considerations for extending our approach to three dimensions.