F. Giardina | Searcharxiv

arXiv2023

Zonal flows and reversals of cortically confined active suspensions

At sufficiently high concentrations, motile bacteria suspended in fluids exhibit a range of ordered and disordered collective motions. Here we explore the combined effects of confinement, periodicity and curvature induced by the active motion of E. coli bacteria in a thin spherical shell (cortex) of an oil-water-oil (O/B/O) double emulsion drop. Confocal microscopy of the bacterial flow fields shows that at high density and activity, they exhibit azimuthal zonal flows which oscillate between counterclockwise and clockwise circulating states. We characterize these oscillatory patterns via their Fourier spectra and the distributions of their circulation persistence times. To explain our observations, we used numerical simulations of active particles and characterize the two-dimensional phase space of bacterial packing fraction and activity associated with persistent collective motions. All together, our study shows how geometric effects lead to new types of collective dynamics.

cond-mat.soft↗

arXiv2020

Models of benthic bipedalism

Walking is a common bipedal and quadrupedal gait and is often associated with terrestrial and aquatic organisms. Inspired by recent evidence of the neural underpinnings of primitive aquatic walking in the little skate Leucoraja erinacea, we introduce a theoretical model of aquatic walking that reveals robust and efficient gaits with modest requirements for body morphology and control. The model predicts undulatory behavior of the system body with a regular foot placement pattern which is also observed in the animal, and additionally predicts the existence of gait bistability between two states, one with a large energetic cost for locomotion and another associated with almost no energetic cost. We show that these can be discovered using a simple reinforcement learning scheme. To test these theoretical frameworks, we built a bipedal robot and show that its behaviors are similar to those of our minimal model: its gait is also periodic and exhibits bistability, with a low efficiency gait separated from a high efficiency gait by a "jump" transition. Overall, our study highlights the physical constraints on the evolution of walking and provides a guide for the design of efficient biomimetic robots.

q-bio.QM↗