Persistent noise-driven odd elastic waves in living chiral active matter
Chirality and activity lead to non-reciprocal transverse interaction between neighboring particles. Then peculiar elasticity emerges, where stress and strain that were independent in regular matter become coupled; compression causes spontaneous rotation of the material, for example. This elasticity is called "odd elasticity" because it is represented by off-diagonal asymmetric elements in the elastic modulus tensor. Although odd elastic waves are predicted to exist, they are always damped, which makes them difficult to experimentally observe. Inspired by the works on fluctuation-driven pattern formations in population dynamics, we generate persistent odd elastic waves driven by noise due to self-propulsion. We focus on the model of starfish embryos Patiria miniata, which form a living crystal during a certain stage of their development. This living crystal has been experimentally shown to be odd-elastic. By adding the noise from collision between neighbors due to self-propulsion, which has been neglected in previous works, we demonstrate that the wave persists even in the presence of damping. With the spectral method using current correlation functions, we also show that the oscillatory behavior reported in the living crystal of starfish embryos is due to the self-circling motion from combination of self-spinning and self-propulsion of the embryos, rather than a wave. Our derivation of the new criterion for persistent noise-driven odd elastic wave suggests that strong enough, but not too strong, noise and transverse force relative to longitudinal force are needed for the persistent wave. Although our work focuses on the starfish embryos, our framework is general and is applicable to other chiral active systems.
The scaling behavior in honeybee and human social interaction
Burstiness of an inter-event time distribution has been much studied because of its relation to information or disease spreading. On the other hand, the distribution of contact duration has received less attention. We measured the duration of honeybee trophallaxis, or mouth-to-mouth liquid food transfer, and face-to-face encounters by using high-resolution tracking in collaboration with Robinson Lab in the University of Illinois and found out that the distribution of event durations is heavy-tailed. Human data recorded by the SocioPatterns collaboration show that human face-to-face interactions in various settings also exhibit a heavy-tailed interaction time distribution. Such similarity across different systems suggests unexpected universality in social interactions. We have derived the power-law form by building a minimal model that treats the termination of an interaction as a particle escaping over an energy barrier; the variability in the population leads to a distribution of energy barriers which is determined by the extreme value theory and is a key to achieving the final power-law form. Honeybees indeed exhibit individual differences in interactivity although they are less different than humans. Our work demonstrates how individual differences can lead to universal patterns of social interaction that transcend species, context and specific mechanisms.
S.H. Choi, V.D. Rao, T. Gernat, A.R. Hamilton, G.E. Robinson and N. Goldenfeld. "Individual variations lead to universal and cross species patterns of social behavior," Proc. Natl. Acad. Sci. 117, 31754-31759 (2020) [link]