Short-time dynamics in active systems: the Vicsek model.

We study the short-time dynamics (STD) of the Vicsek model (VM) with vector noise. The study of STD has proved to be very useful in the determination of the critical point, critical exponents and spinodal points in equilibrium phase transitions. Here we aim is to test its applicability in active systems. We find that, despite the essential non-equilibrium characteristics of the VM (absence of detailed balance, activity), the STD presents qualitatively the same phenomenology as in equilibrium systems. From the STD one can distinguish whether the transition is continuous or discontinuous (which we have checked also computing the Binder cumulant). When the transition is continuous, one can determine the critical point and the critical exponents.

A rheological approach to identify efficient biopolymer producing bacteria.

This study investigates the relationship between collective motion and propulsion of bacterial consortia and their biopolymer production efficiency. Rheological tests were conducted for suspensions of two different methanotrophic bacterial consortia obtained after enrichment of sediment samples from mangrove sites in Brazil. We considered the linear viscoelasticity region and analyzed the values of storage and loss moduli as functions of days of cultivation, for different values of the volume fraction. The suspensions' rheological behaviors reflected the bacterial growth stage. We found that the formation of structures over time in some types of consortia can hinder the movement of bacteria in the search for nutrients. The change in complex viscosity of the two consortia followed a different and rich behavior that appears to be closely related to their capacity to capture methane. Our analysis showed a possible correlation between collective motion, viscosity reduction, and biopolymer production. The pieces of evidence from this study suggest that the efficiency of bacterial motion is directly related to biopolymer production, and this could facilitate the process of identifying the best consortium of biopolymer producing bacteria.