Abstract
We report a novel state of active matter—a swirlonic state. It is comprised of swirlons, formed by groups of active particles orbiting their common center of mass. These quasi-particles demonstrate a surprising behavior: In response to an external load they move with a constant velocity proportional to the applied force, just as objects in viscous media. The swirlons attract each other and coalesce forming a larger, joint swirlon. The coalescence is extremely slow, decelerating process, resulting in a rarified state of immobile quasi-particles. In addition to the swirlonic state, we observe gaseous, liquid and solid states, depending on the inter-particle and self-driving forces. Interestingly, in contrast to molecular systems, liquid and gaseous states of active matter do not coexist. We explain this unusual phenomenon by the lack of fast particles in active matter. We perform extensive numerical simulations and theoretical analysis. The predictions of the theory agree qualitatively and quantitatively with the simulation results.
Highlights
We report a novel state of active matter—a swirlonic state
To common molecular systems active matter undergo different phase transitions – separation into dense and dilute p hase[9,10] and crystallization[14,15,16]; these phenomena my be described within the framework of conventional t hermodynamics[14,15,16,17,18]
The system of dynamic equations for active particles forms a set of very stiff ordinary differential equations (ODEs)
Summary
The most prominent feature of systems of active (self-propelled) particle is the formation of self-organized coherent structures, see e.g.2,3,19–29 Among these are intriguing milling patterns emerging in circular motion, when a group of individuals follow one another around an empty core. We apply the Gaussian dependence of the potential on the inter-particle distance, instead of previously used exponential dependence This simple modification leads to a drastic reduction of the stiffness of the ODEs describing the system and keeps, at the same time, qualitatively the same behavior. The reduced ODE stiffness allowed to release very severe restrictions for the size of the computation time step and simulate relatively large systems, up to few tens of thousand particles Simulation of such large system gives a new insight into the properties of active matter. The results have been obtained for a system in liquid, gaseous and swirlonic state confined in a circular region
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