Phases and homogeneous ordered states in alignment-based self-propelled particle models.

We study a set of models of self-propelled particles that achieve collective motion through similar alignment-based dynamics, considering versions with and without repulsive interactions that do not affect the heading directions. We explore their phase space within a broad range of values of two nondimensional parameters (coupling strength and Peclet number), characterizing their polarization and degree of clustering. The resulting phase diagrams display equivalent, similarly distributed regions for all models with repulsion.

Quantitative kinetic theory of flocking with three-particle closure.

We consider aligning self-propelled particles in two dimensions. Their motion is given by Langevin equations and includes nonadditive N-particle interactions. The qualitative behavior is as for the famous Vicsek model.

Dry Active Matter Exhibits a Self-Organized Cross Sea Phase.

The Vicsek model of self-propelled particles is known in three different phases: a polar ordered homogeneous phase, also called the Toner-Tu phase, a phase of polar ordered regularly arranged high density bands with surrounding low density regions without polar order, and a homogeneous phase without polar order. Here, we show that the standard Vicsek model has a fourth phase for large system sizes: a polar ordered cross sea phase. We demonstrate that the cross sea phase is not just a superposition of two waves, but it is an independent complex pattern with an inherently selected crossing angle.

Multiple Particle Correlation Analysis of Many-Particle Systems: Formalism and Application to Active Matter.

We introduce a fast spatial point pattern analysis technique that is suitable for systems of many identical particles giving rise to multiparticle correlations up to arbitrary order. The obtained correlation parameters allow us to quantify the quality of mean field assumptions or theories that incorporate correlations of limited order. We study the Vicsek model of self-propelled particles and create a correlation map marking the required correlation order for each point in phase space incorporating up to ten-particle correlations.

Transport coefficients of self-propelled particles: Reverse perturbations and transverse current correlations.

The reverse perturbation method [Phys. Rev. E 59, 4894 (1999)1063-651X10.

Influence of sensorial delay on clustering and swarming.

We show that sensorial delay alters the collective motion of self-propelling agents with aligning interactions: In a two-dimensional Vicsek model, short delays enhance the emergence of clusters and swarms, while long or negative delays prevent their formation. In order to quantify this phenomenon, we introduce a global clustering parameter based on the Voronoi tessellation, which permits us to efficiently measure the formation of clusters. Thanks to its simplicity, sensorial delay might already play a role in the organization of living organisms and can provide a powerful tool to engineer and dynamically tune the behavior of large ensembles of autonomous robots.

Polarized Ukraine 2014: opinion and territorial split demonstrated with the bounded confidence XY model, parametrized by Twitter data.

Multiple countries have recently experienced extreme political polarization, which, in some cases, led to escalation of hate crime, violence and political instability. Besides the much discussed presidential elections in the USA and France, Britain's Brexit vote and Turkish constitutional referendum showed signs of extreme polarization. Among the countries affected, Ukraine faced some of the gravest consequences.

Giant Kovacs-Like Memory Effect for Active Particles.

Dynamical properties of self-propelled particles obeying a bounded confidence rule are investigated by means of kinetic theory and agent-based simulations. While memory effects are observed in disordered systems, we show that they also occur in active matter systems. In particular, we find that the system exhibits a giant Kovacs-like memory effect that is much larger than predicted by a generic linear theory.

Active matter beyond mean-field: ring-kinetic theory for self-propelled particles.

Recently, Hanke et al. [Phys. Rev.

Tricritical points in a Vicsek model of self-propelled particles with bounded confidence.

We study the orientational ordering in systems of self-propelled particles with selective interactions. To introduce the selectivity we augment the standard Vicsek model with a bounded-confidence collision rule: a given particle only aligns to neighbors who have directions quite similar to its own. Neighbors whose directions deviate more than a fixed restriction angle α are ignored.

Invasion-wave-induced first-order phase transition in systems of active particles.

An instability near the transition to collective motion of self-propelled particles is studied numerically by Enskog-like kinetic theory. While hydrodynamics breaks down, the kinetic approach leads to steep solitonlike waves. These supersonic waves show hysteresis and lead to an abrupt jump of the global order parameter if the noise level is changed.

Kinetic theory for systems of self-propelled particles with metric-free interactions.

A model of self-driven particles similar to the Vicsek model [Phys. Rev. Lett.

Kinetic theory of flocking: derivation of hydrodynamic equations.

It is shown how to explicitly coarse-grain the microscopic dynamics of the rule-based Vicsek model for self-propelled agents. The hydrodynamic equations are derived by means of an Enskog-type kinetic theory. Expressions for all transport coefficients are given.

Chapman-Enskog expansion for multi-particle collision models.

Particle-based simulation methods for fluid flow follow a discrete time dynamics of subsequent streaming and collision events. The algorithm considered here, called stochastic rotation dynamics, involves collisions between an arbitrary number of partners; all particles that happen to be in the same cell of a randomly positioned grid interact at once by prescribed rules. I show, in two dimensions, how a multi-particle generalization of the Enskog equation can be derived from the Liouville equation and how the hydrodynamic equations can be obtained by a Chapman-Enskog expansion.

Transport coefficients of multi-particle collision algorithms with velocity-dependent collision rules.

Detailed calculations of the transport coefficients of a recently introduced particle-based model for fluid dynamics with a non-ideal equation of state are presented. Excluded volume interactions are modeled by means of biased stochastic multi-particle collisions which depend on the local velocities and densities. Momentum and energy are exactly conserved locally.

Simulation of claylike colloids.

We investigate the properties of dense suspensions and sediments of small spherical silt particles by means of a combined molecular dynamics and stochastic rotation dynamics (SRD) simulation. We include van der Waals and effective electrostatic interactions between the colloidal particles, as well as Brownian motion and hydrodynamic interactions which are calculated in the SRD part. We present the simulation technique and first results.