We study the dynamics and the phases of self-propelled disk-shaped particles of different sizes with soft repulsive potential in two dimensions. We observe enhanced dynamics for large size diversity among the particles. Size diversity is defined by the polydispersity index , which is the width of the uniform distribution of the particle's radius. We calculate the steady state diffusion coefficient D ef f and for high self-propulsion speed v0, it follows a scaling function, where α and β are the two exponents and independent of the self-propulsion speed and the polydispersity index. The phase diagram exhibits a liquid phase with a large number-fluctuations for high self-propulsion speed v0 and a jammed phase at low v0 with small number fluctuation. Further, we categorize the phases into solid-jammed and MIPS-liquid for low polydispersity ( ) where the particles form periodic clusters. In contrast, we call it liquid-jammed and liquid phase for high where particles form non-periodic clusters. We study the system for three different packing densities of the particles, and the system responds in the same fashion for the polydispersity in the particles' size. Our study can help understand the behavior of cells of various sizes in a tissue, artificial self-driven granular particles, or living organisms of different sizes in a dense environment.
A collection of self-propelled particles (SPPs) shows coherent motion and exhibits a true long range ordered (LRO) state in two dimensions. Various studies show that the presence of spatial inhomogeneities can destroy the usual long range ordering in the system. However, effects of inhomogeneity due to the intrinsic properties of the particles are barely addressed. In this paper we consider a collection of polar SPPs moving with inhomogeneous speed (IS) on a two dimensional substrate, which can arise due to varying physical strength of the individual particle. To our surprise the IS not only preserves the usual long range ordering present in the homogeneous speed models, but also induces faster ordering in the system. Furthermore, The response of the flock to an external perturbation is also faster, compared to Vicsek like model systems, due to the frequent update of neighbors of each SPP in the presence of the IS. Therefore, our study shows that the IS can help in faster information transfer in the moving flock.
We study the collective behavior of binary mixture of self-propelled particles. Particles moves along their heading direction with variable speed and interact through short range alignment interaction. A variable speed parameter γ > 0 is introduced such that for γ = 0.0 model reduces to constant speedVicsek's model. We mix the particles with two different γ's and study the steady state behavior of the mixture for different choice of γ's and noise strength. One of the γ is kept fixed to 1.0 and another one is varied from small 0.0 to larger values 8.0. Properties of system is characterise by two types of order parameters (i) orientation order parameter, which is a measure of ordering in the system and (ii) density order parameter, which measures the phase separation is the system. For all set of γ's, system shows a transition from disorder-to-ordered state on the variation of noise strength. The nature of transition and critical noise is independent of value of γ, which is also supported from coarse-grained hydrodynamic study.On the variation of system parameters, (γ's, η), we find four distinct phases, (i) ordered phase separated, (ii) ordered mixed, (iii) disordered mixed and (iv) disordered phase segragated. Our study shade light on different phases of mixture of different types of active particles. * jayps.rs.phy16@itbhu.ac.in † smishra.phy@itbhu.ac.in arXiv:1902.00296v1 [cond-mat.soft]
We model a binary mixture of passive and active Brownian particles in two dimensions using the effective interaction between passive particles in the active bath. The activity of active particles and the size ratio of two types of particles are two control parameters in the system. The effective interaction is calculated from the average force on two particles generated by the active particles. The effective interaction can be attractive or repulsive, depending on the system parameters. The passive particles form four distinct structural orders for different system parameters viz; homogeneous structures (HS), disordered cluster (DC), ordered cluster (OC), and crystalline structure (CS). The change in structure is dictated by the change in nature of the effective interaction. We further confirm the four structures using full microscopic simulation of active and passive mixture. Our study is useful to understand the different collective behaviour in non-equilibrium systems.
In this study, we introduce a minimal model for a collection of polar self-propelled particles (SPPs) on a two-dimensional substrate where each particle has a different ability to interact with its neighbors. The SPPs interact through a short-range alignment interaction and interaction strength of each particle is obtained from a uniform distribution. Moreover, the volume exclusion among the SPPs is taken care of by introducing a repulsive interaction among them. We characterise the ordered steady state and kinetics of the system for different strengths of the disorder. We find that the presence of the disorder does not destroy the usual long-range ordering in the system. To our surprise, we note that the density clustering is enhanced in the presence of the disorder. Moreover, the disorder leads to the formation of a random network of different interaction strengths, which makes the alignment weaker and it results in the slower dynamics. Hence, the disorder leads to more cohesion among the particles. Furthermore, we note that the kinetics of the ordered state remains unaffected in the presence of the disorder. Size of orientationally ordered domains and density clusters grow with time with dynamic growth exponents z o ∼ 2 and z ρ ∼ 4, respectively.
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