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    Phase separation and super diffusion of binary mixtures ofactive and passive particles
    Yan Wang(王艳), Zhuanglin Shen(谌庄琳), Yiqi Xia(夏益祺), Guoqiang Feng(冯国强), Wende Tian(田文得)
    Chin. Phys. B, 2020, 29 (5): 053103.   DOI: 10.1088/1674-1056/ab81f4
    Abstract641)   HTML    PDF (1895KB)(211)      
    Computer simulations were performed to study the dense mixtures of passive particles and active particles in two dimensions. Two systems with different kinds of passive particles (e.g., spherical particles and rod-like particles) were considered. At small active forces, the high-density and low-density regions emerge in both systems, indicating a phase separation. At higher active forces, the systems return to a homogeneous state with large fluctuation of particle area in contrast with the thermo-equilibrium state. Structurally, the rod-like particles accumulate loosely due to the shape anisotropy compared with the spherical particles at the high-density region. Moreover, there exists a positive correlation between Voronoi area and velocity of the particles. Additionally, a small number of active particles capably give rise to super-diffusion of passive particles in both systems when the self-propelled force is turned on.
    Constraint dependence of average potential energy of a passive particle in an active bath
    Simin Ye(叶思敏), Peng Liu(刘鹏), Zixuan Wei(魏子轩), Fangfu Ye(叶方富), Mingcheng Yang(杨明成), Ke Chen(陈科)
    Chin. Phys. B, 2020, 29 (5): 058201.   DOI: 10.1088/1674-1056/ab7d9b
    Abstract687)   HTML    PDF (624KB)(265)      
    We quantify the mean potential energy of a passive colloidal particle harmonically confined in a bacterial solution using optical traps. We find that the average potential energy of the passive particle depends on the trap stiffness, in contrast to the equilibrium case where energy partition is independent of the external constraints. The constraint dependence of the mean potential energy originates from the fact that the persistent collisions between the passive particle and the active bacteria are influenced by the particle relaxation dynamics. Our experimental results are consistent with the Brownian dynamics simulations, and confirm the recent theoretical prediction.
    Symmetry properties of fluctuations in an actively driven rotor
    He Li(李赫), Xiang Yang(杨翔), Hepeng Zhang(张何朋)
    Chin. Phys. B, 2020, 29 (6): 060502.   DOI: 10.1088/1674-1056/ab862b
    Abstract652)   HTML    PDF (11591KB)(246)      
    We investigate rotational dynamics of an actively driven rotor through experiments and numerical simulations. While probability density distributions of rotor angular velocity are strongly non-Gaussian, relative probabilities of observing rotation in opposite directions are shown to be linearly related to the angular velocity magnitude. We construct a stochastic model to describe transitions between different states from rotor angular velocity data and use the stochastic model to show that symmetry properties in probability density distributions are related to the detailed fluctuation relation (FR) of entropy productions.
    Self-assembled vesicle-colloid hybrid swimmers: Non-reciprocal strokes with reciprocal actuation
    Jaime Agudo-Canalejo, Babak Nasouri
    Chin. Phys. B, 2020, 29 (6): 064704.   DOI: 10.1088/1674-1056/ab892b
    Abstract474)   HTML    PDF (598KB)(171)      
    We consider a self-assembled hybrid system, composed of a bilayer vesicle to which a number of colloids are adhered. Based on known results of membrane curvature elasticity, we predict that, for sufficiently deflated prolate vesicles, the colloids can self-assemble into a ring at a finite distance away from the vesicle equator, thus breaking the up-down symmetry in the system. Because the relative variation of the position of the colloidal ring along the vesicle endows the system with an effective elasticity, periodic cycles of inflation and deflation can lead to non-reciprocal shape changes of the vesicle-colloid hybrid, allowing it to swim in a low Reynolds number environment under reciprocal actuation. We design several actuation protocols that allow control over the swimming direction.
    Diffusion and collective motion of rotlets in 2D space
    Daiki Matsunaga, Takumi Chodo, Takuma Kawai
    Chin. Phys. B, 2020, 29 (6): 064705.   DOI: 10.1088/1674-1056/ab8ac3
    Abstract473)   HTML    PDF (1065KB)(167)      
    We investigate the collective motion of rotlets that are placed in a single plane. Due to the hydrodynamic interactions, the particles move through the two-dimensional (2D) plane and we analyze these diffusive motions. By analyzing the scaling of the values, we predict that the diffusion coefficient scales with φ0.5, the average velocity with φ, and relaxation time of the velocity autocorrelation function with φ-1.5, where φ is the area fraction of the particles. In this paper, we find that the predicted scaling could be seen only when the initial particle position is homogeneous. The particle collective motions are different by starting the simulation from random initial positions, and the diffusion coefficient is the largest at a minimum volume fraction of our parameter range, φ=0.05. The deviations based on two initial positions can be explained by the frequency of the collision events. The particles collide during their movements and the inter-particle distances gradually increase. When the area fraction is large, the particles will result in relatively homogeneous configurations regardless of the initial positions because of many collision events. When the area fraction is small (φ < 0.25), on the other hand, two initial positions would fall into different local solutions because the rare collision events would not modify the inter-particle distances drastically. By starting from the homogeneous initial positions, the particles show the maximum diffusion coefficient at φ≈0.20. The diffusion coefficient starts to decrease from this area fraction because the particles start to collide and hinder each other from a critical fraction ~23%. We believe our current work contributes to a basic understanding of the collective motion of rotating units.
    Simulation of microswimmer hydrodynamics with multiparticle collision dynamics
    Andreas Z?ttl
    Chin. Phys. B, 2020, 29 (7): 074701.   DOI: 10.1088/1674-1056/ab943f
    Abstract415)   HTML    PDF (1297KB)(196)      
    In this review we discuss the recent progress in the simulation of soft active matter systems and in particular the hydrodynamics of microswimmers using the method of multiparticle collision dynamics, which solves the hydrodynamic flows around active objects on a coarse-grained level. We first present a brief overview of the basic simulation method and the coupling between microswimmers and fluid. We then review the current achievements in simulating flexible and rigid microswimmers using multiparticle collision dynamics, and briefly conclude and discuss possible future directions.
    Regulation of microtubule array inits self-organized dense active crowds
    Xin-Chen Jiang(蒋新晨), Yu-Qiang Ma(马余强), Xiaqing Shi(施夏清)
    Chin. Phys. B, 2020, 29 (7): 078201.   DOI: 10.1088/1674-1056/ab9430
    Abstract546)   HTML    PDF (919KB)(136)      
    Microtubule self-organization under mechanical and chemical regulations plays a central role in cytokinesis and cellular transportations. In plant-cells, the patterns or phases of cortical microtubules organizations are the direct indicators of cell-phases. The dense nematic pattern of cortical microtubule array relies on the regulation of single microtubule dynamics with mechanical coupling to steric interaction among the self-organized microtubule crowds. Building upon previous minimal models, we investigate the effective microtubule width, microtubule catastrophe rate, and zippering angle as factors that regulate the self-organization of the dense nematic phase. We find that by incorporating the effective microtubule width, the transition from isotropic to the highly ordered nematic phase (NI phase) with extremely long microtubules will be gapped by another nematic phase which consists of relative short microtubules (N phase). The N phase in the gap grows wider with the increase of the microtubule width. We further illustrate that in the dense phase, the collision-induced catastrophe rate and an optimal zippering angle play an important role in controlling the order-disorder transition, as a result of the coupling between the collision events and ordering. Our study shows that the transition to dense microtubule array requires the cross-talk between single microtubule growth and mechanical interactions among microtubules in the active crowds.
    Phoretic self-assembly of active colloidal molecules
    Lijie Lei(雷李杰), Shuo Wang(王硕), Xinyuan Zhang(张昕源), Wenjie Lai(赖文杰), Jinyu Wu(吴晋宇), and Yongxiang Gao(高永祥)
    Chin. Phys. B, 2021, 30 (5): 056112.   DOI: 10.1088/1674-1056/abc2bd
    Abstract461)   HTML1)    PDF (3360KB)(135)      
    We simulate the self-assembly of active colloidal molecules from binary mixtures of spherical particles using a Brownian dynamics algorithm. These particles interact via phoretic interactions, which are determined by two independently tunable parameters, surface activity and surface mobility. In systems composed of equal-size particles, we observe the formation of colloidal molecules with well-defined coordination numbers and spatial arrangement, which also display distinct dynamic functions, such as resting, translating, and rotating. By changing the size ratio to 2:1 between the two species, we further observe the formation of colloidal molecules with new structures arising from breaking the size symmetry. By tuning the mutual interactions between the smaller species via their surface mobility, we are able to control their spacing as well as the coordination number of the colloidal molecules. This study highlights the importance of tuning surface parameters and size asymmetry in controlling the structure and the active dynamics of colloidal molecules.