Emergence of coherent structures and large-scale flows in motile suspensions

The emergence of coherent structures, large-scale flows and correlated dynamics in suspensions of motile particles such as swimming micro-organisms or artificial microswimmers is studied using direct particle simulations. A detailed model is proposed for a slender rod-like particle that propels itself in a viscous fluid by exerting a prescribed tangential stress on its surface, and a method is devised for the efficient calculation of hydrodynamic interactions in large-scale suspensions of such particles using slender-body theory and a smooth particle-mesh Ewald algorithm. Simulations are performed with periodic boundary conditions for various system sizes and suspension volume fractions, and demonstrate a transition to large-scale correlated motions in suspensions of rear-actuated swimmers, or Pushers, above a critical volume fraction or system size. This transition, which is not observed in suspensions of head-actuated swimmers, or Pullers, is seen most clearly in particle velocity and passive tracer statistics. These observations are consistent with predictions from our previous mean-field kinetic theory, one of which states that instabilities will arise in uniform isotropic suspensions of Pushers when the product of the linear system size with the suspension volume fraction exceeds a given threshold. We also find that the collective dynamics of Pushers result in giant number fluctuations, local alignment of swimmers and strongly mixing flows. Suspensions of Pullers, which evince no large-scale dynamics, nonetheless display interesting deviations from the random isotropic state.     

Experiments and theory of undulatory locomotion in a simple structured medium

Undulatory locomotion of micro-organisms through geometrically complex, fluidic environments is ubiquitous in nature and requires the organism to negotiate both hydrodynamic effects and geometrical constraints. To understand locomotion through such media, we experimentally investigate swimming of the nematode Caenorhabditis elegans through fluid-filled arrays of micro-pillars and conduct numerical simulations based on a mechanical model of the worm that incorporates hydrodynamic and contact interactions with the lattice. We show that the nematode''s path, speed and gait are significantly altered by the presence of the obstacles and depend strongly on lattice spacing. These changes and their dependence on lattice spacing are captured, both qualitatively and quantitatively, by our purely mechanical model. Using the model, we demonstrate that purely mechanical interactions between the swimmer and obstacles can produce complex trajectories, gait changes and velocity fluctuations, yielding some of the life-like dynamics exhibited by the real nematode. Our results show that mechanics, rather than biological sensing and behaviour, can explain some of the observed changes in the worm''s locomotory dynamics.     

Numerical study of microscopic particle arrangement of suspension flow in a narrow channel for the estimation of macroscopic rheological properties

In a narrow channel, the apparent relative viscosity of a suspension with finite-size particles is strongly dependent on its microscopic particle arrangement. Relative viscosity increases when suspended particles flow near the channel wall; thus, a suspension in a narrow channel does not always exhibit the same rheological properties even if the concentration is the same. In this study, we focus on the inertia and concentration of particles in a narrow channel and consider their effects on the microscopic particle arrangement and macroscopic suspension rheology. Two-dimensional pressure-driven suspension flow simulations were performed using a two-way coupling scheme, and normalized particle density distribution (PDD) were implemented to consider their particle arrangements. The results demonstrated that the velocity profiles for the particle suspension were changed by the Reynolds number and particle concentration because of the interactions between particles according to the power-law index. These changes affected the particle equilibrium positions in the channel, and the subsequent changes in solvent layer thickness caused changes in the macroscopic apparent viscosity. The behavior of microscopic particles played important roles in determining macroscopic rheology. Thus, we have confirmed that a normalized PDD can be used to estimate and assess the macroscopic rheology of a suspension.     

The phonon structure of layered ultrathin coherent structures

The new artificial materials characterized as layered ultrathin coherent structures (LUCS) have attracted a great deal of research interest. They are important not only for the novel physical properties they exhibit, but also for possible additional phenomena, such as excitonic superconductivity. In previous publications, we developed a tight-binding theory for the electronic structures of these materials. The present paper studies the phonon structures of LUCSs for a special lattice model, including nearest- and next-nearest-neighbor harmonic interactions.     

Design of untethered soft material micromachine for life-like locomotion

Living organisms composed of composite materials with complex structures support autonomous and intelligent behaviors, such as motility, perception and response to changes of the environment. By studying the biological structures and their environmental interactions, researchers are now using these natural systems as models for building soft material machines. In this review, we discuss materials and machine engineering principles to achieve life-like locomotion and functionalities in untethered soft micromachines. Through the various mechanochemical or physical mechanisms, we show how molecular motion can be collectively amplified into versatile macroscopic deformation by materials engineering across multiple length scales. In controlled ways, mobile micromachines are made to crawl, roll or jump and adaptive to various terrains, typically inspired by the terrestrial animals while propulsion of swimming micromachines are guided by aquatic organisms. Besides, out-of-equilibrium behaviors of living systems, such as cell cycling, have stimulated the design of autonomous movement. Furthermore, we review the recent efforts on robotic locomotion intelligence to achieve adaptive, functional locomotion and navigation in complex environment. We finally provide a critical perspective for the field of soft micromachines, and highlight the key challenges of different material systems that need to be overcome to realize practical use.     

三维地震动作用下适用于高耸结构的地震动强度指标

烟囱、水塔和广播电视塔等高耸混凝土结构属于高柔的悬臂结构。现有的地震动强度指标多集中于一维或二维地震动作用下的建筑结构，而高耸混凝土结构对竖向地震极其敏感，有必要开展考虑竖向地震动作用的地震动强度指标研究。该文针对混凝土烟囱和水塔结构，提出了一种同时考虑高阶振型和周期延长效应的复合型地震动强度指标，并分别在三维远场地震动和近断层脉冲地震动输入条件下，对该指标的充分性和有效性进行分析和检验。通过和既有的15种地震动强度指标的有效性对比，发现该文提出的指标在三维条件下对这类结构具有较强的适用性。因此，该指标可作为评价高耸混凝土结构抗震性能的一个合理的指标。     

Role of coherent structures in turbulent shear flows

A definition for the large-scale coherent structure is presented, and the nature and role of coherent structures in turbulent shear flows are examined. The equations governing the coherent motions and the experimental considerations as well as constraints in the investigations of coherent structures in wall-bounded and free turbulent shear flows are discussed. Results from a few of our recent and ongoing studies of coherent structures in excited and unexcited free turbulent shear flows are reviewed. These results show that coherent structures are dominant in transport in the early stages of their formation, but not in the self-preserving regions of turbulent shear flows.     

Meso-scale dislocations and friction of shape-complementary soft interfaces

The interface between two surfaces patterned with complementary shapes such as arrays of ridge–channel structures or pillars accommodates relative misorientation and lattice mismatch by spontaneous production of dislocation arrays. Here, we show that the relative sliding of such an interface is accomplished by dislocation glide on the interfacial plane. An exception is the singular case where the lattices are perfectly matched across the sample dimension, in which case sliding is accompanied by motion of edge-nucleated defects. These are meso-scale analogues of molecular sliding friction mechanisms between crystalline interfaces. The dislocations, in addition to the long-range elastic energy associated with their Burgers vectors, also cause significant out-of-plane dilation, which props open the interface locally. For this reason, the sliding friction is strongly pressure dependent; it also depends on the relative orientation of the patterns. Sliding friction can be strongly enhanced compared with a control, showing that shape-complementary interfaces can be engineered for strongly enhanced pressure- and orientation-dependent frictional properties in soft solids.     

Multiparticle entanglement and the Schrödinger cat state using ground-state coherences

In this paper we show how ground-state coherences and dispersive interactions of single photons with a collective system produces a variety of multiparticle entangled states and mesoscopic superpositions. Further single photons act as a carrier of information and can entangle macroscopic systems and can produce large phase shifts. Our work produces states as considered by Schrödinger in the cat-paradox, though in our case cat is replaced by the collective atomic system.     

Electrorheological fluids — structure and dynamics

Electrorheological fluids are colloidal suspensions that solidify under the influence of electric fields, due to the fact that electric fields induce interactions between particles arising from either the dielectric or the conductivity response of the particles. These interactions are principally dipolar at long distances. However, because of the image forces induced by constant potential electrodes, the long range dipolar repulsion is suppressed. It follows that the ground state of the system consists of a macroscopic phase separation into regions of high and low particle concentrations. The mechanism by which the suspension approaches this phase separation may be strongly dependent on thermal fluctuations. In hydrodynamical flows, these suspensions behave as shear-thinning “Bingham plastics”.     

Settling of bidispersed small solid particles in a dilute suspension containing gyrotactic micro-organisms

The motivation of this research is to investigate the feasibility of utilizing bioconvection for enhancing mixing in a suspension of small solid particles. This may be important in micro-fluidic applications relevant to biotechnology and medicine, such as analyses of DNA or drugs, screening of patients, and combinatorial synthesis. Traditionally, the mixing of fluids in micro-volumes has been limited to diffusion. Due to the microscopic size of the organisms involved in bioconvection, bioconvective flows are a prospective and novel alternative for micro-fluidic mixing. This paper considers a bidispersed suspension of small solid particles that have different densities and settling velocities in a fluid that contains motile gyrotactic micro-organisms. The particles are assumed to be sufficiently small so that their Brownian diffusion is not negligible. It is found that the number density distribution of solid particles of one type impacts that of particles of the other type as well as that of micro-organisms.     

钢筋混凝土柱偏心受压力学性能的细观数值研究

钢筋混凝土构件的宏观力学性能由其组分-钢筋和混凝土两部分的力学性能决定。结合混凝土细观结构形式,认为混凝土是由骨料颗粒、砂浆基质及界面过渡区组成的复合材料,假定钢筋与混凝土之间完好粘结,基于钢筋混凝土柱偏心受压试验,建立了钢筋混凝土柱偏心受压加载下力学特性及破坏行为研究的细观尺度力学分析模型。通过对混凝土方形和矩形试件进行受压力学特性模拟,采用反演法确定了界面的力学参数,进而模拟了钢筋混凝土柱偏心受压加载下的宏观力学性能。结果表明,相比于宏观尺度模型,细观数值分析模型能够充分体现材料的非均质性,能够较好的模拟试件的宏观力学性能,并且能够细致的描述裂缝发展及试件破坏过程,与试验结果吻合良好。该文建立的细观尺度分析模型与方法,为钢筋混凝土构件层次宏观力学非线性及其尺寸效应研究提供了理论支持。     

Fast,high-throughput measurement of collective behaviour in a bacterial population

Swimming bacteria explore their environment by performing a random walk, which is biased in response to, for example, chemical stimuli, resulting in a collective drift of bacterial populations towards ‘a better life’. This phenomenon, called chemotaxis, is one of the best known forms of collective behaviour in bacteria, crucial for bacterial survival and virulence. Both single-cell and macroscopic assays have investigated bacterial behaviours. However, theories that relate the two scales have previously been difficult to test directly. We present an image analysis method, inspired by light scattering, which measures the average collective motion of thousands of bacteria simultaneously. Using this method, a time-varying collective drift as small as 50 nm s⁻¹ can be measured. The method, validated using simulations, was applied to chemotactic Escherichia coli bacteria in linear gradients of the attractant α-methylaspartate. This enabled us to test a coarse-grained minimal model of chemotaxis. Our results clearly map the onset of receptor methylation, and the transition from linear to logarithmic sensing in the bacterial response to an external chemoeffector. Our method is broadly applicable to problems involving the measurement of collective drift with high time resolution, such as cell migration and fluid flows measurements, and enables fast screening of tactic behaviours.     

Understanding how bacterial collectives organize on surfaces by tracking surfactant flow

Swarming is a collective bacterial behavior in which a dense population of bacterial cells moves over a porous surface, resulting in the expansion of the population. This collective behavior can guide bacteria away from potential stressors such as antibiotics and bacterial viruses. However, the mechanisms responsible for the organization of swarms are not understood. Here, we briefly review models that are based on bacterial sensing and fluid mechanics that are proposed to guide swarming in the pathogenic bacterium Pseudomonas aeruginosa. To provide further insight into the role of fluid mechanics in P. aeruginosa swarms, we track the movement of tendrils and the flow of surfactant using a novel technique that we have developed, Imaging of Reflected Illuminated Structures (IRIS). Our measurements show that tendrils and surfactants form distinct layers that grow in lockstep with each other. The results raise new questions about existing swarming models and the possibility that the flow of surfactants impacts tendril development. These findings emphasize that swarm organization involves an interplay between biological processes and fluid mechanics.     

Carbon nanotube filters

Over the past decade of nanotube research, a variety of organized nanotube architectures have been fabricated using chemical vapour deposition. The idea of using nanotube structures in separation technology has been proposed, but building macroscopic structures that have controlled geometric shapes, density and dimensions for specific applications still remains a challenge. Here we report the fabrication of freestanding monolithic uniform macroscopic hollow cylinders having radially aligned carbon nanotube walls, with diameters and lengths up to several centimetres. These cylindrical membranes are used as filters to demonstrate their utility in two important settings: the elimination of multiple components of heavy hydrocarbons from petroleum-a crucial step in post-distillation of crude oil-with a single-step filtering process, and the filtration of bacterial contaminants such as Escherichia coli or the nanometre-sized poliovirus ( approximately 25 nm) from water. These macro filters can be cleaned for repeated filtration through ultrasonication and autoclaving. The exceptional thermal and mechanical stability of nanotubes, and the high surface area, ease and cost-effective fabrication of the nanotube membranes may allow them to compete with ceramic- and polymer-based separation membranes used commercially.     

Periodic Squeezing in the Tavis-Cummings Model

Abstract

By utilizing our previous operator solution [17], we have investigated the squeezing in the radiation field of the Tavis-Cummings model (collective N ? 1 two-level atoms interacting with a resonant single cavity quantized mode). With field and atoms initially in coherent field state strong or weak and atomic coherent state (of few excited atoms), periodic time-dependent squeezing in the field and the macroscopic polarization is expressed in terms of Jacobian elliptic functions of the first kind. The statistical investigations are carried out for the quasiprobability distribution functions (Wigner function and Q function). The distribution function of the field quadrature has a variance less (greater) than that for a coherent state if this quadrature is squeezed (unsqueezed).     

Meso-structure evolution in a 2D granular material during biaxial loading

Multi-scale approaches of constitutive modeling require an intermediate scale linking the variables in macroscopic scale (incremental stress and strain) to variables in microscopic scale (contact force and contact displacement). In this paper, we introduce a mesoscopic scale, in which the granular material is tessellated into small loops by contact network. Then numerical biaxial tests from different initial states by DEM modeling, is performed to investigate how the meso-structure (mesoscopic loops) evolves along the drained biaxial loading path. Results suggest that the procedure of the biaxial test is accompanied with the exchange between small, dense structures and big, loose structures. The macroscopic dilatancy primarily originates from this exchange. In dense and intermediate specimens the meso-structure evolution is found not to be consistent with the evolution of the macroscopic volumetric strain during contractancy phases. This inconsistency has led to interpret the elastic and the plastic parts of the volumetric strain from a meso-scale viewpoint. It is shown that the initial contractancy in dense and intermediate specimens is largely an elastic process, which is highly dependent on elastic parameters of the material.     

逆压梯度下近壁湍流的喷射和扫掠

采用Fourier谱展开和紧致有限差分格式，选用两组共振三波为相干结构的初值，计算了其在零压和逆压梯度作用下的演化。对演化后期流场的2，4象限的运动进行了详细的分析。结果发现，在逆压梯度下，扫掠对雷诺应力的贡献要强于喷射。无论是在零压梯度还是逆压梯度下，uv和u2在法向的输运主要是靠Q2和Q4这两种运动来完成的。零压梯度下喷射部分对输运的贡献大于扫掠的部分。而在逆压梯度下喷射部分对输运的贡献明显减少，扫掠的作用要强于喷射。     

Triple-scale failure estimation for a composite-reinforced structure based on integrated modeling approaches. Part 2: Meso- and macroscopic scale analysis

In this article, the meso- and macroscopic failure features are discussed considering the identical composite component as in the foregoing article. In the meso-scale failure analysis, the risk of plastic instability of the composite tube was estimated considering the shakedown boundary as a failure criterion. The meso-scale stresses of the composite tube were computed using micromechanical homogenization and compared with the shakedown boundary of the composite obtained from the direct shakedown analysis. The stress states were close to the shakedown boundary indicating no critical danger of plastic failure. In the macro-scale failure analysis, the mechanical influence of the local composite integration was investigated with regard to the brittle failure risk of the neutron-embrittled component. To this end, a probabilistic failure analysis code was applied which was based on the fracture mechanics and the weakest-link failure theory. Various fracture criteria were considered. It was found that the failure risk of the tungsten block was strongly reduced by the composite reinforcement of the tube due to the intensification of compressive stress fields.     

Fish and robots swimming together: attraction towards the robot demands biomimetic locomotion

The integration of biomimetic robots in a fish school may enable a better understanding of collective behaviour, offering a new experimental method to test group feedback in response to behavioural modulations of its ‘engineered’ member. Here, we analyse a robotic fish and individual golden shiners (Notemigonus crysoleucas) swimming together in a water tunnel at different flow velocities. We determine the positional preference of fish with respect to the robot, and we study the flow structure using a digital particle image velocimetry system. We find that biomimetic locomotion is a determinant of fish preference as fish are more attracted towards the robot when its tail is beating rather than when it is statically immersed in the water as a ‘dummy’. At specific conditions, the fish hold station behind the robot, which may be due to the hydrodynamic advantage obtained by swimming in the robot''s wake. This work makes a compelling case for the need of biomimetic locomotion in promoting robot–animal interactions and it strengthens the hypothesis that biomimetic robots can be used to study and modulate collective animal behaviour.