arXiv subjects

C. Reichhardt

Publications and source records attributed to C. Reichhardt.

At least 199 records · Page 11Linked to original sources

arXiv2013

Dynamics and Separation of Circularly Moving Particles in Asymmetric Patterned Arrays

There are many examples of driven and active matter systems containing particles that exhibit circular motion with different chiralities, such as swimming bacteria near surfaces or certain types of self-driven colloidal particles. Circular motion of passive particles can also be induced with an external rotating drive. Here we examine particles that move in circles and interact with a periodic array of asymmetric L-shaped obstacles. We find a series of dynamical phases as a function of swimming radius, including regimes where the particle motion is rectified, producing a net dc motion. The direction of the rectification varies with the swimming radius, permitting the separation of particles with different swimming radii. Particles with the same swimming radius but different chirality can also move in different directions over the substrate and be separated. The rectification occurs for specific windows of swimming radii corresponding to periodic orbits in which the particles interact one or more times with the barriers per rotation cycle. The rectification effects are robust against the addition of thermal or diffusive effects, and are in some cases even enhanced by these effects.

cond-mat.stat-mech↗

arXiv2013

Dynamic Regimes for Driven Colloidal Particles on a Periodic Substrate at Commensurate and Incommensurate Fillings

We examine colloidal particles driven over a periodic muffin tin substrate using numerical simulations. In the absence of a driving force this system exhibits a rich variety of commensurate and incommensurate static phases in which topological defects can form domain walls, ordered stripes, superlattices, and disordered patchy regimes as a function of the filling fraction. When an external drive is applied, these different static phases generate distinct dynamical responses. At incommensurate fillings the flow generally occurs in the form of localized pulses or solitons correlated with the motion of the topological defect structures. We also find dynamic transitions between different types of moving states that are associated with changes in the velocity force curves, structural transitions in the topological defect arrangements, and modifications of the velocity distributions and particle trajectories. We find that the dynamic transitions between ordered and disordered flows exhibit hysteresis, while in strongly disordered regimes there is no hysteresis and the velocity force curves are smooth. When stripe patterns are present, transport can occur along the stripe direction rather than along the driving direction. Structural dynamic transitions can also occur within the pinned regimes when the applied drive causes distortions of the interstitially pinned particles while the particles trapped at pinning sites remain immobile.

cond-mat.soft↗

arXiv2013

Frustrated colloidal ordering and fully packed loops in arrays of optical traps

We propose that a system of colloidal particles interacting with a honeycomb array of optical traps that each contain three wells can be used to realize a fully packed loop model. One of the phases in this system can be mapped to Baxter's three-coloring problem, offering an easily accessible physical realization of this problem. As a function of temperature and interaction strength, we find a series of phases, including long range ordered loop or stripe states, stripes with sliding symmetries, random packed loop states, and disordered states in which the loops break apart. Our geometry could be constructed using ion trap arrays, BEC vortices in optical traps, or magnetic vortices in nanostructured superconductors.

cond-mat.soft↗

arXiv2012

Domain and Stripe Formation Between Hexagonal and Square Ordered Fillings of Colloidal Particles on Periodic Pinning Substrates

Using large scale numerical simulations, we examine the ordering of colloidal particles on square periodic two-dimensional muffin-tin substrates consisting of a flat surface with localized pinning sites. We show that when there are four particles per pinning site, the particles adopt a hexagonal ordering, while for five particles per pinning site, a square ordering appears. For fillings between four and five particles per pinning site, we identify a rich variety of distinct ordering regimes, including disordered grain boundaries, crystalline stripe structures, superlattice orderings, and disordered patchy arrangements. We characterize the different regimes using Voronoi analysis, energy dispersion, and ordering of the domains. We show that many of the boundary formation features we observe occur for a wide range of other fillings. Our results demonstrate that grain boundary tailoring can be achieved with muffin-tin periodic pinning substrates.

cond-mat.soft↗

arXiv2012

Colloidal Lattice Shearing and Rupturing with a Driven Line of Particles

We examine the dynamics of two-dimensional colloidal systems using numerical simulations of a system with a drive applied to a thin region in the middle of the sample to produce a local shear. For a monodisperse colloidal assembly, we find a well defined decoupling transition separating a regime of elastic motion from a plastic phase where the particles in the driven region break away or decouple from the particles in the bulk, producing a shear band. For a bidisperse assembly, we find that the onset of a bulk disordering transition coincides with the broadening of the shear band. We identify several distinct dynamical regimes that are correlated with features in the velocity-force curves. As a function of bidispersity, the decoupling force shows a nonmonotonic behavior associated with features in the noise fluctuations, power spectra, and bulk velocity profiles. When pinning is added in the bulk, we find that the shear band regions can become more localized, causing a decoupling of the driven particles from the bulk particles. For a system with thermal noise and no pinning, the shear band region becomes more extended and the average velocity of the driven particles drops at the thermal disordering transition of the bulk system.

cond-mat.soft↗

arXiv2012

Strongly Enhanced Vortex Pinning by Conformal Crystal Arrays

Conformal crystals are non-uniform structures created by a conformal transformation of regular two-dimensional lattices. We show that gradient-driven vortices interacting with a conformal pinning array exhibit substantially stronger pinning effects over a much larger range of field than found for random or periodic pinning arrangements. The pinning enhancement is partially due to matching of the critical flux gradient with the pinning gradient, but the preservation of the sixfold ordering in the conformally transformed hexagonal lattice plays a crucial role. Our results can be generalized to a wide class of gradient-driven interacting particle systems such as colloids on optical trap arrays.

cond-mat.supr-con↗

arXiv2012

Dynamic Phases, Pinning, and Pattern Formation for Driven Dislocation Assemblies

We show that driven dislocation assemblies exhibit a set of dynamical phases remarkably similar to those of driven systems with quenched disorder such as vortices in superconductors, magnetic domain walls, and charge density wave materials. These phases include jammed, fluctuating, and dynamically ordered states, and each produces distinct dislocation patterns as well as specific features in the noise fluctuations and transport properties. Our work suggests that many of the results established for systems with quenched disorder undergoing depinning transitions can be applied to dislocation systems, providing a new approach for understanding dislocation pattern formation and dynamics.

cond-mat.mtrl-sci↗

arXiv2012

Static and Dynamic Phases for Vortex Matter with Attractive Interactions

Exotic vortex states with long range attraction and short range repulsion have recently been proposed to arise in superconducting hybrid structures and multi-band superconductors. Using large scale simulations we examine the static and dynamic properties of such vortex states interacting with random and periodic pinning. In the absence of pinning this system does not form patterns but instead completely phase separates. When pinning is present there is a transition from inhomogeneous to homogeneous vortex configurations similar to a wetting phenomenon. Under an applied drive, a dynamical dewetting process can occur from a strongly pinned homogeneous state into pattern forming states. We show that a signature of the exotic vortex interactions under transport measurements is a robust double peak feature in the differential conductivity curves.

cond-mat.supr-con↗

arXiv2012

Jamming in Systems With Quenched Disorder

We numerically study the effect of adding quenched disorder in the form of randomly placed pinning sites on jamming transitions in systems that jam at a well defined point J in the clean limit. Quenched disorder decreases the jamming density and introduces a depinning threshold. The onset of a finite threshold coincides with point J at the lowest pinning densities, but for higher pinning densities there is always a finite threshold even well below jamming. We find that proximity to point J strongly affects the transport curves and noise fluctuations, and observe a change from plastic behavior below jamming, where the system is highly heterogeneous, to elastic depinning above jamming. Many of the general features we find are related to other systems containing quenched disorder, including the peak effect observed in vortex systems.

cond-mat.soft↗

arXiv2012

Statics and Dynamics of Yukawa Cluster Crystals on Ordered Substrates

We examine the statics and dynamics of particles with repulsive Yukawa interactions in the presence of a two-dimensional triangular substrate for fillings of up to twelve particles per potential minimum. We term the ordered states Yukawa cluster crystals and show that they are distinct from the colloidal molecular crystal states found at low fillings. As a function of substrate and interaction strength at fixed particle density we find a series of novel crystalline states that we characterize using the structure factor. For fillings greater than four, shell and ring structures form at each potential minimum and can exhibit sample-wide orientational order. A disordered state can appear between ordered states as the substrate strength varies. Under an external drive, the onsets of different orderings produce clear changes in the critical depinning force, including a peak effect phenomenon that has generally only previously been observed in systems with random substrates. We also find a rich variety of dynamic ordering transitions that can be observed via changes in the structure factor and features in the velocity-force curves. The dynamical states encompass a variety of moving structures including one-dimensional stripes, smectic ordering, polycrystalline states, triangular lattices, and symmetry locking states. Despite the complexity of the system, we identify several generic features of the dynamical phase transitions which we map out in a series of phase diagrams. Our results have implications for the structure and depinning of colloids on periodic substrates, vortices in superconductors and Bose-Einstein condensates, Wigner crystals, and dusty plasmas.

cond-mat.soft↗

arXiv2011

Multi-Step Ordering in Kagome and Square Artificial Spin Ice

We show that in colloidal models of artificial kagome and modified square ice systems, a variety of ordering and disordering regimes occur as a function of biasing field, temperature, and colloid-colloid interaction strength, including ordered monopole crystals, biased ice rule states, thermally induced ice rule ground states, biased triple states, and disordered states. We describe the lattice geometries and biasing field protocols that create the different states and explain the formation of the states in terms of sublattice switching thresholds. For a system prepared in a monopole lattice state, we show that a sequence of different orderings occurs for increasing temperature. Our results also explain several features observed in nanomagnetic artificial ice systems under an applied field.

cond-mat.mes-hall↗

arXiv2011

Bidirectional Sorting of Flocking Particles in the Presence of Asymmetric Barriers

We numerically demonstrate bidirectional sorting of flocking particles interacting with an array of asymmetric barriers. Each particle aligns with the average swimming direction of its neighbors according to the Vicsek model and experiences additional steric interactions as well as repulsion from the fixed barriers. We show that particles preferentially localize to one side of the barrier array over time, and that the direction of this rectification can be reversed by adjusting the particle-particle exclusion radius or the noise term in the equations of motion. These results provide a conceptual basis for isolation and sorting of single- and multi-cellular organisms which move collectively according to flocking-type interaction rules.

cond-mat.soft↗

arXiv2011

Active Matter on Asymmetric Substrates

For collections of particles in a thermal bath interacting with an asymmetric substrate, it is possible for a ratchet effect to occur where the particles undergo a net dc motion in response to an ac forcing. Ratchet effects have been demonstrated in a variety of systems including colloids as well as magnetic vortices in type-II superconductors. Here we examine the case of active matter or self-driven particles interacting with asymmetric substrates. Active matter systems include self-motile colloidal particles undergoing catalysis, swimming bacteria, artificial swimmers, crawling cells, and motor proteins. We show that a ratchet effect can arise in this type of system even in the absence of ac forcing. The directed motion occurs for certain particle-substrate interaction rules and its magnitude depends on the amount of time the particles spend swimming in one direction before turning and swimming in a new direction. For strictly Brownian particles there is no ratchet effect. If the particles reflect off the barriers or scatter from the barriers according to Snell's law there is no ratchet effect; however, if the particles can align with the barriers or move along the barriers, directed motion arises. We also find that under certain motion rules, particles accumulate along the walls of the container in agreement with experiment. We also examine pattern formation for synchronized particle motion. We discuss possible applications of this system for self-assembly, extracting work, and sorting as well as future directions such as considering collective interactions and flocking models.

cond-mat.stat-mech↗

arXiv2011

Hysteresis and Return Point Memory in Artificial Spin Ice Systems

We investigate hysteresis loops and return point memory for artificial square and kagome spin ice systems by cycling an applied bias force and comparing microscopic effective spin configurations throughout the hysteresis cycle. Return point memory loss is caused by motion of individual defects in kagome ice or of grain boundaries in square ice. In successive cycles, return point memory is recovered rapidly in kagome ice. Memory is recovered more gradually in square ice due to the extended nature of the grain boundaries. Increasing the amount of quenched disorder increases the defect density but also enhances the return point memory since the defects become trapped more easily.

cond-mat.stat-mech↗

arXiv2011

Dynamically Induced Locking and Unlocking Transitions in Driven Layered Systems with Quenched Disorder

Using numerical simulations, we examine a simple model of two or more coupled one-dimensional channels of driven particles with repulsive interactions in the presence of quenched disorder. We find that this model exhibits a remarkably rich variety of dynamical behavior as a function of the strength of the quenched disorder, coupling between channels, and external drive. For weaker disorder, the channels depin in a single step. For two channels we find dynamically induced decoupling transitions that result in coexisting pinned and moving phases as well as moving decoupled phases where particles in both channels move at different average velocities and slide past one another. Decoupling can also be induced by changing the relative strength of the disorder in neighboring channels. At higher drives, we observe a dynamical recoupling or locking transition into a state with no relative motion between the channels.This recoupling produces unusual velocity-force signatures, including negative differential conductivity. The depinning threshold shows distinct changes near the decoupling and coupling transitions and exhibits a peak effect phenomenon of the type that has been associated with transitions from elastic to plastic flow in other systems. We map several dynamic phase diagrams showing the coupling-decoupling transitions and the regions in which hysteresis occurs. We also examine the coexistence regime for channels with unequal amounts of quenched disorder. For multiple channels, multiple coupling and decoupling transitions can occur; however, many of the general features found for the two channel system are still present. Our results should be relevant to depinning in layered geometries in systems such as vortices in layered or nanostructured superconductors and Wigner or colloidal particles confined in nano-channels; they are also relevant to the general understating of plastic flow.

cond-mat.stat-mech↗

arXiv2011

Positive and Negative Drag, Dynamic Phases, and Commensurability in Coupled One-Dimensional Channels of Particles with Yukawa Interactions

We introduce a simple model consisting of two or three coupled one-dimensional channels of particles with Yukawa interactions. For the two channel system, when an external drive is applied only to the top or primary channel, we find a transition from locked flow where particles in both channels move together to decoupled flow where the particles in the secondary or undriven channel move at a slower velocity than the particles in the primary or driven channel. Pronounced commensurability effects in the decoupling transition occur when the ratio of the number of particles in the top and bottom channels is varied, and the coupling of the two channels is enhanced when this ratio is an integer or a rational fraction. Near the commensurate fillings, we find additional features in the velocity-force curves caused by the slipping of individual vacancies or incommensurations in the secondary channels. For three coupled channels, when only the top channel is driven we find a remarkably rich variety of distinct dynamic phases, including multiple decoupling and recoupling transitions. These transitions produce pronounced signatures in the velocity response of each channel. We also find regimes where a negative drag effect can be induced in one of the non-driven channels. The particles in this channel move in the opposite direction from the particles in the driven channel due to the mixing of the two different periodic frequencies produced by the discrete motion of the particles in the two other channels. In the two channel system, we also demonstrate a ratchet effect for the particles in the secondary channel when an asymmetric drive is applied to the primary channel. This ratchet effect is similar to that observed in superconducting vortex systems when there is a coupling between two different species of vortices.

cond-mat.soft↗

arXiv2011

Jamming in Granular Polymers

We examine the jamming transition in a two-dimensional granular polymer system using compressional simulations. The jamming density ϕ_c decreases with increasing length of the granular chain due to the formation of loop structures, in excellent agreement with recent experiments. The jamming density can be further reduced in mixtures of granular chains and granular rings, also as observed in experiment. We show that the nature of the jamming in granular polymer systems has pronounced differences from the jamming behavior observed for polydisperse two-dimensional disk systems at Point J. This result provides further evidence that there is more than one type of jamming transition.

cond-mat.soft↗

arXiv2011

Structural Transitions and Dynamical Regimes for Directional Locking of Particles Driven over Periodic Substrates

We numerically investigate collective ordering and disordering effects for vortices in type-II superconductors interacting with square and triangular substrate arrays under a dc drive that is slowly rotated with respect to the fixed substrate. A series of directional locking transitions occur when the particle motion locks to symmetry directions of the substrate, producing a series of steps in the velocity-force curves. The locking transitions coincide with structural transitions between triangular, square, smectic, or disordered particle arrangements, which can be identified using the structure factor. We show that the widths of the locking steps pass through local minima and maxima as a function of the ratio of the number of particles to the number of substrate minima. Unlike a static system, where matching effects occur for simple integer commensuration ratios, our system exhibits dynamical commensuration effects where an integer number of particle chains flow between one-dimensional lines of substrate minima. As the system enters and exits the locking steps, order-disorder transitions in the structure of the moving particle assembly occur. We identify two distinct symmetry locking regimes as a function of substrate strength which produce different locking step characteristics. For weak substrates, all the particles are in motion and a portion of the particles flow through the substrate minima, leading to structural transitions at certain driving angles. For strong substrates, some particles are permanently pinned while the remaining particles flow around them. At the crossover between these two regimes of substrate strength, the locking steps are destroyed due to the onset of chaotic plastic flow which produces pronounced changes in the transport characteristics. Similar effects occur for colloidal particles driven over square and triangular substrate arrays.

cond-mat.supr-con↗