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1. Collective motion of driven semiflexible filaments tuned by soft repulsion and stiffness

   https://doi.org/10.1039/D0SM01036G  

   Moore, Jeffrey M.; Thompson, Tyler N.; Glaser, Matthew A.; Betterton, Meredith D. (October 2020, Soft Matter)

   null (Ed.)

   In active matter systems, self-propelled particles can self-organize to undergo collective motion, leading to persistent dynamical behavior out of equilibrium. In cells, cytoskeletal filaments and motor proteins form complex structures important for cell mechanics, motility, and division. Collective dynamics of cytoskeletal systems can be reconstituted using filament gliding experiments, in which cytoskeletal filaments are propelled by surface-bound motor proteins. These experiments have observed diverse dynamical states, including flocks, polar streams, swirling vortices, and single-filament spirals. Recent experiments with microtubules and kinesin motor proteins found that the collective behavior of gliding filaments can be tuned by altering the concentration of the crowding macromolecule methylcellulose in solution. Increasing the methylcellulose concentration reduced filament crossing, promoted alignment, and led to a transition from active, isotropically oriented filaments to locally aligned polar streams. This emergence of collective motion is typically explained as an increase in alignment interactions by Vicsek-type models of active polar particles. However, it is not yet understood how steric interactions and bending stiffness modify the collective behavior of active semiflexible filaments. Here we use simulations of driven filaments with tunable soft repulsion and rigidity in order to better understand how the interplay between filament flexibility and steric effects can lead to different active dynamic states. We find that increasing filament stiffness decreases the probability of filament alignment, yet increases collective motion and long-range order, in contrast to the assumptions of a Vicsek-type model. We identify swirling flocks, polar streams, buckling bands, and spirals, and describe the physics that govern transitions between these states. In addition to repulsion and driving, tuning filament stiffness can promote collective behavior, and controls the transition between active isotropic filaments, locally aligned flocks, and polar streams. 

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2. Period multiplication cascade at the order-by-disorder transition in uniaxial random field XY magnets

   https://doi.org/10.1038/s41467-020-18270-6  

   Basak, S.; Dahmen, K. A.; Carlson, E. W. (December 2020, Nature Communications)

   null (Ed.)

   Abstract Uniaxial random field disorder induces a spontaneous transverse magnetization in the XY model. Adding a rotating driving field, we find a critical point attached to the number of driving cycles needed to complete a limit cycle, the first discovery of this phenomenon in a magnetic system. Near the critical drive, time crystal behavior emerges, in which the period of the limit cycles becomes an integer n  > 1 multiple of the driving period. The period n can be engineered via specific disorder patterns. Because n generically increases with system size, the resulting period multiplication cascade is reminiscent of that occurring in amorphous solids subject to oscillatory shear near the onset of plastic deformation, and of the period bifurcation cascade near the onset of chaos in nonlinear systems, suggesting it is part of a larger class of phenomena in transitions of dynamical systems. Applications include magnets, electron nematics, and quantum gases. 

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3. Why Does the Deep Western Boundary Current “Leak” around Flemish Cap?

   https://doi.org/10.1175/JPO-D-19-0247.1  

   Solodoch, Aviv; McWilliams, James C.; Stewart, Andrew L.; Gula, Jonathan; Renault, Lionel (July 2020, Journal of Physical Oceanography)

   null (Ed.)

   Abstract The southward-flowing deep limb of the Atlantic meridional overturning circulation is composed of both the deep western boundary current (DWBC) and interior pathways. The latter are fed by “leakiness” from the DWBC in the Newfoundland Basin. However, the cause of this leakiness has not yet been explored mechanistically. Here the statistics and dynamics of the DWBC leakiness in the Newfoundland Basin are explored using two float datasets and a high-resolution numerical model. The float leakiness around Flemish Cap is found to be concentrated in several areas (hot spots) that are collocated with bathymetric curvature and steepening. Numerical particle advection experiments reveal that the Lagrangian mean velocity is offshore at these hot spots, while Lagrangian variability is minimal locally. Furthermore, model Eulerian mean streamlines separate from the DWBC to the interior at the leakiness hot spots. This suggests that the leakiness of Lagrangian particles is primarily accomplished by an Eulerian mean flow across isobaths, though eddies serve to transfer around 50% of the Lagrangian particles to the leakiness hot spots via chaotic advection, and rectified eddy transport accounts for around 50% of the offshore flow along the southern face of Flemish Cap. Analysis of the model’s energy and potential vorticity budgets suggests that the flow is baroclinically unstable after separation, but that the resulting eddies induce modest modifications of the mean potential vorticity along streamlines. These results suggest that mean uncompensated leakiness occurs mostly through inertial separation, for which a scaling analysis is presented. Implications for leakiness of other major boundary current systems are discussed. 

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4. New Insights into the Formation of Aggregates of Bidisperse Nano- and Microplastics in Water Based on the Analysis of In Situ Microscopy and Molecular Simulation

   https://doi.org/10.1021/acs.langmuir.4c01216  

   Hammond, Christian Bentum; Faeli_Qadikolae, Abolfazl; Aghaaminiha, Mohammadreza; Sharma, Sumit; Wu, Lei (July 2024, Langmuir)

   Walker, Gilbert (Ed.)

   Microplastics (MPs) and nanoplastics (NPs) in water pose a global threat to human health and the environment. To develop efficient removal strategies, it is crucial to understand how these particles behave as they aggregate. However, our knowledge of the process of aggregate formation from primary particles of different sizes is limited. In this study, we analyzed the growth kinetics and structures of aggregates formed by polystyrene MPs in mono- and bidisperse systems using in situ microscopy and image analysis. Our findings show that the scaling behavior of aggregate growth remains unaffected by the primary particle size distribution, but it does delay the onset of rapid aggregation. We also performed a structural analysis that reveals the power law dependence of aggregate fractal dimension (df) in both mono- and bidisperse systems, with mean df consistent with diffusion-limited cluster aggregation (DLCA) aggregates. Our results also suggest that the df of aggregates is insensitive to the shape anisotropy. We simulated molecular forces driving aggregation of polystyrene NPs of different sizes under high ionic strength conditions. These conditions represent salt concentration in ocean water and wastewater, where the DLVO theory does not apply. Our simulation results show that the aggregation tendency of the NPs increases with the ionic strength. The increase in the aggregation is caused by the depletion of clusters of ions from the NPs surface. 

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5. Reversible to irreversible transitions in periodic driven many-body systems and future directions for classical and quantum systems

   https://doi.org/10.1103/PhysRevResearch.5.021001  

   Reichhardt, C; Regev, Ido; Dahmen, K; Okuma, S; Reichhardt, C_J O (May 2023, Physical Review Research)

   Reversible to irreversible (R-IR) transitions arise in numerous periodically driven collectively interacting systems that, after a certain number of driving cycles, organize into a reversible state where the particle trajectories repeat during every or every few cycles. On the irreversible side of the transition, the motion is chaotic. R-IR transitions were first systematically studied for periodically sheared dilute colloids, and have now been found in a wide variety of both soft and hard matter periodically driven systems, including amorphous solids, crystals, vortices in type-II superconductors, and magnetic textures. It has been shown that in several of these systems, the transition to a reversible state is an absorbing phase transition with a critical divergence in the organization timescale at the transition. The same systems are capable of storing multiple memories and may exhibit return point memory. We give an overview of R-IR transitions including recent advances in the field and discuss how the general framework of R-IR transitions could be applied to a much broader class of nonequilibrium systems in which periodic driving occurs, including not only soft and hard condensed matter systems, but also astrophysics, biological systems, and social systems. In particular, some likely candidate systems are commensurate-incommensurate states, systems exhibiting hysteresis or avalanches, nonequilibrium pattern forming states, and other systems with absorbing phase transitions. Periodic driving could be applied to hard condensed matter systems to see if organization into reversible states occurs for metal-insulator transitions, semiconductors, electron glasses, electron nematics, cold atom systems, or Bose-Einstein condensates. R-IR transitions could also be examined in dynamical systems where synchronization or phase locking occurs. We also discuss the possibility of using complex periodic driving, such as changing drive directions or using multiple frequencies, to determine whether these systems can still organize to reversible states or retain complex multiple memories. Finally, we describe features of classical and quantum time crystals that could suggest the occurrence of R-IR transitions in these systems. 

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