Emerging Mesoscale Flows and Chaotic Advection in Dense Active Matter.

We study two models of overdamped self-propelled disks in two dimensions, with and without aligning interactions. Both models support active mesoscale flows, leading to chaotic advection and transport over large length scales in their homogeneous dense fluid states, away from dynamical arrest. They form streams and vortices reminiscent of multiscale flow patterns in turbulence. We show that the characteristics of these flows do not depend on the specific details of the active fluids, and result from the competition between crowding effects and persistent propulsions. This observation suggests that dense active suspensions of self-propelled particles present a type of "active turbulence" distinct from collective flows reported in other types of active systems.

Partial and complete wetting of droplets of active Brownian particles.

We study wetting droplets formed of active Brownian particles in contact with a repulsive potential barrier, in a wedge geometry. Our numerical results demonstrate a transition between partially wet and completely wet states, as a function of the barrier height, analogous to the corresponding surface phase transition in passive fluids. We analyse partially wet configurations characterised by a nonzero contact angle θ between the droplet surface and the barrier including the average density profile and its fluctuations. These findings are compared with two equilibrium systems: a Lennard-Jones fluid and a simple contour model for a liquid-vapour interface. We locate the wetting transition where cos(θ) = 1, and the neutral state where cos(θ) = 0. We discuss the implications of these results for possible definitions of surface tensions in active fluids.

Tricritical Behavior in Dynamical Phase Transitions.

We identify a new scenario for dynamical phase transitions associated with time-integrated observables occurring in diffusive systems described by the macroscopic fluctuation theory. It is characterized by the pairwise meeting of first- and second-order bias-induced phase transition curves at two tricritical points. We formulate a simple, general criterion for its appearance and derive an exact Landau theory for the tricritical behavior. The scenario is demonstrated in three examples: the simple symmetric exclusion process biased by an activity-related structural observable; the Katz-Lebowitz-Spohn lattice gas model biased by its current; and in an active lattice gas biased by its entropy production.

Intermittent relaxation and avalanches in extremely persistent active matter.

We use numerical simulations to study the dynamics of dense assemblies of self-propelled particles in the limit of extremely large, but finite, persistence times. In this limit, the system evolves intermittently between mechanical equilibria where active forces balance interparticle interactions. We develop an efficient numerical strategy allowing us to resolve the statistical properties of elastic and plastic relaxation events caused by activity-driven fluctuations. The system relaxes via a succession of scale-free elastic events and broadly distributed plastic events that both depend on the system size. Correlations between plastic events lead to emergent dynamic facilitation and heterogeneous relaxation dynamics. Our results show that dynamical behaviour in extremely persistent active systems is qualitatively similar to that of sheared amorphous solids, yet with some important differences.

Necking and failure of a particulate gel strand: signatures of yielding on different length scales.

"Sticky" spheres with a short-ranged attraction are a basic model of a wide range of materials from the atomic to the granular length scale. Among the complex phenomena exhibited by sticky spheres is the formation of far-from-equilibrium dynamically arrested networks which comprise "strands" of densely packed particles. The aging and failure of such gels under load is a remarkably challenging problem, given the simplicity of the model, as it involves multiple length- and time-scales, making a single approach ineffective. Here we tackle this challenge by addressing the failure of a single strand with a combination of methods. We study the mechanical response of a single strand of a model gel-former to deformation, both numerically and analytically. Under elongation, the strand breaks by a necking instability. We analyse this behaviour at three different length scales: a rheological continuum model of the whole strand; a microscopic analysis of the particle structure and dynamics; and the local stress tensor. Combining these different approaches gives a coherent picture of the necking and failure. The strand has an amorphous local structure and has large residual stresses from its initialisation. We find that neck formation is associated with increased plastic flow, a reduction in the stability of the local structure, and a reduction in the residual stresses; this indicates that the system loses its solid character and starts to behave more like a viscous fluid. These results will inform the development of more detailed models that incorporate the heterogeneous network structure of particulate gels.

Heterogeneous nucleation in the random field Ising model.

We investigate the nucleation dynamics of the three-dimensional random field Ising model under an external field. We use umbrella sampling to compute the free-energy cost of a critical nucleus and use forward flux sampling for the direct estimation of nucleation rates. For moderate to strong disorder, our results indicate that the size of the nucleating cluster is not a good reaction coordinate, contrary to the pure Ising model. We rectify this problem by introducing a coordinate that also accounts for the location of the nucleus. Using the free energy barrier to predict the nucleation rate, we find reasonable agreement, although deviations become stronger as disorder increases. We attribute this effect to cluster shape fluctuations. We also discuss finite-size effects on the nucleation rate.

Disordered Collective Motion in Dense Assemblies of Persistent Particles.

We explore the emergence of nonequilibrium collective motion in disordered nonthermal active matter when persistent motion and crowding effects compete, using simulations of a two-dimensional model of size polydisperse self-propelled particles. In stark contrast with monodisperse systems, we find that polydispersity stabilizes a homogeneous active liquid at arbitrary large persistence times, characterized by remarkable velocity correlations and irregular turbulent flows. For all persistence values, the active fluid undergoes a nonequilibrium glass transition at large density. This is accompanied by collective motion, whose nature evolves from near-equilibrium spatially heterogeneous dynamics at small persistence, to a qualitatively different intermittent dynamics when persistence is large. This latter regime involves a complex time evolution of the correlated displacement field.

Dimensionality reduction of local structure in glassy binary mixtures.

We consider unsupervised learning methods for characterizing the disordered microscopic structure of supercooled liquids and glasses. Specifically, we perform dimensionality reduction of smooth structural descriptors that describe radial and bond-orientational correlations and assess the ability of the method to grasp the essential structural features of glassy binary mixtures. In several cases, a few collective variables account for the bulk of the structural fluctuations within the first coordination shell and also display a clear connection with the fluctuations of particle mobility. Fine-grained descriptors that characterize the radial dependence of bond-orientational order better capture the structural fluctuations relevant for particle mobility but are also more difficult to parameterize and to interpret. We also find that principal component analysis of bond-orientational order parameters provides identical results to neural network autoencoders while having the advantage of being easily interpretable. Overall, our results indicate that glassy binary mixtures have a broad spectrum of structural features. In the temperature range we investigate, some mixtures display well-defined locally favored structures, which are reflected in bimodal distributions of the structural variables identified by dimensionality reduction.

Direct imaging of contacts and forces in colloidal gels.

Colloidal dispersions are prized as model systems to understand the basic properties of materials and are central to a wide range of industries from cosmetics to foods to agrichemicals. Among the key developments in using colloids to address challenges in condensed matter is to resolve the particle coordinates in 3D, allowing a level of analysis usually only possible in computer simulations. However, in amorphous materials, relating mechanical properties to microscopic structure remains problematic. This makes it rather hard to understand, for example, mechanical failure. Here, we address this challenge by studying the contacts and the forces between particles as well as their positions. To do so, we use a colloidal model system (an emulsion) in which the interparticle forces and local stress can be linked to the microscopic structure. We demonstrate the potential of our method to reveal insights into the failure mechanisms of soft amorphous solids by determining local stress in a colloidal gel. In particular, we identify "force chains" of load-bearing droplets and local stress anisotropy and investigate their connection with locally rigid packings of the droplets.

Multilevel simulation of hard-sphere mixtures.

We present a multilevel Monte Carlo simulation method for analyzing multi-scale physical systems via a hierarchy of coarse-grained representations, to obtain numerically exact results, at the most detailed level. We apply the method to a mixture of size-asymmetric hard spheres, in the grand canonical ensemble. A three-level version of the method is compared with a previously studied two-level version. The extra level interpolates between the full mixture and a coarse-grained description where only the large particles are present-this is achieved by restricting the small particles to regions close to the large ones. The three-level method improves the performance of the estimator, at fixed computational cost. We analyze the asymptotic variance of the estimator and discuss the mechanisms for the improved performance.

IL-10 signaling in dendritic cells is required for tolerance induction in a murine model of allergic airway inflammation.

Allergen specific tolerance induction efficiently ameliorates subsequent allergen induced inflammatory responses. The underlying regulatory mechanisms have been attributed mainly to interleukin (IL)-10 produced by diverse hematopoietic cells, while targets of IL-10 in allergen specific tolerance induction have not yet been well defined. Here, we investigate potential cellular targets of IL-10 in allergen specific tolerance induction using mice with a cell type specific inactivation of the IL-10 receptor gene. Allergic airway inflammation was effectively prevented by tolerance induction in mice with IL-10 receptor (IL-10R) deficiency in T or B cells. Similarly, IL-10R on monocytes/macrophages and/or neutrophils was not required for tolerance induction. In contrast, tolerance induction was impaired in mice that lack IL-10R on dendritic cells: those mice developed an allergic response characterized by a pronounced neutrophilic lung infiltration, which was not ameliorated by tolerogenic treatment. In conclusion, our results show that allergen specific tolerance can be effectively induced without a direct impact of IL-10 on cells of the adaptive immune system, and highlight dendritic cells, but not macrophages nor neutrophils, as the main target of IL-10 during tolerance induction.

Interleukin-10 signaling in regulatory T cells is required for suppression of Th17 cell-mediated inflammation.

Effector CD4+ T cell subsets, whose differentiation is facilitated by distinct cytokine cues, amplify the corresponding type of inflammatory response. Regulatory T (Treg) cells integrate environmental cues to suppress particular types of inflammation. In this regard, STAT3, a transcription factor essential for T helper 17 (Th17) cell differentiation, is necessary for Treg cell-mediated control of Th17 cell responses. Here, we showed that anti-inflammatory interleukin-10 (IL-10), and not proinflammatory IL-6 and IL-23 cytokine signaling, endowed Treg cells with the ability to suppress pathogenic Th17 cell responses. Ablation of the IL-10 receptor in Treg cells resulted in selective dysregulation of Th17 cell responses and colitis similar to that observed in mice harboring STAT3-deficient Treg cells. Thus, Treg cells limit Th17 cell inflammation by serving as principal amplifiers of negative regulatory circuits operating in immune effector cells.

Assessing the structural heterogeneity of supercooled liquids through community inference.

We present an information-theoretic approach inspired by distributional clustering to assess the structural heterogeneity of particulate systems. Our method identifies communities of particles that share a similar local structure by harvesting the information hidden in the spatial variation of two- or three-body static correlations. This corresponds to an unsupervised machine learning approach that infers communities solely from the particle positions and their species. We apply this method to three models of supercooled liquids and find that it detects subtle forms of local order, as demonstrated by a comparison with the statistics of Voronoi cells. Finally, we analyze the time-dependent correlation between structural communities and particle mobility and show that our method captures relevant information about glassy dynamics.

Performance and Return to Sport After Hip Arthroscopy in the National Basketball Association.

PURPOSE: To determine: (1) return to sport (RTS) rate in National Basketball Association (NBA) players following hip arthroscopy, (2) postoperative career length and games per season, (3) pre- and postoperative performance, and (4) postoperative performance compared with control players. METHODS: NBA athletes who underwent hip arthroscopy and matched controls were identified. RTS was defined as playing in at least 1 game after surgery. Player efficiency ratings were used for performance evaluation. Continuous variables of each group were compared using a 2-tailed paired samples Student t test for normally distributed data. χ2 was used to analyze categorical data. RTS was used as the primary outcome with statistical significance defined by a P value < .05. A Bonferroni correction was used to control for the remaining multiple comparisons with statistical significance defined by a P value ≤.008. RESULTS: Twenty-three players (24 hips) were analyzed (mean age 27.5 ± 3.1 years; mean experience in the NBA 5.8 ± 2.8 years at time of surgery). Small forwards (n = 8, 33.3%) represented the largest proportion of players that underwent hip arthroscopy. Twenty players (21 surgeries, 87.5%) were able to RTS in NBA at an average of 5.7 ± 2.6 months. The overall 1-year NBA career survival rate of players undergoing hip arthroscopy was 79.2%. Players in the control group (5.2 ± 3.5 years) had a similar career length as (P = .068) players who underwent surgery (4.4 ± 3.0 years). There was no significant (P = .045) decrease in games per season following surgery. There was no significant difference in performance postoperatively compared with preoperatively (P = .017) and compared with matched controls following surgery (P = .570). CONCLUSIONS: The RTS rate for NBA athletes after hip arthroscopy is high. There was no decrease in games played, career lengths, or performance following hip arthroscopy in NBA players versus preoperatively and matched controls. LEVEL OF EVIDENCE: Level III case-control study.

Unraveling the Large Deviation Statistics of Markovian Open Quantum Systems.

We analyze dynamical large deviations of quantum trajectories in Markovian open quantum systems in their full generality. We derive a quantum level-2.5 large deviation principle for these systems, which describes the joint fluctuations of time-averaged quantum jump rates and of the time-averaged quantum state for long times. Like its level-2.5 counterpart for classical continuous-time Markov chains (which it contains as a special case), this description is both explicit and complete, as the statistics of arbitrary time-extensive dynamical observables can be obtained by contraction from the explicit level-2.5 rate functional we derive. Our approach uses an unraveled representation of the quantum dynamics which allows these statistics to be obtained by analyzing a classical stochastic process in the space of pure states. For quantum reset processes we show that the unraveled dynamics is semi-Markovian and derive bounds on the asymptotic variance of the number of quantum jumps which generalize classical thermodynamic uncertainty relations. We finish by discussing how our level-2.5 approach can be used to study large deviations of nonlinear functions of the state, such as measures of entanglement.

Microscopic analysis of thermo-orientation in systems of off-centre Lennard-Jones particles.

When fluids of anisotropic molecules are placed in temperature gradients, the molecules may align themselves along the gradient: this is called thermo-orientation. We discuss the theory of this effect in a fluid of particles that interact by a spherically symmetric potential, where the particles' centres of mass do not coincide with their interaction centres. Starting from the equations of motion of the molecules, we show how a simple assumption of local equipartition of energy can be used to predict the thermo-orientation effect, recovering the result of Wirnsberger et al. [Phys. Rev. Lett. 120, 226001 (2018)]. Within this approach, we show that for particles with a single interaction centre, the thermal centre of the molecule must coincide with the interaction centre. The theory also explains the coupling between orientation and kinetic energy that is associated with this non-Boltzmann distribution. We discuss deviations from this local equipartition assumption, showing that these can occur in linear response to a temperature gradient. We also present numerical simulations showing significant deviations from the local equipartition predictions, which increase as the centre of mass of the molecule is displaced further from its interaction centre.

Correction of coarse-graining errors by a two-level method: Application to the Asakura-Oosawa model.

We present a method that exploits self-consistent simulation of coarse-grained and fine-grained models in order to analyze properties of physical systems. The method uses the coarse-grained model to obtain a first estimate of the quantity of interest, before computing a correction by analyzing properties of the fine system. We illustrate the method by applying it to the Asakura-Oosawa model of colloid-polymer mixtures. We show that the liquid-vapor critical point in that system is affected by three-body interactions which are neglected in the corresponding coarse-grained model. We analyze the size of this effect and the nature of the three-body interactions. We also analyze the accuracy of the method as a function of the associated computational effort.

Shear-wave elastography of the ulnar collateral ligament of the elbow in healthy volunteers: a pilot study.

OBJECTIVE: To estimate the intra-observer repeatability of shear wave elastography in the UCL of the elbow, and to compare shear wave velocities between dominant and non-dominant arms. MATERIALS AND METHODS: Twenty elbows in ten healthy volunteers were evaluated [five males, five females; mean age, 31.8 ± 10.3 years]. Shear wave velocity was measured on three separate days during the span of 1 week utilizing a linear 18-MHz transducer. Elastograms were obtained until ten ROIs were drawn, not drawing more than two ROIs on any elastogram. Elastograms were considered diagnostic if any portion of the UCL was colored in and free of boundary artifacts. Median velocity and interquartile range were recorded. A result was considered reliable if the IQR/median ratio of the ten measurements was < 0.3. RESULTS: IQR/median was < 0.3 in 88% of sessions, although in 28% of sessions fewer than 60% of elastograms were diagnostic. The ICC was 0.05 (95% CI; - 0.18-0.36; poor). Repeatability coefficient (95% limits of agreement) was 1.95 m/s (95% CI; 1.61-2.37 m/s). Mean velocity in dominant arms was 5.14 ± 0.53 m/s and 5.24 ± 0.39 m/s in non-dominant (p = 0.558). CONCLUSIONS: Mean shear wave velocity was similar between dominant and non-dominant arms. Although repeatability was poor as assessed by ICC, the repeatability coefficient may be a more useful indicator of clinical utility once shear wave velocities in diseased ligaments are explored. Future studies should therefore evaluate velocities in diseased ligaments and develop techniques to improve elastogram quality.

Performance and Return to Sport After Hip Arthroscopy for Femoroacetabular Impingement in Professional Athletes Differs Between Sports.

PURPOSE: To determine (1) return-to-sport rates for National Football League, Major League Baseball, National Basketball Association, and National Hockey League (NHL) athletes after hip arthroscopy for femoroacetabular impingement syndrome, (2) postoperative return-to-sport rate differences between sports, (3) differences in postoperative career length and games per season, (4) differences in preoperative and postoperative performance, and (5) postoperative performance compared with that of matched control players. METHODS: Professional athletes who underwent hip arthroscopy for femoroacetabular impingement syndrome were identified. Matched control players were identified by position, age, experience, and performance. Return to sport was defined as playing in at least 1 game after surgery. Continuous variables for each group were compared by using a 2-tailed paired-samples Student t test or χ2 test. A Bonferroni correction was used to control for multiple comparisons with statistical significance defined by a P value < .002. RESULTS: One hundred seventy-two players (86.4%) (mean age, 28.8 ± 5.2 years) were able to return to sport at an average of 7.1 ± 4.1 months. Athletes played 3.5 ± 2.4 years after surgery without significant differences between sports (P > .002). NHL players who underwent surgery played significantly fewer years (4.4 vs 3.3 years) (P < .001) and fewer games per season (4 fewer games) (P <.001) after surgery compared with control players. NHL players also had a significant decrease in performance after surgery compared with their performance before surgery (P < .001). In National Football League, Major League Baseball, and National Basketball Association athletes, no significant differences were found in games per season, career length, or preoperative performance compared with postoperative performance and performance of matched control players (P > .002). CONCLUSION: The RTS rate for professional athletes after surgery for femoroacetabular impingement syndrome is high. Only NHL athletes had significantly shorter careers and played significantly fewer games per season compared with matched control players, with no difference between sports. NHL athletes had significantly worse postoperative performance compared with preoperative performance, with all other sports demonstrating a career-related decline similar to that of matched control players.

Selective expression of the chemokine receptor XCR1 on cross-presenting dendritic cells determines cooperation with CD8+ T cells.

The expression of the chemokine receptor XCR1 and the function of its ligand XCL1 (otherwise referred to as ATAC, lymphotactin, or SCM-1) remained elusive to date. In the present report we demonstrated that XCR1 is exclusively expressed on murine CD8(+) dendritic cells (DCs) and showed that XCL1 is a potent and highly specific chemoattractant for this DC subset. CD8(+) T cells abundantly secreted XCL1 8-36 hr after antigen recognition on CD8(+) DCs in vivo, in a period in which stable T cell-DC interactions are known to occur. Functionally, XCL1 increased the pool of antigen-specific CD8(+) T cells and their capacity to secrete IFN-gamma. Absence of XCL1 impaired the development of cytotoxicity to antigens cross-presented by CD8(+) DCs. The XCL1-XCR1 axis thus emerges as an integral component in the development of efficient cytotoxic immunity in vivo.