Defect turbulence in a dense suspension of polar, active swimmers.

We study the effects of inertia in dense suspensions of polar swimmers. The hydrodynamic velocity field and the polar order parameter field describe the dynamics of the suspension. We show that a dimensionless parameter R (ratio of the swimmer self-advection speed to the active stress invasion speed [Phys. Rev. X 11, 031063 (2021)2160-330810.1103/PhysRevX.11.031063]) controls the stability of an ordered swimmer suspension. For R smaller than a threshold R_{1}, perturbations grow at a rate proportional to their wave number q. Beyond R_{1} we show that the growth rate is O(q^{2}) until a second threshold R=R_{2} is reached. The suspension is stable for R>R_{2}. We perform direct numerical simulations to characterize the steady-state properties and observe defect turbulence for R

Erratum: Oriented Active Solids [Phys. Rev. Lett. 123, 238001 (2019)].

This corrects the article DOI: 10.1103/PhysRevLett.123.238001.

Two-temperature activity induces liquid-crystal phases inaccessible in equilibrium.

In equilibrium hard-rod fluids, and in effective hard-rod descriptions of anisotropic soft-particle systems, the transition from the isotropic (I) phase to the nematic phase (N) is observed above the rod aspect ratio L/D=3.70 as predicted by Onsager. We examine the fate of this criterion in a molecular dynamics study of a system of soft repulsive spherocylinders rendered active by coupling half the particles to a heat bath at a higher temperature than that imposed on the other half. We show that the system phase-separates and self-organizes into various liquid-crystalline phases that are not observed in equilibrium for the respective aspect ratios. In particular, we find a nematic phase for L/D=3 and a smectic phase for L/D=2 above a critical activity.

Therapeutic and Safety Outcomes of Intravenous Ketamine for Treatment-refractory Depression in a Veteran Population: A Case Series.

INTRODUCTION: Major depressive disorder is a serious, recurrent, and disabling psychiatric illness. Despite many proven treatments with multiple medications or therapies, approximately 30% of patients fail to achieve remission and are considered to have treatment-refractory depression (TRD). Recently, there has been a growing interest in the use of intravenous (IV) ketamine for the treatment of TRD. There is limited yet increasing evidence to support the use of ketamine, a glutamate receptor antagonist, in the management of depression; however, the lack of data regarding the safety and tolerability of therapy has limited its clinical use. By analyzing a cohort of veterans with TRD and comorbid psychiatric conditions treated with IV ketamine infusions for a 24-month study period, we aim to provide critical information about ketamine's clinical effectiveness and safety. MATERIALS AND METHODS: Based on a retrospective chart review, we identified eight veterans with TRD receiving treatment with repeated-dose IV ketamine from 2018 to 2020. The magnitude of clinical response was based on the Beck Depression Inventory self-report scale and the Patient Health Questionnaire-9, both measured at the initial patient consultation and before the beginning of each ketamine infusion treatment. Safety analysis included changes to pre- and post-ketamine infusion on vital signs, effects on alertness and sedation, and potential psychosis-like effects. For all outcomes, we estimated a linear mixed-effects model that allowed heterogeneous residual variances for each veteran. The effect of continuous predictor variables was estimated using restricted cubic splines with knot points specified at the 5th, 35th, 65th, and 95th percentiles. All the analyses were conducted using SAS v.9.4, with P < .05 indicating the statistical significance. This study had institutional review board approval: 1220. RESULTS: During the study period, the median number of ketamine infusions was 15 across a median of 164 days of treatment follow-up with a median time between ketamine infusions of 4 days. For both Beck Depression Inventory and Patient Health Questionnaire-9 scores, there was a statistically significant reduction across infusions (both P < .001), but the strongest reduction occurred before day 40. The change was statistically significant for decreased heart rate (P = .019) but not for systolic blood pressure (P = .612), diastolic blood pressure (P = .942), respiratory rate (P = .822), oxygen saturation (P = .070), and temperature (P = .943). Side effects were reported in six patients (75%); however, the only side effect reported was excessive sedation or dizziness immediately after infusion. CONCLUSIONS: In this study, repeated-dose IV ketamine infusions over a 24-month study period resulted in a significant reduction in depression scores in a group of veterans with TRD. The rapid onset of significant response, absence of psychosis-like effects or dissociative symptoms despite psychiatric comorbidities, and minimal effects on vital signs support the clinical efficacy and safety of this exciting new treatment option for patients with TRD. Limitations include a 2-year study period, lack of information on long-term effects, and the retrospective nature of the study. Prospective studies of longer duration are needed to assess the long-term efficacy and safety of IV ketamine for TRD.

Posttraumatic Stress Disorder in a Physician Assistant After Working in an ICU During COVID-19.

INTRODUCTION: We present a case report of a physician assistant who experiences posttraumatic stress disorder (PTSD) from providing care to patients affected with COVID-19. We believe this case is important as it will reveal the unfortunate impact COVID-19 has on the mental health of health care professionals. CASE PRESENTATION: A 51-year-old White woman presented to our clinic with a 1-year history of panic attacks, mood swings, difficulty sleeping, nightmares, social withdrawal, guilt, and depression. DISCUSSION: Cross-sectional, survey-based studies have highlighted PTSD rates in health care workers during the pandemic, but these studies have not explored how exactly PTSD presents on the individual level. CONCLUSIONS: This case presents a compelling reflection on what could be a larger trend of increasing mental health issues as a direct result of the COVID-19 pandemic and emphasizes the need for better mental health support and infrastructure to be in place for the well-being of the health care workers in this country.

Active nonreciprocal attraction between motile particles in an elastic medium.

We show from experiments and simulations on vibration-activated granular matter that self-propelled polar rods in an elastic medium on a substrate turn and move towards each other. We account for this effective attraction through a coarse-grained theory of a motile particle as a moving point-force density that creates elastic strains in the medium that reorient other particles. Our measurements confirm qualitatively the predicted features of the distortions created by the rods, including the |x|^{-1/2} tail of the trailing displacement field and nonreciprocal sensing and pursuit. A discrepancy between the magnitudes of displacements along and transverse to the direction of motion remains. Our theory should be of relevance to the interaction of motile cells in the extracellular matrix or in a supported layer of gel or tissue.

Impact of delirium on mortality in patients hospitalized for heart failure.

OBJECTIVE: Heart Failure (HF) is one of the leading causes of hospitalization in the United States accounting for ≈800,000 hospital discharges and $11 billion in annual costs. Delirium occurs in approximately 30% of elderly hospitalized patients and its incidence is significantly higher among those admitted to the critical care units. Despite this, there has been limited exploration of the clinical and economic impact of delirium in patients hospitalized with acute HF. We hypothesized that delirium in HF is associated with excess mortality and hospital costs. METHODS: We queried the 2001-2014 Nationwide Inpatient Sample to identify hospitalizations that included a primary discharge diagnosis of HF (ICD-9-CM: 428.xx) and stratified them by presence or absence of delirium (ICD-9-CM: 239.0, 290.41, 293.0, 293.1, 348.31). Differences in in-hospital mortality, length of stay (LOS), and hospital costs were assessed using propensity-score matched cohorts. RESULTS: Major predictors of delirium included advanced age, Caucasian race, underlying dementia or psychiatric diagnoses, higher Elixhauser Comorbidity Index, renal failure, cardiogenic shock, and coronary artery bypass surgery. In the propensity-score matched analysis of 76,411 hospitalization with delirium compared to 76,612 without delirium, in-hospital mortality (odds ratio: 1.67, 95% CI: 1.51-1.77), LOS (rate ratio [RR]: 1.47, 95% CI: 1.45-1.51), and hospital costs (RR: 1.44, 95% CI: 1.41-1.48) were all statistically higher in the presence of delirium (all p < 0.001). CONCLUSION: In patients hospitalized with HF, delirium is an independent predictor of increased in-hospital mortality, longer LOS, and excess hospital costs despite adjustment for baseline characteristics.

Heating leads to liquid-crystal and crystalline order in a two-temperature active fluid of rods.

We report phase separation and liquid-crystal ordering induced by scalar activity in a system of soft repulsive spherocylinders (SRSs) of shape anisotropy L/D=5 using molecular dynamics (MD) simulations. Activity is introduced by increasing the temperature of half of the SRSs (labeled hot) while maintaining the temperature of the other half constant at a lower value (labeled cold). The difference between the two temperatures scaled by the lower temperature provides a measure of the activity. Starting from different equilibrium initial phases, we find that activity leads to segregation of the hot and cold particles. Activity also drives the cold particles through a phase transition to a more ordered state and the hot particles to a state of less order compared to the initial equilibrium state. The cold components of a homogeneous isotropic structure acquire nematic and, at higher activity, crystalline order. Similarly, the cold zone of a nematic initial state undergoes smectic and crystal ordering above a critical value of activity while the hot component turns isotropic. We find that the hot particles occupy a larger volume and exert an extra kinetic pressure, confining, compressing, and provoking an ordering transition of the cold-particle domains.

Strong confinement of active microalgae leads to inversion of vortex flow and enhanced mixing.

Microorganisms swimming through viscous fluids imprint their propulsion mechanisms in the flow fields they generate. Extreme confinement of these swimmers between rigid boundaries often arises in natural and technological contexts, yet measurements of their mechanics in this regime are absent. Here, we show that strongly confining the microalga Chlamydomonas between two parallel plates not only inhibits its motility through contact friction with the walls but also leads, for purely mechanical reasons, to inversion of the surrounding vortex flows. Insights from the experiment lead to a simplified theoretical description of flow fields based on a quasi-2D Brinkman approximation to the Stokes equation rather than the usual method of images. We argue that this vortex flow inversion provides the advantage of enhanced fluid mixing despite higher friction. Overall, our results offer a comprehensive framework for analyzing the collective flows of strongly confined swimmers.

Layered Chiral Active Matter: Beyond Odd Elasticity.

In equilibrium liquid crystals, chirality leads to a variety of spectacular three-dimensional structures, but chiral and achiral phases with the same broken continuous symmetries have identical long-time, large-scale dynamics. In this Letter, starting from active model H^{*}, the general hydrodynamics of a pseudoscalar in a momentum-conserving fluid, we demonstrate that chirality qualitatively modifies the dynamics of layered liquid crystals in active systems in both two and three dimensions due to an active "odder" elasticity. In three dimensions, we demonstrate that the hydrodynamics of active cholesterics differs fundamentally from smectic-A liquid crystals, unlike their equilibrium counterpart. This distinction can be used to engineer a columnar array of vortices, with an antiferromagnetic vorticity alignment, that can be switched on and off by external strain. A two-dimensional chiral layered state-an array of lines on an incompressible, freestanding film of chiral active fluid with a preferred normal direction-is generically unstable. However, this instability can be tuned in easily realizable experimental settings when the film is either on a substrate or in an ambient fluid.

Adjunctive Minocycline for Treatment of Posttraumatic Stress Disorder.

INTRODUCTION: Posttraumatic stress disorder (PTSD) is a chronic, debilitating anxiety disorder. While there is evidence that antibiotics such as minocycline may help to improve symptoms in some psychiatric disorders, no human studies have evaluated their potential as a treatment for PTSD. METHODS: We present results from 4 men aged 33 to 59 years who completed a 12-week pilot, prospective, nonrandomized, open-label clinical trial of adjunctive minocycline for veterans diagnosed with PTSD. RESULTS: All 4 patients showed reduction in PTSD symptoms at the end of the 12-week study, and 3 patients showed reduction in depression symptoms. Observed changes in inflammatory biomarkers are discussed. DISCUSSION: Previous studies have reported increased inflammation in PTSD, though evidence of a potential therapeutic effect of minocycline for PTSD has not been reported previously in humans. CONCLUSION: These findings suggest that antibiotics like minocycline may help to reduce symptoms of PTSD, though further investigation is needed to confirm these findings.

Nonmutual torques and the unimportance of motility for long-range order in two-dimensional flocks.

As the constituent particles of a flock are polar and in a driven state, their interactions must, in general, be fore-aft asymmetric and nonreciprocal. Within a model that explicitly retains the classical spin angular momentum field of the particles we show that the resulting asymmetric contribution to interparticle torques, if large enough, leads to a buckling instability of the flock. More precisely, this asymmetry also yields a natural mechanism for a difference between the speed of advection of polarization information along the flock and the speed of the flock itself, concretely establishing that the absence of detailed balance, and not merely the breaking of Galilean invariance, is crucial for this distinction. To highlight this we construct a model of asymmetrically interacting spins fixed to lattice points and demonstrate that the speed of advection of polarization remains nonzero. We delineate the conditions on parameters and wave number for the existence of the buckling instability. Our theory should be consequential for interpreting the behavior of real animal groups as well as experimental studies of artificial flocks composed of polar motile rods on substrates.

Phases and excitations of active rod-bead mixtures: simulations and experiments.

We present a large-scale numerical study, supplemented by experimental observations, on a quasi-two-dimensional active system of polar rods and spherical beads confined between two horizontal plates and energised by vertical vibration. For a low rod concentration Φr, our observations are consistent with a direct phase transition, as bead concentration Φb is increased, from the isotropic phase to a homogeneous flock. For Φr above a threshold value, an ordered band dense in both rods and beads occurs between the disordered phase and the homogeneous flock, in both experiments and simulations. Within the size ranges accessible, we observe only a single band, whose width increases with Φr. Deep in the ordered state, we observe broken-symmetry "sound" modes and giant number fluctuations. The direction-dependent sound speeds and the scaling of fluctuations are consistent with the predictions of field theories of flocking; sound damping rates show departures from such theories, but the range of wavenumbers explored is modest. At very high densities, we see phase separation into rod-rich and bead-rich regions, both of which move coherently.

Swimmer Suspensions on Substrates: Anomalous Stability and Long-Range Order.

We present a comprehensive theory of the dynamics and fluctuations of a two-dimensional suspension of polar active particles in an incompressible fluid confined to a substrate. We show that, depending on the sign of a single parameter, a state with polar orientational order is anomalously stable (or anomalously unstable), with a nonzero relaxation (or growth) rate for angular fluctuations, not parallel to the ordering direction, at zero wave number. This screening of the broken-symmetry mode in the stable state does lead to conventional rather than giant number fluctuations as argued by Bricard et al., Nature 503, 95 (2013), but their bend instability in a splay-stable flock does not exist and the polar phase has long-range order in two dimensions. Our theory also describes confined three-dimensional thin-film suspensions of active polar particles as well as dense compressible active polar rods, and predicts a flocking transition without a banding instability.

Oriented Active Solids.

We present a complete analysis of the linearized dynamics of active solids with uniaxial orientational order, taking into account a hitherto overlooked consequence of rotation invariance. Our predictions include a purely active response of two-dimensional orientationally ordered solids to shear, the possibility of stable active solids with quasi-long-range order in two dimensions and long-range order in three dimensions, generic instability of the solid for one sign of active forcing, and the instability of the uniaxially ordered phase in momentum-conserved systems for large active forcing irrespective of its sign.

Kepler Orbits in Pairs of Disks Settling in a Viscous Fluid.

We show experimentally that a pair of disks settling at negligible Reynolds number (∼10^{-4}) displays two classes of bound periodic orbits, each with transitions to scattering states. We account for these dynamics, at leading far-field order, through an effective Hamiltonian in which gravitational driving endows orientation with the properties of momentum. This treatment is successfully compared against the measured properties of orbits and critical parameters of transitions between types of orbits. We demonstrate a precise correspondence with the Kepler problem of planetary motion for a wide range of initial conditions, find and account for a family of orbits with no Keplerian analog, and highlight the role of orientation as momentum in the many-disk problem.

Inferring critical thresholds of ecosystem transitions from spatial data.

Ecosystems can undergo abrupt transitions between alternative stable states when the driver crosses a critical threshold. Dynamical systems theory shows that when ecosystems approach the point of loss of stability associated with these transitions, they take a long time to recover from perturbations, a phenomenon known as critical slowing down. This generic feature of dynamical systems can offer early warning signals of abrupt transitions. However, these signals are qualitative and cannot quantify the thresholds of drivers at which transition may occur. Here, we propose a method to estimate critical thresholds from spatial data. We show that two spatial metrics, spatial variance and autocorrelation of ecosystem state variable, computed along driver gradients can be used to estimate critical thresholds. First, we investigate cellular-automaton models of ecosystem dynamics that show a transition from a high-density state to a bare state. Our models show that critical thresholds can be estimated as the ecosystem state and the driver values at which spatial variance and spatial autocorrelation of the ecosystem state are maximum. Next, to demonstrate the application of the method, we choose remotely sensed vegetation data (Enhanced Vegetation Index, EVI) from regions in central Africa and northeast Australia that exhibit alternative states in woody cover. We draw transects (8 × 90 km) that span alternative stable states along rainfall gradients. Our analyses of spatial variance and autocorrelation of EVI along transects yield estimates of critical thresholds. These estimates match reasonably well with those obtained by an independent method that uses large-scale (250 × 200 km) spatial data sets. Given the generality of the principles that underlie our method, our method can be applied to a variety of ecosystems that exhibit alternative stable states.

Stress fluctuations in transient active networks.

Inspired by experiments on dynamic extensile gels of biofilaments and motors, we propose a model of a network of linear springs with kinetics consisting of growth at a prescribed rate, death after a lifetime drawn from a distribution, and birth at a randomly chosen node. The model captures features such as the build-up of self-stress, that are not easily incorporated into hydrodynamic theories. We study the model numerically and show that our observations can largely be understood through a stochastic effective-medium model. The resulting dynamically extending force-dipole network displays many features of yielded plastic solids, and offers a way to incorporate strongly non-affine effects into theories of active solids. A rather distinctive form for the stress distribution, and a Herschel-Bulkley dependence of stress on activity, are our major predictions.

Trapping and sorting active particles: Motility-induced condensation and smectic defects.

We present an experimental realization of the collective trapping phase transition [Kaiser et al., Phys. Rev. Lett. 108, 268307 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.268307], using motile polar granular rods in the presence of a V-shaped obstacle. We offer a theory of this transition based on the interplay of motility-induced condensation and liquid-crystalline ordering and show that trapping occurs when persistent influx overcomes the collective expulsion of smectic defect structures. In agreement with the theory, our experiments find that a trap fills to the brim when the trap angle θ is below a threshold θ_{c}, while all particles escape for θ>θ_{c}. Our simulations support a further prediction, that θ_{c} goes down with increasing rotational noise. We exploit the sensitivity of trapping to the persistence of directed motion to sort particles based on the statistical properties of their activity.

Defect Unbinding in Active Nematics.

We formulate the statistical dynamics of topological defects in the active nematic phase, formed in two dimensions by a collection of self-driven particles on a substrate. An important consequence of the nonequilibrium drive is the spontaneous motility of strength +1/2 disclinations. Starting from the hydrodynamic equations of active nematics, we derive an interacting particle description of defects that includes active torques. We show that activity, within perturbation theory, lowers the defect-unbinding transition temperature, determining a critical line in the temperature-activity plane that separates the quasi-long-range ordered (nematic) and disordered (isotropic) phases. Below a critical activity, defects remain bound as rotational noise decorrelates the directed dynamics of +1/2 defects, stabilizing the quasi-long-range ordered nematic state. This activity threshold vanishes at low temperature, leading to a reentrant transition. At large enough activity, active forces always exceed thermal ones and the perturbative result fails, suggesting that in this regime activity will always disorder the system. Crucially, rotational diffusion being a two-dimensional phenomenon, defect unbinding cannot be described by a simplified one-dimensional model.