Active matter beyond mean-field: Ring-kinetic theory for self-propelled particles

A ring-kinetic theory for Vicsek-style models of self-propelled agents is derived from the exact N-particle evolution equation in phase space. The theory goes beyond mean-field and does not rely on Boltzmann's approximation of molecular chaos. It can handle pre-collisional correlations and cluster formation which both seem important to understand the phase transition to collective motion. We propose a diagrammatic technique to perform a small density expansion of the collision operator and derive the first two equations of the BBGKY-hierarchy. An algorithm is presented that numerically solves the evolution equation for the two-particle correlations on a lattice. Agent-based simulations are performed and informative quantities such as orientational and density correlation functions are compared with those obtained by ring-kinetic theory. Excellent quantitative agreement between simulations and theory is found at not too small noises and mean free paths. This shows that there is parameter ranges in Vicsek-like models where the correlated closure of the BBGKY-hierarchy gives correct and nontrivial results. We calculate the dependence of the orientational correlations on distance in the disordered phase and find that it seems to be consistent with a power law with exponent around -1.8, followed by an exponential decay. General limitations of the kinetic theory and its numerical solution are discussed

Kinetic theory for systems of self-propelled particles with metric-free interactions

A model of self-driven particles similar to the Vicsek model [Phys. Rev. Lett. 75 (1995) 1226] but with metric-free interactions is studied by means of a novel Enskog-type kinetic theory. In this model, N particles of constant speed v0 try to align their travel directions with the average direction of a fixed number of closest neighbors. At strong alignment a global flocking state forms. The alignment is defined by a stochastic rule, not by a Hamiltonian. The corresponding interactions are of genuine multi-body nature. The theory is based on a Master equation in 3N-dimensional phase space, which is made tractable by means of the molecular chaos approximation. The phase diagram for the transition to collective motion is calculated and compared to direct numerical simulations. A linear stability analysis of a homogeneous ordered state is performed using the kinetic but not the hydrodynamic equations in order to achieve high accuracy. In contrast to the regular metric Vicsek-model no instabilities occur. This confirms previous direct simulations that for Vicsek-like models with metric-free interactions, there is no formation of density bands and that the flocking transition is continuous.Comment: 21 pages, 17 figure

Flow field predictions for a slab delta wing at incidence

Theoretical results are presented for the structure of the hypersonic flow field of a blunt slab delta wing at moderately high angle of attack. Special attention is devoted to the interaction between the boundary layer and the inviscid entropy layer. The results are compared with experimental data. The three-dimensional inviscid flow is computed numerically by a marching finite difference method. Attention is concentrated on the windward side of the delta wing, where detailed comparisons are made with the data for shock shape and surface pressure distributions. Surface streamlines are generated, and used in the boundary layer analysis. The three-dimensional laminar boundary layer is computed numerically using a specially-developed technique based on small cross-flow in streamline coordinates. In the rear sections of the wing the boundary layer decreases drastically in the spanwise direction, so that it is still submerged in the entropy layer at the centerline, but surpasses it near the leading edge. Predicted heat transfer distributions are compared with experimental data

HIV neutralization through use of antibodies and pharmacokinetics of topical applications

Thesis (M.A.)--Boston UniversityThe Human Immunodeficiency Virus Type 1 (HIV-1), a sexually transmitted retrovirus that causes the Acquired Immunodeficiency Syndrome (AIDS), infects over two million people a year. Several methods introduced to prevent HIV-1 transmission, such as condoms, circumcision and antiretroviral drugs, have proven to be partially effective, but more effective approaches are being sought. Topical microbicides are being developed to provide a women-controlled method to prevent the transmission of HIV-1. Unfortunately, most of the candidate microbicide compounds tested to date have either elicited undesirable mucosal inflammation and epithelial lesions leading to increased seroconversions, or have been ineffective. One novel approach currently being explored is the use of monoclonal antibodies as components of topical microbicides. Monoclonal antibodies can be produced inexpensively by transfection into Nicotiana plants. We hypothesize that anti-HIV monoclonal antibodies produced in Nicotiana (MAb-N) will be effective in neutralizing HIV- 1 when used as topical microbicides at mucosal sites, and set out to test whether they retain their efficacy under physiological conditions. We tested the pharmacodynamics of anti-HIV MAb-N efficacy in Cynomolgus macaques following application of the antibodies in the vaginal compartment. We further studied the ability of MAb-N to cross through the vaginal epithelium using an EpiVaginal tissue model. To determine the pharmacodynamics of HIV neutralizing activity after the application of anti-HIV MAb-Ns to the vaginal mucosa, we used a neutralization assay based on HIV-expression in the TZM-bl cell line to test the efficacy of various doses of MAbs in a time course after they had been administered intravaginally in gel form to Cynomolgus macaques. To determine the pharmacokinetics of Mab-N transport across the vaginal epithelium, monoclonal antibodies were added to the apical surface, and a human-IgG ELISA was used to detect Mab-N that had crossed the epithelium into the basal supernatant. Immunohistology was used to confirm and validate ELISA data for evidence of transfer of Mabs across the epithelial layer. Our results show that anti-HIV MAb-Ns were effective in neutralizing cell-free HIV in TZM-bl neutralization assays. We found that MAb-Ns retained their anti-viral efficacy in monkeys after 4-hours. However, neutralizing activity was decreased after 24- hours and 72-hours, with wide variability in effectiveness between individual macaques. Mab-ns tested in the EpiVaginal tissue model showed minimal transfer of antibodies across the epithelium, ranging from 0.005% to 0.09%. Immunohistological data showed that antibodies applied apically to tissue models concentrated only in the superficial layers of the stratum corneum and did not penetrate the epithelium. Our data indicate that anti-HIV MAb-Ns are effective in neutralizing HIV-1 following vaginal application for at least 4 hours, and that they do not pass through the vaginal epithelium in significant amounts. Our data support their further development as vaginal microbicides

Coupled vortex oscillations in spatially separated permalloy squares

We experimentally study the magnetization dynamics of pairs of micron-sized permalloy squares coupled via their stray fields. The trajectories of the vortex cores in the Landau-domain patterns of the squares are mapped in real space using time-resolved scanning transmission x-ray microscopy. After excitation of one of the vortex cores with a short magnetic-field pulse, the system behaves like coupled harmonic oscillators. The coupling strength depends on the separation between the squares and the configuration of the vortex-core polarizations. Considering the excitation via a rotating in-plane magnetic field, it can be understood that only a weak response of the second vortex core is observed for equal core polarizations

Magnetic antivortex-core reversal by circular-rotational spin currents

Topological singularities occur as antivortices in ferromagnetic thin-film microstructures. Antivortices behave as two-dimensional oscillators with a gyrotropic eigenmode which can be excited resonantly by spin currents and magnetic fields. We show that the two excitation types couple in an opposing sense of rotation in the case of resonant antivortex excitation with circular-rotational currents. If the sense of rotation of the current coincides with the intrinsic sense of gyration of the antivortex, the coupling to the Oersted fields is suppressed and only the spin-torque contribution locks into the gyrotropic eigenmode. We report on the experimental observation of purely spin-torque induced antivortex-core reversal. The dynamic response of an isolated antivortex is imaged by time-resolved scanning transmission x-ray microscopy on its genuine time and length scale

Control of a virtual ambulation influences body movement and motion sickness

Drivers typically are less susceptible to motion sickness than passengers. The influence of vehicle control has theoretical implications for the etiology of motion sickness, and has practical implications for the design of virtual environments. In the present study, participants either controlled or did not control a nonvehicular virtual avatar (i.e., an ambulatory character in a console video game). We examined the incidence of motion sickness and patterns of movement of the head and torso as participants either played or watched the game. Motion sickness incidence was lower when controlling the virutal avatar than when watching an avatar that was controlled by someone else. Patterns of head and torso movement differed between particpants who did and did not control the avatar. Indepenently, patterns of movement differed between participants who reported motion sickness and those who did not. The results suggest that motion sickness is influenced by control of stimulus motion, whether that motion arises from a vehicle or from any other source. We consider implications for the design of humancomputer interfaces