Odd Diffusivity of Chiral Random Motion.

Diffusive transport is characterized by a diffusivity tensor which may, in general, contain both a symmetric and an antisymmetric component. Although the latter is often neglected, we derive Green-Kubo relations showing it to be a general characteristic of random motion breaking time-reversal and parity symmetries, as encountered in chiral active matter. In analogy with the odd viscosity appearing in chiral active fluids, we term this component the odd diffusivity. We show how odd diffusivity emerges in a chiral random walk model, and demonstrate the applicability of the Green-Kubo relations through molecular dynamics simulations of a passive tracer particle diffusing in a chiral active bath.

Time reversal symmetry breaking and odd viscosity in active fluids: Green-Kubo and NEMD results.

Active fluids, which are driven at the microscale by non-conservative forces, are known to exhibit novel transport phenomena due to the breaking of time reversal symmetry. Recently, Epstein and Mandadapu [arXiv:1907.10041 (2019)] obtained Green-Kubo relations for the full set of viscous coefficients governing isotropic chiral active fluids, including the so-called odd viscosity, invoking Onsager's regression hypothesis for the decay of fluctuations in active non-equilibrium steady states. In this Communication, we test these Green-Kubo relations using molecular dynamics simulations of a canonical model system consisting of actively torqued dumbbells. We find the resulting odd and shear viscosity values from the Green-Kubo relations to be in good agreement with values measured independently through non-equilibrium molecular dynamics flow simulations. This provides a test of the Green-Kubo relations and lends support to the application of the Onsager regression hypothesis in relation to viscous behaviors of active matter systems.

Time-reversal symmetry breaking in two-dimensional nonequilibrium viscous fluids.

We study the rheological signatures of departure from equilibrium in two-dimensional viscous fluids with and without internal spin. Under the assumption of isotropy, we provide the most general linear constitutive relations for stress and couple stress in terms of the velocity and spin fields. Invoking Onsager's regression hypothesis for fluctuations about steady states, we derive the Green-Kubo formulas relating the transport coefficients to time-correlation functions of the fluctuating stress. In doing so, we show that one of the nonequilibrium transport coefficients, the odd viscosity, requires time-reversal symmetry breaking in the case of systems without internal spin. However, the Green-Kubo relations for systems with internal spin also show that there is a possibility for nonvanishing odd viscosity even when time-reversal symmetry is preserved. Furthermore, we find that breakdown of equipartition in nonequilibrium steady states results in the decoupling of the two rotational viscosities relating the vorticity and the internal spin.

Statistical mechanics of transport processes in active fluids. II. Equations of hydrodynamics for active Brownian particles.

We perform a coarse-graining analysis of the paradigmatic active matter model, active Brownian particles, yielding a continuum description in terms of balance laws for mass, linear and angular momentum, and energy. The derivation of the balance of linear momentum reveals that the active force manifests itself directly as a continuum-level body force proportional to an order parameter-like director field, which therefore requires its own evolution equation to complete the continuum description of the system. We derive this equation, demonstrating in the process that bulk currents may be sustained in homogeneous systems only in the presence of interparticle aligning interactions. Furthermore, we perform a second coarse-graining of the balance of linear momentum and derive the expression for active or swim pressure in the case of mechanical equilibrium.

CD36 in the periphery and brain synergizes in stroke injury in hyperlipidemia.

OBJECTIVE: Hyperlipidemia exacerbates ischemic stroke outcome and increases CD36 expression in the postischemic brain as well as in peripheral monocytes/macrophages. By exchanging bone marrow-derived cells between CD36-expressing and CD36-deficient mice, this study investigates the contribution of peripheral CD36 in comparison with that of brain CD36 to stroke pathology in hyperlipidemia. METHODS: Following bone marrow transplantation, mice were fed a high-fat diet for 11 weeks and then subjected to ischemic stroke. Stroke outcome, expression of brain CD36, monocyte chemoattractant protein-1 (MCP-1), CCR2, and plasma MCP-1 levels were determined at 3 days postischemia. CD36 and CCR2 expression were also determined in splenocytes incubated with serum obtained from CD36-expressing or CD36-deficient mice. RESULTS: Infiltrating immune cells from the periphery are the major source of CD36 in the postischemic brain and contribute to stroke-induced brain injury. This CD36 effect was dependent on the modulation of MCP-1 and CCR2 expression in peripheral immune cells as well as CD36-expressing cells in the host brain. INTERPRETATION: This study demonstrates that CD36 expressed in the periphery and brain synergize in ischemic brain injury through regulation of the MCP-1/CCR2 chemokine axis in hyperlipidemic conditions.