arXiv subjects

Stefano Steffenoni

Publications and source records attributed to Stefano Steffenoni.

5 recordsLinked to original sources

arXiv2020

Active Brownian Heat Engines

When do non-equilibrium forms of disordered energy qualify as heat? \textcolor{blue}{We address this question in the context of cyclically operating heat engines in contact with a non-equilibrium energy reservoir that defies the zeroth law of thermodynamics. To consistently address the latter as a heat bath requires the existence of a precise mapping to an equivalent cycle with an equilibrium bath at a time-dependent effective temperature. We identify the most general setup for which this can generically be ascertained and thoroughly discuss an analytically tractable, experimentally relevant scenario}: a Brownian particle confined in a \textcolor{blue}{periodically} modulated harmonic potential and coupled to some non-equilibrium bath of variable activity. We deduce formal limitations for its thermodynamic performance, including maximum efficiency, efficiency at maximum power, and maximum efficiency at fixed power. They can guide the design of new micro-machines and clarify how much these can outperform passive-bath designs, which has been a debated issue for recent experimental realizations. To illustrate the general principles for practical quasi-static and finite-rate protocols, we further analyze a specific realization of such an active heat engine based on the paradigmatic Active Brownian Particle (ABP) model. This reveals some non-intuitive features of the explicitly computed dynamical effective temperature, illustrates various conceptual and practical limitations of the effective-equilibrium mapping, and clarifies the operational relevance of various coarse-grained measures of dissipation.

cond-mat.stat-mech↗

arXiv2019

Physically consistent numerical solver for time-dependent Fokker-Planck equations

We present a simple thermodynamically consistent method for solving time-dependent Fokker--Planck equations (FPE) for over-damped stochastic processes, also known as Smoluchowski equations. It yields both transition and steady-state behavior and allows for computations of moment-generating and large-deviation functions of observables defined along stochastic trajectories, such as the fluctuating particle current, heat and work. The key strategy is to approximate the FPE by a Master equation with transition rates in configuration space that obey a local detailed balance condition for arbitrary discretization. Its time-dependent solution is obtained by a direct computation of the time-ordered exponential, representing the propagator of the FPE, by summing over all possible paths in the discretized space. The method thus not only preserves positivity and normalization of the solutions, but also yields a physically reasonable total entropy production, regardless of the discretization. To demonstrate the validity of the method and to exemplify its potential for applications, we compare it against Brownian-dynamics simulations of a heat engine based on an active Brownian particle trapped in a time-dependent quartic potential.

cond-mat.stat-mech↗

arXiv2016

Microscopic derivation of the hydrodynamics of active-Brownian-particle suspensions

We derive the hydrodynamic equations of motion for a fluid of active particles described by under- damped Langevin equations that reduce to the Active-Brownian-Particle model, in the overdamped limit. The contraction into the hydrodynamic description is performed by locally averaging the par- ticle dynamics with the non-equilibrium many-particle probability density, whose formal expression is found in the physically relevant limit of high-friction through a multiple-time-scale analysis. This approach permits to identify the conditions under which self-propulsion can be subsumed into the fluid stress tensor and thus to define systematically and unambiguously the local pressure of the active fluid.

cond-mat.stat-mech↗

arXiv2016

Interacting Brownian dynamics in a nonequilibrium particle bath

We set up a mesoscopic theory for interacting Brownian particles embedded in a nonequilibrium environment, starting from the microscopic interacting many-body theory. Using nonequilibrium linear response theory, we characterize the effective dynamical interactions on the mesoscopic scale and the statistics of the nonequilibrium environmental noise, arising upon integrating out the fast degrees of freedom. As hallmarks of nonequilibrium, the breakdown of the fluctuation-dissipation and action-reaction relations for Brownian degrees of freedom are exemplified with two prototypical models for the environment, namely, active Brownian particles and stirred colloids.

cond-mat.stat-mech↗

arXiv2014

Friction and noise for a probe in a nonequilibrium fluid

We investigate the fluctuation dynamics of a probe around a deterministic motion induced by interactions with driven particles. The latter constitute the nonequilibrium medium in which the probe is immersed and is modelled as overdamped Langevin particle dynamics driven by nonconservative forces. The expansion that yields the friction and noise expressions for the reduced probe dynamics is based on linear response around a time dependent nonequilibrium condition of the medium. The result contains an extension of the second fluctuation dissipation relation between friction and noise for probe motion in a nonequilibrium fluid

cond-mat.stat-mech↗