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Register a new userMemory effects in a gas of viscoelastic particles
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Added by
Francisco Vega Reyes
Publication date
2020
fields
Physics
and research's language is
English
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We study a granular gas of viscoelastic particles (kinetic energy loss upon collision is a function of the particles relative velocities at impact) subject to a stochastic thermostat. We show that the system displays anomalous cooling and heating rates during thermal relaxation processes, this causing the emergence of thermal memory. In particular, a significant textit{Mpemba effect} is present; i.e., an initially hotter/cooler granular gas can cool down/heat up faster than an in comparison cooler/hotter granular gas. Moreover, a textit{Kovacs effect} is also observed; i.e., a non-monotonic relaxation of the granular temperature --if the gas undergoes certain sudden temperature changes before fixing its value. Our results show that both memory effects have distinct features, very different and eventually opposed to those reported in theory for granular fluids under simpler collisional models. We study our system via three independent methods: approximate solution of the kinetic equation time evolution and computer simulations (both molecular dynamics simulations and Direct Simulation Monte Carlo method), finding good agreement between them.
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We analyze the dynamics of an active tracer particle embedded in a thermal lattice gas. All particles are subject to exclusion up to third nearest neighbors on the square lattice, which leads to slow dynamics at high densities. For the case with no rotational diffusion of the tracer, we derive an analytical expression for the resulting drift velocity v of the tracer in terms of non-equilibrium density correlations involving the tracer particle and its neighbors, which we verify using numerical simulations. We show that the properties of the passive system alone do not adequately describe even this simple system of a single non-rotating active tracer. For large activity and low density, we develop an approximation for v. For the case where the tracer undergoes rotational diffusion independent of its neighbors, we relate its diffusion coefficient to the thermal diffusion coefficient and v. Finally we study dynamics where the rotation of the tracer is limited by the presence of neighboring particles. We find that the effect of this rotational locking may be quantitatively described in terms of a reduction of the rotation rate.
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We analyse the dynamical evolution of a fluid with non-linear drag, for which binary collisions are elastic, at the kinetic level of description. When quenched to low temperatures, the system displays a really complex behaviour. The glassy response of the system is controlled by a long-lived non-equilibrium state, and includes non-exponential, algebraic, relaxation and strong memory effects in the time evolution of its kinetic temperature. Moreover, the observed behaviour is universal, in the sense that the time evolution of the temperature -- for both relaxation and memory effects -- falls onto a master curve, regardless of the details of the experiment. Our theoretical predictions are checked against simulations of the kinetic equation, and an excellent agreement is found.
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