Statistical Physics and soft matter

Christian Maes (KUL, Leuven)

Response functions out of equilibrium

The linear response of nonequilibrium systems to an external stimulus can systematically be represented as a sum of two correlation functions in the unperturbed system.  One is the dissipative term, known from equilibrium response, and the second involves the dynamical activity, very much distinct from the first when away from equilibrium.  We illustrate the new fluctuation-response relations for both inertial systems (coupled oscillators) and for jump processes (boundary driven ion channel).

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Martin van der Hoef (Universiteit Twente)

The effect of air on granular matter: why do light particles sink to the bottom, and sandheaps grow when shaken?

The behavior of granular matter (like sand) is currently one of the fastest growing fields in contemporary physics, owing to its wealth of beautiful phenomena as well as to its direct relevance for innumerable industrial applications. Most of the current research is concerned with relatively large particles (> 1 mm), where the effects of the ambient air can be safely ignored. However, when particles get smaller, the effects of air cannot be neglected, and a whole new range of interesting and even unexpected phenomena may occur.
After a brief introduction into the physics of granular matter, I want to share two examples of phenomena that are directly caused by the presence of air: i) segregation in vibrated beds of equal-sized bronze and glass spheres, and ii) formation of heaps (known as "Faraday heaps") for shallow vibrated sand beds. These systems are not only studied experimentally, but also by numerical simulations. The importance of the simulations is that they allow for a detailed visualization of the airflow through the granular bed - which proves to be the key to understanding the phenomena.

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Massimiliano Esposito (ULB, Brussels)

Entropy production as correlation between system and reservoir

I will present an exact (classical and quantum) expression for the entropy production of a finite system placed in contact with one or several finite reservoirs, each of which is initially described by a canonical equilibrium distribution. Although the total entropy of system plus reservoirs is conserved, we show that system entropy production is always positive and is a direct measure of system-reservoir correlations and/or entanglements. I will illustrate this novel interpretation of the Second Law using various model systems and I will explicitly show that this exact formulation leads to the standard description of irreversibility in the limit of a large reservoir.