Thermodynamics of Nonequilibrium Systems, from Maxwell Demons to Active Microswimmers in Confined Flows
Marc Lagoin (LOMA, U. Bordeaux)
Many physical systems can be described using a few coarse variables, ignoring most microscopic details. This idea, formalized in Langevin’s theory of Brownian motion [3], allows us to understand how fluctuations and dissipation produce predictable behaviours even in systems far from equilibrium.In this talk, I will present two experiments that illustrate this approach.
First, in a driven granular gas, we built a macroscopic version of a Feynman-type ratchet [4,7] and measured how fluctuations can be rectified to produce motion. This work is described in Ref. [2] and provides a simple experimental test of stochastic models.Second, we studied Chlamydomonas reinhardtii microswimmers in oscillatory Poiseuille flows. By tracking their motion, we measured their long-time dispersion and interpreted it using a coarse-grained Brownian-particle description [8]. This leads to an active version of the classical Taylor–Aris dispersion [5,6], as shown in Ref. [1].
These two studies show that simple stochastic descriptions can help in understanding the behaviour of nonequilibrium systems, from macroscopic mechanical devices to suspensions of active microorganisms.
- M.Lagoin, J. Lacherez, G. de Tournemire, A. Badr, Y. Amarouchene, A. Allard, and T. Salez, Enhanced dispersion of active microswimmers in confined flows, PNAS (2025); arXiv:2507.08369.
- M. Lagoin, C. Crauste-Thibierge, A. Naert, A human-scale Brownian ratchet, Phys. Rev. Lett. (2022); HAL: hal-03650213.
- P. Langevin, Sur la théorie du mouvement brownien, C. R. Acad. Sci. (1908).
- R. P. Feynman, The Feynman Lectures on Physics, Vol. I, Ch. 46: Ratchet and Pawl (1964).
- G. I. Taylor, Dispersion of soluble matter in solvent flowing slowly through a tube, Proc. R. Soc. A (1953).
- R. Aris, On the dispersion of a solute in a fluid flowing through a tube, Proc. R. Soc. A (1956).
- M. von Smoluchowski, early works on fluctuation-induced transport (1912).
- H. Risken, The Fokker–Planck Equation: Methods of Solution and Applications.