## ACTUALITES

Dans cette rubrique sont regroupées quelques faits marquants qui se sont déroulés dernièrement au laboratoire (séminaires, publications importantes, parution de livres, colloques...).

Les archives sont également accessibles ici .

A video produced by The Lutetium Project, an outreach YouTube channel co-founded by Mathias Kasiulis, a PhD student at LPTMC, just won the 2017 edition of the Milton Van Dyke Award. This prize is awarded every year by the Division of Fluid Dynamics of the American Physical Society (APSDFD) during its annual meeting, to reward pictures and videos of current research in fluid dynamics.

The video itself displays an experiment that was carried out in the PMMH laboratory, at ESPCI, and was published in Physical Review Letters in early 2017. It shows a drop of a mixture of water and alcohol bursting into a myriad of smaller droplets when it is deposited onto a bath of sunflower oil. This bursting is a rather uncommon example of a hydrodynamical phenomenon caused by the interplay between evaporation and surface tension. The video itself is part of a larger collection called "Experiments in Music", a series of videos by The Lutetium Project that aims at showing the aesthetic value of research experiments, accompanying it with an original music composed by Julien Mazet, a student at Conservatoire National Supérieur de Musique et de Danse de la Ville de Paris.

You can find all the other videos by The Lutetium Project on their YouTube channel: https://www.youtube.com/TheLutetiumProject

G. Vaccario, C. Antoine, and J. Talbot

Phys. Rev. Lett. 115, 240601 – Published 9 December 2015

Abstract

Although there are many theoretical studies of the mean first-passage time (MFPT), most neglect the diffusive heterogeneity of real systems. We present exact analytical expressions for the MFPT and residence times of a pointlike particle diffusing in a spherically symmetric d-dimensional heterogeneous system composed of two concentric media with different diffusion coefficients with an absorbing inner boundary (target) and a reflecting outer boundary. By varying the convention, e.g., Itō, Stratonovich, or isothermal, chosen to interpret the overdamped Langevin equation with multiplicative noise describing the diffusion process, we find different predictions and counterintuitive results for the residence time in the outer region and hence for the MFPT, while the residence time in the inner region is independent of the convention. This convention dependence of residence times and the MFPT could provide insights about the heterogeneous diffusion in a cell or in a tumor, or for animal and insect searches inside their home range.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.240601

F. Léonard and B. Delamotte

Phys. Rev. Lett. 115, 200601 – Published 10 November 2015

Abstract

We present models where γ+ and γ−, the exponents of the susceptibility in the high- and low-temperature phases, are generically different. In these models, continuous symmetries are explicitly broken down by discrete anisotropies that are irrelevant in the renormalization-group sense. The Zq-invariant models are the simplest examples for two-component order parameters (N=2) and the model with icosahedral symmetry for N=3. We accurately compute γ+−γ− as well as the ratio ν/ν′ of the exponents of the two correlation lengths present for T<Tc.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.200601

**Evaluation of departure from equilibrium at cellular scale**

Carlo Bianca and Annie Lemarchand

J. Chem. Phys. 141, 144102 (2014)

The complex spatiotemporal structures that appear in biological systems require farfrom equilibrium conditions which lead to the circulation of reaction fluxes. Recent developments in nonequilibrium statistical physics propose a theoretical framework for estimating these reaction fluxes. In particular, the time asymmetry of fluctuations could be a priori exploited. Fluorescence imaging gives directly access to the observation of the dynamics of cellular events. The temporal ordering of three proteins in the endocytic pathway has been extracted and reaction flux has been estimated for an assumed mechanism with linear dynamics [D. R. Sisan, D. Yarar, C. M. Waterman, and J. S. Urbach, Biophys. J. 98, 2432 (2010)]. However, endocytosis is known to involve complex regulation mechanisms and our aim is to warn against the blind use of the simple relation between reaction flux and crosscorrelation function of concentration fluctuations found for linear deterministic dynamics [W. J. Heuett and H. Qian, J. Chem. Phys. 124, 044110 (2006)].

In the biologically relevant case of a reactive system which may admit periodic oscillations, our results show that the amplitude of the crosscorrelation functions is not only proportional to the reaction flux but also to a specific parameterdependent function which diverges as the Hopf bifurcation approaches. In order to harvest the determination of correlation functions for reaction flux estimation in a given chemical system, it is therefore essential first to identify the reactionmechanism and secondly to evaluate the associated rate constants. If these demanding requirements may be fulfilled, the stochastic differential equations of Langevin type governing the fluctuating

dynamics of concentrations provides a reliable, analytical formula relating the reaction flux and the correlations of fluctuations. From the theoretical viewpoint, the interplay between fluctuations and nonlinearities of deterministic dynamics is subtle and leads to specific formulas for the time crosscorrelations

of concentration fluctuations, that depend on the details of dynamics.

Difference of time cross-correlation functions of concentration fluctuations, *I*(t)=<(*X*(0)-*X*_{S})(Y(t)-Y_{S})>-<(X(t)-X_{S})(Y(0)-Y_{S})>,

in the case of the Brusselator model, known to possess a stationary state (*X*_{S},*Y*_{S})* *and a Hopf bifurcation. The concentrations of species A, B, and C are fixed and the concentrations *X*(t) and *Y*(t) of species X and Y are variable. The results of the numerical solution of the master equation (blue dashed line) confirm the analytical approach by Langevin equations (red dashed line) and invalidate the result obtained when assuming that dynamics is linear (black dotted line).