Sylvain Joubaud

Wringing Out DNA

The chiral nature of DNA plays a crucial role in cellular processes. Here we use magnetic tweezers to explore one of the signatures of this chirality, the coupling between stretch and twist deformations. We show that the extension of a stretched DNA molecule increases linearly by 0.42 nm per excess turn applied to the double helix. This result contradicts the intuition that DNA should lengthen as it is unwound and get shorter with overwinding. We then present numerical results of energy minimizations of torsionally restrained DNA that display a behavior similar to the experimental data and shed light on the molecular details of this surprising effect.

Entropy production and time asymmetry in nonequilibrium fluctuations.

The time-reversal symmetry of nonequilibrium fluctuations is experimentally investigated in two out-of-equilibrium systems: namely, a Brownian particle in a trap moving at constant speed and an electric circuit with an imposed mean current. The dynamical randomness of their nonequilibrium fluctuations is characterized in terms of the standard and time-reversed entropies per unit time of dynamical systems theory. We present experimental results showing that their difference equals the thermodynamic entropy production in units of Boltzmanns constant.

Work fluctuation theorems for harmonic oscillators.

We study experimentally the thermal fluctuations of energy input and dissipation in a harmonic oscillator driven out of equilibrium, and search for fluctuation relations. Both the transient evolution from the equilibrium state, and non-equilibrium steady states are analyzed. Fluctuation relations are obtained experimentally for both the work and the heat, for the stationary and transient evolutions. A stationary state fluctuation theorem is verified for various time dependences of the imposed external torque. The transient fluctuation theorem is satisfied for the work given to the system but not for the heat dissipated by the system in the case of linear forcing. Experimental observations on the statistical and dynamical properties of the position fluctuations of the torsion pendulum allow us to derive analytical expressions for the probability density functions of the work and the heat. We obtain for the first time an analytic expression for the probability density function of the heat. The agreement between experiments and our predictions is excellent.

Fluctuations in out-of-equilibrium systems: from theory to experiment

We introduce from an experimental point of view the main concepts of fluctuation theorems for work, heat and entropy production in out-of- equilibrium systems. We will discuss the important difference between the applications of these concepts to stochastic systems and to a second class of systems (chaotic systems) where the fluctuations are induced either by chaotic flows or by fluctuating driving forces. We will mainly analyze the stochastic systems using the measurements performed in two experiments: (a) a harmonic oscillator driven out of equilibrium by an external force; (b) a colloidal particle trapped in a time-dependent double-well potential. We will rapidly describe some consequences of fluctuation theorems and some useful applications to the analysis of experimental data. As an example, the case of a molecular motor will be analyzed in some detail. Finally we will discuss the problems related to the applications of fluctuation theorems to chaotic systems.

Experimental study of parametric subharmonic instability for internal plane waves

Internal waves are believed to be of primary importance as they affect ocean mixing and energy transport. Several processes can lead to the breaking of internal waves and they usually involve non linear interactions between waves. In this work, we study experimentally the parametric subharmonic instability (PSI), which provides an efficient mechanism to transfer energy from large to smaller scales. It corresponds to the destabilization of a primary plane wave and the spontaneous emission of two secondary waves, of lower frequencies and different wave vectors. Using a time-frequency analysis, we observe the time evolution of the secondary waves, thus measuring the growth rate of the instability. In addition, a Hilbert transform method allows the measurement of the different wave vectors. We compare these measurements with theoretical predictions, and study the dependence of the instability with primary wave frequency and amplitude, revealing a possible effect of the confinement due to the finite size of the beam, on the selection of the unstable mode.

Thermodynamic time asymmetry in non-equilibrium fluctuations

We here present the complete analysis of experiments on driven Brownian motion and electric noise in an RC circuit, showing that thermodynamic entropy production can be related to the breaking of time-reversal symmetry in the statistical description of these non-equilibrium systems. The symmetry breaking can be expressed in terms of dynamical entropies per unit time, one for the forward process and the other for the time-reversed process. These entropies per unit time characterize dynamical randomness, i.e., temporal disorder, in time series of the non-equilibrium fluctuations. Their difference gives the well-known thermodynamic entropy production, which thus finds its origin in the time asymmetry of dynamical randomness, alias temporal disorder, in systems driven out of equilibrium.

Experimental parametric subharmonic instability in stratified fluids

Internal gravity waves contribute to fluid mixing and energy transport, not only in oceans but also in the atmosphere and in astrophysical bodies. An efficient way to transfer energy from large scale to smaller scale is the parametric subharmonic instability. We provide here the first experimental measurement of the growth rate of this instability. We make careful and quantitative comparisons with theoretical predictions for propagating vertical modes in laboratory experiments.

Fluctuations of the total entropy production in stochastic systems

Fluctuations of the total entropy are experimentally investigated in two stochastic systems in a non-equilibrium steady state: an electric circuit with an imposed mean current and a harmonic oscillator driven out of equilibrium by a periodic torque. In these two linear systems, we study the total entropy production which is the entropy created to maintain the system in the non-equilibrium steady state. The fluctuation theorem holds for the total entropy production in the two experimental systems, both for all observation times and for all fluctuation magnitudes.

Experimental observation of a strong mean flow induced by internal gravity waves

We report the experimental observation of a robust horizontal mean flow induced by internal gravity waves. A wave beam is forced at the lateral boundary of a tank filled with a linearly stratified fluid initially at rest. After a transient regime, a strong jet appears in the wave beam, with horizontal recirculations outside the wave beam. Using multiple scale analysis, we present a simple physical mechanism predicting the growth rate of the mean flow and its initial spatial structure. We find good agreement with experimental results. These results show that a mean flow with non-zero vertical vorticity can be generated by Reynolds stresses if the wave fulfils two conditions: (1) the wave amplitude must vary along its propagation direction, which is the case in the presence of viscosity; (2) the wave amplitude must vary in the lateral direction, which is the case when the wave generator is localized in space.

Finite-size effects in parametric subharmonic instability

The parametric subharmonic instability in stratified fluids depends on the frequency and the amplitude of the primary plane wave. In this paper, we present experimental and numerical results emphasizing that the finite width of the beam also plays an important role on this triadic instability. A new theoretical approach based on a simple energy balance is developed and compared to numerical and experimental results. Because of the finite width of the primary wave beam, the secondary pair of waves can leave the interaction zone which affects the transfer of energy. Experimental and numerical results are in good agreement with the prediction of this theory, which brings new insights on energy transfers in the ocean where internal waves with finite-width beams are dominant.