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We present an experimental realization at macroscopic scale of the storied Brownian ratchet, which is an illustration of the Maxwell's demon. In our mechanism, the rotation of a centimeter-scale 1D Brownian object in a granular gas is detected by an electromechanical converter (dynamo), generating a voltage proportional to its angular velocity. The current generated by this random rotation is rectified by an electronic device (demon), such that only positive current passes. Eventually, work can be produced. The advantage of such a macroscopic setup is to allow measurement of all the observables with time: useful power (work), heat taken from the bath, and finally the efficiency of the equivalent heat engine. The feedback allowing the conversion from heat into work expresses as a bias on the Brownian motion.

We report experimental evidence that a polymer stretched at constant strain rate $\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\lambda}}$ presents complex memory effects after $\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\lambda}}$ is set to zero at a specific strain ${\ensuremath{\lambda}}_{w}$ for a duration ${t}_{w}$, ranging from $100\phantom{\rule{4pt}{0ex}}\mathrm{s}$ to $2.2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{4pt}{0ex}}\mathrm{s}$. When the strain rate is resumed, both the stress and the dielectric constant relax to the unperturbed state nonmonotonically. The relaxations depend on the observable, on ${\ensuremath{\lambda}}_{w}$ and on ${t}_{w}$. Relaxation master curves are obtained by scaling the time and the amplitudes by $ln({t}_{w})$. The dielectric evolution also captures the distribution of the relaxation times, so the results impose strong constraints on the relaxation models of polymers under stress and they can be useful for a better understanding of memory effects in other disorder materials.

The polymer relaxation dynamic of a sample, stretched up to the stress hardening regime, is measured, at room temperature, as a function of the strain λ for a wide range of the strain ratė γ, by an original dielectric spectroscopy set up. The mechanical stress modies the shape of the dielectric spectra mainly because it aects the dominant polymer relaxation time τ , which depends on λ and is a decreasing function ofγ. The fastest dynamics is not reached at yield but in the softening regime. The dynamics slows down during the hardening, with a progressive increase of τ. A small inuence ofγ and λ on the dielectric strength cannot be excluded. Mechanical and dynamical properties of polymers are intensively studied, due to their fundamental and technological importance [1]. When strained at a given strain rate, beyond the elastic regime in which the stress is proportional to strain, glassy polymers exhibit a maximum in the stress-strain curves (yield point) at a strain of a few percents [2] and the deformation becomes irreversible (see Fig.1). At larger strains, depending on the history of the sample [3], the stress drops (strain-softening regime) before reaching a plateau corresponding to plastic ow. Strain-hardening may then occur at even larger strains, depending on the molecular weight and on the cross-linking of the polymer [4]. The key new insights obtained either by numerical simulations [57] or experiments [8 11] are that strain hardening appears to be controlled by the same mechanisms that control plastic ow [1215]. However the microscopic mechanisms leading to such a mechanical behavior are not fully understood [16, 17]. For example it is unclear to what extent the relaxation dynamics in polymer glasses is modied when the sample is stretched into the plastic region (see for example refs.[1821]). The purpose of this letter is to bring new insight into this problem by presenting the results of experiments in which we performed dielectric spectroscopy of polymer samples stretched till the strain hardening regime. Di-electric spectroscopy, allows the investigation of the dynamics of relaxation processes by means of the polarization of molecular dipoles. It is directly sensitive to polymer mobility and probes directly the segmental motion. It can be used to quantify the mobile fraction of polymers. Measuring the dielectric response of polymers in situ had been pioneered by Venkataswamy et al. [22]. It is complementary to other techniques, such as Nuclear Magnetic Resonance [23] and the diusion of probe molecules [2426], used to study the molecular dynamics of polymers under stress. The dielectric spectroscopy has already been used in combination with mechanical deformation to study the dynamics in the amorphous phase of polymer under stress [27, 28]. The results of these experiments were limited to the yield point whereas the experimental studies of the microscopic behavior and processes FIG. 1. Stress evolution over a wide range of λ at several constant strain rates : 2.5 × 10 −3 s −1 (black), 2.5 × 10 −4 s −1 (red), 2.5 × 10 −5 s −1 (green) and 2.5 × 10 −6 s −1 (blue). Several identical specimens have been measured for each strain rate. Inset: tensile machine for dielectric measurements under stress : motor (A), load cell (B), linear transducer (C), sample fastening cylinders with the stretched polymer lm (D) and electrodes (E) connected to the dielectric spectrometer (not sketched). during strain hardening are more scarce. In this letter we present the results obtained by our original experimental apparatus which can measure with high accuracy the evolution of the Dielectric Spec-trum(DS) of a sample stretched, till the strain hardening regime, at dierent strain rates. Thus we can precisely compare the DS s obtained as a function of stress at room temperature with those (named DS T) obtained as a function of temperature in an unstressed sample. This comparison allows us to extract useful informations on the relaxation dynamics under stress at various strain rates. Our experimental results bring new important informa-tions because they clearly show an acceleration of the dynamics which reaches a maximum in the softening regions. Instead the molecular mobility slows down again during the strain hardening regimes.

We have measured the ac nonlinear dielectric response χ3 (ω, T) of an archetypical glassformer (glycerol) just above its glass transition temperature Tg ∼190 K. Our measurements were performed at temperatures between Tg + 4 K and Tg + 35 K, which corresponds to 5 decades of variation of the relaxation time τ (T) of the liquid. We find that χ3(ω, T) is peaked for ωτ ∼ 0.2 and that the height of the peak grows as one approaches Tg. A powerlaw in frequency beyond the peak of χ3 is observed and χ3 (ω, T) displays scaling as a function of ωτ. These features correspond to the main predictions of some recent theoretical works relating the nonlinear susceptibility of supercooled liquids to the average number Ncorr of dynamically correlated molecules. We thus interpret the experimental increase of the peak of nonlinear susceptibility when T decreases, as an increase of Ncorr when T Tg. This strongly reinforces the collective picture of the dynamics of supercooled liquids close to the glass transition. Finally, we discuss some heating effects and show that they do not affect significantly our conclusions. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; We present a detailed analysis of the mixing process in an associating system, the water-tert-butanol (2-methyl-2-propanol) mixture. Using molecular dynamics simulations, together with neutron, X-ray diffraction experiments, and pulsed gradient spin-echo NMR, we study the local structure and dynamic properties over the full concentration range, and thereby provide quantitative data that reveal relationships between local structure and macroscopic behavior. For an alcohol-rich mixture, diffraction patterns from both neutron and X-ray experiments exhibit a peak at low wavelength vector (q approximate to 0.7 angstrom(-1)) characteristic of supermolecular structures. On increasing the water content, this ``prepeak'' progressively flattens and shifts to low wave vector. We identify hydrogen bonds in the system as the driving force for the specific organization that appears in mixtures, and provide an analysis of the variation of the cluster size distribution with composition. We find that the sizes of local hydrogen-bonded clusters observed in alcohol-rich mixtures become larger as the mole fraction, x(w), of water is increased; a nanophase separation is seen for x(w) in the range 0.6-0.7. This corresponds to several changes in some macroscopic properties of the liquid mixture. Thus, we propose a microscopic description of the effect of water addition in alcohol, which is in agreement with both neutron diffraction pattern and mobility of water and alcohol species. In summary we present a full and comprehensive description of miscibility at its limit in an associated system.

Interacting magnetic nanoparticles display a wide variety of magnetic behaviors that are now being gathered in the emerging field of 'supermagnetism.' We have investigated how the out-of-equilibrium dynamics in the disordered superspin glass (SSG) state of a frozen ferrofluid sample is affected by texturation. Via magnetization relaxation experiments at low temperatures, we were able to estimate superspin correlation lengths for both textured and non-textured samples. The comparison with simulations and experiments on atomic spin glasses shows that the dynamic correlations in SSG's appear to develop in a way reminiscent to those in atomic spin glasses at intermediate time/length scales., 12 pages with 4 figures included

We give an overview of our recent works in which the a.c. nonlinear dielectric response of an archetypical glassformer (glycerol) was measured close to its glass transition temperature T g . The purpose was to investigate the prediction that the nonlinear susceptibility is directly related to the number of dynamically correlated molecules N { corr} (T). We explain that two nonlinear susceptibilities are available, namely χ3 (3) and χ3 (1), which correspond respectively to the nonlinear cubic response at the third harmonics and at the first harmonics. We describe how to measure these nonlinear responses, even if they yield signals much smaller than that of the linear response. We show that both \(\vert {\chi }_{3}^{(3)}(\omega,T)\vert\) and \(\vert {\chi }_{3}^{(1)}(\omega,T)\vert\) are peaked as a function of the angular frequency ω and mainly obeys critical scaling as a function of ωτα(T), where τα(T) is the relaxation time of the liquid. Both χ3 (3) and χ3 (1) decay with the same power-law of ω beyond the peak. The height of the peak increases as the temperature approaches T g : This yields an accurate determination of the temperature dependence of N { corr} (T), once the contribution of saturation of dipoles is disentangled from that of dynamical glassy correlations.

International audience; The box model, originally introduced to account for the nonresonant hole burning (NHB) dielectricexperiments in supercooled liquids, is compared to the measurements of the third harmonics P 3 of the polarisation, reported recently in glycerol, close to the glass transition temperature T g [C. Crauste-Thibierge, C. Brun, F. Ladieu, D. L’Hôte, G. Biroli, and J.-P. Bouchaud, Phys. Rev. Lett.104, 165703 (2010)]. In this model, each box is a distinct dynamical relaxing entity (hereafter called dynamical heterogeneity (DH)) which follows a Debye dynamics with its own relaxation time τ dh . When it is submitted to a strong electric field, the model posits that a temperature increase δT dh , depending on τ dh , arises due to the dissipation of the electrical power. Each DH has thus its own temperature increase, on top of the temperature increase of the phonon bath δT ph . Contrary to the “fast” hole burning experiments where δT ph is usually neglected, the P 3 measurements are, from a thermal point of view, fully in a stationary regime, which means that δT ph can no longer be neglected a priori. This is why the version of the box model that we study here takes δT ph into account, which implies that the δT dh of the DHs are all coupled together. The value of P 3, including both the “intrinsic” contribution of each DH as well as the “spurious” one coming from δT ph , is computed within this box model and compared to the P 3 measurements for glycerol, in the same range of frequencies and temperatures T. Qualitatively, we find that this version of the box model shares with experiments some nontrivial features, e.g., the existence of a peak at finite frequency in the modulus of P 3 as well as its order of magnitude. Quantitatively, however, some experimental features are not accounted for by this model. We show that these differences between the model and the experiments do not come from δT ph but from the “intrinsic” contribution of the DHs. Finally, we show that the interferences between the 3ω response of the various DHs are the most important issue leading to the discrepancies between the box model prediction and the experiments. We argue that this could explain why the box model is quite successful to account for some kinds of nonlinear experiments (such as NHB) performed close to T g , even if it does not completely account for all of them (such as the P 3 measurements). This conclusion is supported by an analytical argument which helps understanding how a “space-free” model as the box model is able to account for some of the experimental nonlinear features.

We present a detailed study of the heating effects in dielectric measurements carried out on a liquid. Such effects come from the dissipation of the electric power in the liquid and give a contribution to the nonlinear third harmonics susceptibility chi_3 which depends on the frequency and temperature. This study is used to evaluate a possible `spurious' contribution to the recently measured nonlinear susceptibility of an archetypical glassforming liquid (Glycerol). Those measurements have been shown to give a direct evaluation of the number of dynamically correlated molecules temperature dependence close to the glass transition temperature T_g~190K (Crauste-Thibierge et al., Phys. Rev. Lett 104,165703(2010)). We show that the heating contribution is totally negligible (i) below 204K at any frequency; (ii) for any temperature at the frequency where the third harmonics response chi_3 is maximum. Besides, this heating contribution does not scale as a function of f/f_{\alpha}, with f_{\alpha}(T) the relaxation frequency of the liquid. In the high frequency range, when f/f_{\alpha} >= 1, we find that the heating contribution is damped because the dipoles cannot follow instantaneously the temperature modulation due to the heating phenomenon. An estimate of the magnitude of this damping is given., Comment: 25 pages, 10 figures, Accepted for publication in Journal of Chemical Physics

International audience; The existence of a growing correlation length associated with the strong increase of relaxation times of glass-formers close to the glass transition is still a major open question in glass physics. It has been recently proposed that the ac nonlinear susceptibility of a supercooled liquid close to the glass transition temperature T g would be a probe of dynamical correlations. As for spin glasses, where the nonlinear susceptibility diverges at the transition, this quantity is tailored to reveal the possible “hidden” critical behavior of the glass transition. We have developed a high sensitivity experimental device to measure the nonlinear dielectric susceptibility of an insulating material at finite frequency. It measures the third harmonics of the current flowing out of a capacitor with the supercooled liquid as the dielectric layer. It is based on a bridge with two capacitors, and reaches a sensitivity better than 10 −7 (ratio of third to first harmonics). Our first results on supercooled glycerol are presented. They clearly reveal the growing of the correlations close to the glass transition

The effect of magnetic anisotropy-axis alignment of individual nanoparticles on the collective aging behavior in the superspin glass state of a frozen ferrofluid has been investigated. The ferrofluid studied here consists of maghemite nanoparticles (γ-Fe2O3, mean diameter=8.6 nm) dispersed in glycerin at a volume fraction of ∼15%. The low temperature aging behavior has been explored through “zero-field cooled magnetization” (ZFCM) relaxation measurements using superconducting quantum interference device magnetometry. The ZFCM response functions were found to scale with effective age of the system in both textured and nontextured superspin glass states, but with markedly different scaling exponents, μ. The value of μ was found to shift from ∼0.9 in nontextured case to ∼0.6 in the textured case, despite the identical cooling protocol used in both experiments.

The ac nonlinear dielectric response $\chi_3(\omega,T)$ of glycerol was measured close to its glass transition temperature $T_g$ to investigate the prediction that supercooled liquids respond in an increasingly non-linear way as the dynamics slows down (as spin-glasses do). We find that $\chi_3(\omega,T)$ indeed displays several non trivial features. It is peaked as a function of the frequency $\omega$ and obeys scaling as a function of $\omega \tau(T)$, with $\tau(T)$ the relaxation time of the liquid. The height of the peak, proportional to the number of dynamically correlated molecules $N_{corr}(T)$, increases as the system becomes glassy, and $\chi_3$ decays as a power-law of $\omega$ over several decades beyond the peak. These findings confirm the collective nature of the glassy dynamics and provide the first direct estimate of the $T$ dependence of $N_{corr}$., Comment: 22 pages, 6 figures. With respect to v1, a few new sentences were added in the introduction and conclusion, references were updated, some typos corrected.