A spatio-temporal study of rheo-oscillations in a sheared lamellar phase using ultrasound

We present an experimental study of the flow dynamics of a lamellar phase sheared in the Couette geometry. High-frequency ultrasonic pulses at 36 MHz are used to measure time-resolved velocity profiles. Oscillations of the viscosity occur in the vicinity of a shear-induced transition between a high-viscosity disordered fluid and a low-viscosity ordered fluid. The phase coexistence shows up as shear bands on the velocity profiles. We show that the dynamics of the rheological data result from two different processes: (i) fluctuations of slip velocities at the two walls and (ii) flow dynamics in the bulk of the lamellar phase. The bulk dynamics are shown to be related to the displacement of the interface between the two differently sheared regions in the gap of the Couette cell. Two different dynamical regimes are investigated under applied shear stress: one of small amplitude oscillations of the viscosity ($\delta\eta/\eta\simeq 3$%) and one of large oscillations ($\delta\eta/\eta\simeq 25$%). A phenomenological model is proposed that may account for the observed spatio-temporal dynamicsComment: 16 pages, 17 figures, submitted to Eur. Phys. J.

Modularity and Graph Expansion

We relate two important notions in graph theory: expanders which are highly connected graphs, and modularity a parameter of a graph that is primarily used in community detection. More precisely, we show that a graph having modularity bounded below 1 is equivalent to it having a large subgraph which is an expander. We further show that a connected component $H$ will be split in an optimal partition of the host graph $G$ if and only if the relative size of $H$ in $G$ is greater than an expansion constant of $H$. This is a further exploration of the resolution limit known for modularity, and indeed recovers the bound that a connected component $H$ in the host graph~$G$ will not be split if~$e(H)<\sqrt{2e(G)}$.Comment: Accepted to Innovations in Theoretical Computer Science (ITCS) 202

Modélisation de la torréfaction de plaquettes de bois en four tournant et validation expérimentale à l’échelle d’un pilote continu de laboratoire

Torrefaction is a thermal treatment at low temperature (250-300°C) used to improve biomass properties. Torrefied biomass has a higher energy density, it is more hydrophobic and more brittle. In this study, a one-dimensional numerical model of torrefaction in a rotary kiln has been developed. The wood chips flow, the thermal transfers, the drying step and the torrefaction kinetics have been modelled separately. These submodels have been experimentally validated before being implemented together. The model can thus predict the temperature and the mass loss of wood chips along the kiln. These results are in good agreement with values obtained during torrefaction experiments in the pilot-scale rotary kiln. In parallel, torrefied biomass has been analysed in terms of composition, heating value and structural properties with emphasis on the decrease of grinding energy consumption.La torréfaction est un traitement thermique à basse température (250 à 300 °C) en atmosphère inerte qui permet de modifier les propriétés de la biomasse. La biomasse torréfiée est alors plus dense énergétiquement, plus hydrophobe et plus fragile. Dans cette étude, un modèle numérique de torréfaction en four tournant à une dimension a été développé. Le transport des plaquettes de bois, les transferts thermiques, le séchage ainsi que les cinétiques de torréfaction ont été modélisés séparément. Après confrontation aux résultats expérimentaux, ces différents sous-modèles ont été assemblés dans un modèle global. Le modèle prédit alors l’évolution de la température et de la perte de masse des plaquettes le long du four. Les résultats numériques montrent une adéquation satisfaisante avec les valeurs obtenues lors d’expériences de torréfaction sur un four tournant pilote. Les solides torréfiés ont été analysés et leurs propriétés ont été corrélées à la perte de masse. Il a en particulier été démontré que l'énergie de broyage de la biomasse torréfiée était fortement réduite

Towards local rheology of emulsions under Couette flow using Dynamic Light Scattering

We present local velocity measurements in emulsions under shear using heterodyne Dynamic Light Scattering. Two emulsions are studied: a dilute system of volume fraction $\phi=20$ % and a concentrated system with $\phi=75$ %. Velocity profiles in both systems clearly show the presence of wall slip. We investigate the evolution of slip velocities as a function of shear stress and discuss the validity of the corrections for wall slip classically used in rheology. Focussing on the bulk flow, we show that the dilute system is Newtonian and that the concentrated emulsion is shear-thinning. In the latter case, the curvature of the velocity profiles is compatible with a shear-thinning exponent of 0.4 consistent with global rheological data. However, even if individual profiles can be accounted for by a power-law fluid (with or without a yield stress), we could not find a fixed set of parameters that would fit the whole range of applied shear rates. Our data thus raise the question of the definition of a global flow curve for such a concentrated system. These results show that local measurements are a crucial complement to standard rheological tools. They are discussed in light of recent works on soft glassy materials.Comment: 13 pages, 21 figures, submitted to Eur. Phys. J.

Gemini and Lowell observations of 67P/Churyumov−Gerasimenko during the <i>Rosetta</i> mission

We present observations of comet 67P/Churyumov−Gerasimenko acquired in support of the Rosetta mission. We obtained usable data on 68 nights from 2014 September until 2016 May, with data acquired regularly whenever the comet was observable. We collected an extensive set of near-IR J, H and Ks data throughout the apparition plus visible-light images in g', r', i' and z' when the comet was fainter. We also obtained broad-band R and narrow-band CN filter observations when the comet was brightest using telescopes at Lowell Observatory. The appearance was dominated by a central condensation and the tail until 2015 June. From 2015 August onwards, there were clear asymmetries in the coma, which enhancements revealed to be due to the presence of up to three features (i.e. jets). The features were similar in all broad-band filters; CN images did not show these features but were instead broadly enhanced in the southeastern hemisphere. Modelling using the parameters from Vincent et al. replicated the dust morphology reasonably well, indicating that the pole orientation and locations of active areas have been relatively unchanged over at least the last three apparitions. The dust production, as measured by A(0°)fρ peaked ∼30 d after perihelion and was consistent with predictions from previous apparitions. A(0°)fρ as a function of heliocentric distance was well fitted by a power law with slope −4.2 from 35 to 120 d post-perihelion. We detected photometric evidence of apparent outbursts on 2015 August 22 and 2015 September 19, although neither was discernible morphologically in this data set

Power transformer model in railway applications based on bond graph and parameter identification

Validation and verification are the most important issues in railway applications due to cost and security reasons. Therefore, having a model of the system would be necessary in this case. Due to non-ideal test conditions in industrial applications, an accurate parameter identification process has to be defined. In this paper, bond graph method is used to model energy exchanges within components of a traction chain. More precisely, the non-linear transformer model and its parameter identification is studied. In the case of non-ideal test conditions, the usual Jiles-Atherton parameter identification procedure can not be performed. Regarding state of the art, the Jiles-Atherton parameter identification is discussed. It is highlighted that an uncomplete hysteresis cycle, including extremum point and coercive field are mandatory for an accurate parameter identification. The proposed identification process is applied to a real application case. The obtained parameters are then inserted into the overall system model. The consecutive simulations are compared to experimental data obtained through traction chain test bench

Uncertainty-aware Flexibility Envelope Prediction in Buildings with Controller-agnostic Battery Models

Buildings are a promising source of flexibility for the application of demand response. In this work, we introduce a novel battery model formulation to capture the state evolution of a single building. Being fully data-driven, the battery model identification requires one dataset from a period of nominal controller operation, and one from a period with flexibility requests, without making any assumptions on the underlying controller structure. We consider parameter uncertainty in the model formulation and show how to use risk measures to encode risk preferences of the user in robust uncertainty sets. Finally, we demonstrate the uncertainty-aware prediction of flexibility envelopes for a building simulation model from the Python library Energym.Comment: 7 pages, 3 figures. Submitted to the 2023 American Control Conference (ACC

Atomic motifs govern the decoration of grain boundaries by interstitial solutes

Grain boundaries, the two-dimensional (2D) defects between differently oriented crystals, control mechanical and transport properties of materials. Our fundamental understanding of grain boundaries is still incomplete even after nearly a century and a half of research since Sorby first imaged grains. Here, we present a systematic study, over 9 orders of magnitude of size scales, in which we analyze 2D defects between two neighboring crystals across five hierarchy levels and investigate their crystallographic, compositional, and electronic features. The levels are (a) the macroscale interface alignment and grain misorientation (held constant here); (b) the systematic mesoscopic change in the inclination of the grain boundary plane for the same orientation difference; (c) the facets, atomic motifs (structural units), and internal nanoscopic defects within the boundary plane; (d) the grain boundary chemistry; and (e) the electronic structure of the atomic motifs. As a model material, we use Fe alloyed with B and C, exploiting the strong interdependence of interface structure and chemistry in this system. This model system is the basis of the 1.9 billion tons of steel produced annually and has an eminent role as a catalyst. Surprisingly, we find that even a change in the inclination of the GB plane with identical misorientation impacts GB composition and atomic arrangement. Thus, it is the smallest structural hierarchical level, the atomic motifs, which control the most important chemical properties of the grain boundaries. This finding not only closes a missing link between the structure and chemical composition of such defects but also enables the targeted design and passivation of the chemical state of grain boundaries to free them from their role as entry gates for corrosion, hydrogen embrittlement, or mechanical failure

Velocity profiles in shear-banding wormlike micelles

Using Dynamic Light Scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize on the complex, non-Newtonian nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let