Présentation. – Proceedings : https://phdsymp2020.sciencesconf.org/data/pages/Proceedings_phdsymp_2021.pdf
– Youtube presentation : https://www.youtube.com/watch?v=TosfqmLU9oo
Self-sensing concrete, also known as “Smart concrete”, is obtained by including electrically conductive fibers in cement-based materials. These fibers may allow to reduce electrical resistivity and develop a piezoresitive behaviour. Smart Concrete could therefore be simultaneously both a structural and a sensing material, which eliminates the need for external instrumentation in Structural Health Monitoring.
By increasing the fiber volume fraction within the cement matrix, the electrical resistivity of the material is reduced once percolation threshold is reached. Above this percolation threshold, the fiber content is high enough to allow conductive particles to be in contact or very close to each other, thus creating a continuous conductive network within the insulative matrix. Sand’s presence was stated to have an influence on resistivity in case of fibred mortar: a high sand content may prevent the network of conductive fibers from percolating. This phenomenon is referred to as “double percolation”: the cement paste needs to be a continuous phase between sand aggregates in order to allow fibers to maintain their efficiency in reducing the electrical resistivity of composites.
However, little attention has been given to the impact of the size of sand grains on the electrical percolation. This work intends to study the effect of the grain size distribution and volume fraction of sand within mortars containing various fiber volume fractions. The results confirm. the “double percolation” phenomena: when the volume fraction of sand is close to its maximum packing density, the addition of fibres was not as effective in reducing the electrical impedance of mortar samples. In addition, sand’s grain size distribution proved an influence on impedance of mortar: fine sand showed higher impedance compared to standard sand, especially in case of high sand volume fraction. This could be related to the smaller maximum packing density in case of fine sand, where distance between particles would be in average reduced. This effect, combined with the higher number of insulative particles, could probably disrupt the continuity of the conductive network of fibres within mortar.
Séminaire IUSTI – 27 nov. 2020 – 11h salle 250
Liquid-liquid phase separation in living cells: like oil in water?
Pierre Ronceray – LGC, Toulouse
Many intracellular bodies in eukaryotic, both in the nucleus and cytoplasm, are now understood to be membrane-less liquid condensates that separate from the cellular environment through a liquid-liquid demixing, often compared to oil in water. In this talk, I will discuss two of the ways in which this phase separation is different from usual binary liquids. First, the molecular interactions that govern protein phase separation are often strong and specific, which lead to unusual properties such as non-monotonic valence effects and sharp composition-dependence of the viscosity – effects that cells could use to dynamically tune the properties of these organelles. Second, the cellular environment surrounding these droplets has complex mechanical properties, which oppose droplet growth and shifts phase boundaries. I predict that this can result in novel, yet-unobserved phases of size-limited “nanodroplets”.
Séminaire IUSTI – 13 nov. 2020 – 11h salle 250
Titre à venir
Martin Trulsson – Lund University, Sweden
Résumé à venir
Séminaire IUSTI – 16 oct. 2020 – 11h salle 250
Mechanics of cell contacts during tissue morphogenesis
Raphaël Clément – IBDM, AMU, Marseille
Tissue shape changes during embryonic development can be achieved by the local generation of contractile forces at cell contacts. Understanding how such forces are converted into cell shape changes is essential to our understanding of tissue morphogenesis. To that end we analyzed in vivo the deformation dynamics of cell contacts resulting from both endogenous forces (Myosin-II contractile activity) and external forces (optical tweezers). It is consistent with a simple viscoelastic framework, with internal dissipation occurring on the minute timescale. Interestingly this timescale is commensurate with that of endogenous force generation, allowing efficient remodelling of cell contacts. We also show that the turnover of cytoskeletal filaments, by renewing the mechanical structure of the cell, participates in dissipation and therefore affects the reversibility of deformations.
I’ll also discuss recent results on the mechanism of spontaneous symmetry breaking that underlie the asymmetric division of the oocyte during mammalian meiosis.
Séminaire exceptionnel – 08 oct. 2020 – 11h salle 250
Numerical methods for predicting heat and moisture transfer through porous building materials
Suelen Gasparin – LaSIE, La Rochelle
In the current environmental context, it is essential to know how to model the phenomena of heat and mass transfer through porous elements of buildings walls. Therefore, the objective of this seminar is to present innovative numerical methods to efficiently sim- ulate hygrothermal transfer phenomena. First, we describe the physical model and we introduce the scientific issues associated with the development of the numerical models. Then, we present several improved numerical methods applied to real case studies, such as multilayered-walls with nonlinear transfer, parametric problems, optimum insulation thickness, moisture buffer effect and zonal building model. Particularly, a reduced-order model based on spectral methods is presented for the simulation of multidimensional transfer. A comparison with experimental data is also carried out. To summarize, a global overview of the discussed numerical methods is proposed.
Séminaire IUSTI – 2 oct. 2020 – 11h salle 250
Mazi Jalaal – DAMTP, Cambridge, UK
Viscoplastic or yield stress materials can behave like solids or fluids. Such materials, if not sufficiently stressed, behave like an elastic solid, but once the stress exceeds a critical value (the yield stress), the material deforms like a viscous fluid (typically with a nonlinear viscosity).
I will discuss the effect of the yield stress on spreading droplets. I use experiments, asymptotic solutions, and numerical simulations to explain the dynamics and final shape of the droplets. Later, I will show how one can externally control the shape of a droplet, using temperature. For that, I will first present the rheological properties of a thermo-responsive material that undergoes sol(Newtonian)-gel(yield stress) transition upon heating. Then, I show the final diameter of a thermo-responsive droplet can be controlled by simply changing the surface temperature. In the same part of the thesis, we introduce a novel experimental method based on optical coherence tomography to identify the solidified region inside a droplet.
Eventually, I will briefly discuss the other applications of viscoplastic droplets.
Please consult the conference web site for updates http://www.iphmt20.fr
Événement co-organisé par des membres des Axes de Recherches : Combustion, Risques et Génie Civil & Écoulements Compressibles, Ondes de Choc et Interfaces.
Site web : http://www.iphmt20.fr
Séminaire IUSTI – 4 septembre 2020 – 11h salle 250
Titre à venir
Olivier Millet – LaSIE, La Rochelle
Résumé à venir
Séminaire exceptionnel IUSTI – 29 juin 2020 – 11h salle 250
Titre à venir
Dongwhi Choi – ?, Corée du Sud
Résumé à venir
M.Souzy, H.Lhuissier, Y.Méheust, T.Le Borgne and B.Metzger
Aix Marseille Université, CNRS, IUSTI, UMR 7343, 13453 Marseille, France
Geosciences Rennes, UMR 6118, Université de Rennes 1, CNRS, 35042 Rennes, France
(Received 29 July 2019; revised 13 December 2019; accepted 5 February 2020)
Using index-matching and particle tracking, we measure the three-dimensional velocity field in an isotropic porous medium composed of randomly packed solid spheres. This high resolution experimental dataset provides new insights into the dynamics of dispersion and stretching in porous media. Dynamic-range velocity measurements indicate that the distribution of the velocity magnitude, U, is flat at low velocity (p(U) ~ U^0). While such a distribution should lead to a persistent anomalous dispersion process for advected non-diffusive point-particles, we show that the dispersion of non-diffusive tracers nonetheless becomes Fickian beyond a time set by the smallest effective velocity of the tracers. We derive expressions for the onset time of the Fickian regime and the longitudinal and transverse dispersion coefficients as a function of the velocity field properties. The experimental velocity field is also used to study, by numerical advection, the stretching histories of fluid material lines. The mean and the variance of the line elongations are found to grow exponentially in time and the distribution of elongation is log-normal. These results confirm the chaotic nature of advection within three-dimensional porous media. By providing the laws of dispersion and stretching, the present study opens the way to a complete description of mixing in porous media