Recherche

In order to illustrate the team’s research, we selected some highlights.

Microfluidic production of blood platelets

Platelets are small enucleate cells that circulate in blood and are responsible for the arrest of bleeding. They are formed by fragmentation of larger cells called megakaryocytes (MKs). Experiments showed that exposing MKs to shear could enhance platelet production. Mathilde Reyssat has developed a new and rapid method for producing blood platelets in vitro from cultured megakaryocytes based on a microfluidic device. A wide array of specific protein-coated micro-pillars act as anchors on megakaryocytes, which are then subjected to hydrodynamic shear. This in turn induces the elongation of megakaryocytes and finally their rupture into platelets and pro-platelets. For the first time a device is capable of simultaneously handling millions of MKs and leading to their fragmentation into platelets after a simple 2-hour perfusion of MK suspension. This original device is a major breakthrough in the realization of bioreactor dedicated to platelet production.

Fig 5 : Blood platelets production in a micro-fluidic chip

This work has been patented and published in Scientific Reports :
A. Blin, A. Le Goff, A. Magniez, S. Poirault-Chassac, B. Teste, G. Sicot, K. A. Nguyen, F. S. Hamdi, M. Reyssat, D. Baruch. "Microfluidic model of the platelet-generating organ : beyond bone marrow bio mimetics." Scientific Report. 6 : 21700 (2016)

Cholesteric shells : disclosing the inner structure of defects

Confining liquid crystals to spherical geometries imposes topological constrains to the system that typically result in fundamentally new behaviors. Most of the studies have been focused on nematic phases. We have uncovered a number of new shell configurations, varying in number and type of defects, by adding chirality to the nematic phase (cholesteric phase).

We have studied the transition between two limit configurations :
• Monovalent shells (with a single radial defect), analogous to a Dirac monopole
• Bivalent shells (with two defects spanning the shell).

Bringing together experiments and numerical simulations, we have disclosed the complex inner structure associated to those defects and revealed that the transition involves a fascinating waltz dynamics, where the two defects come closer while turning around each other.

Fig 6 : Two +1 topological charge defects winding together to form one single + 2 charge defect in a cholesteric liquid crystal shell

These results have been published in two papers :
• Darmon, M. Benzaquen, D. Seĉ, S. Čopar, O. Dauchot and T. Lopez-Leon "Waltzing route toward double-helix formation in cholesteric shells", PNAS, Vol. 113, 9469 (2016)
• Darmon, M. Benzaquen, S. Čopar, O. Dauchot and T. Lopez-Leon "Topological defects in cholesteric liquid crystal shells” Soft Matter, 12, 9280 (2016)

First experimental evidence of the Gardner transition

Glasses has been an inspiring field of research for the study of all sorts of disordered systems including financial markets, neurons, proteins, and algorithmic problems. Recent breakthrough results predict a glass-to-glass transition, towards the so-called Gardner phase, the existence of which may well root many of these fascinating problems. We have obtained the very first experimental evidence of this transition in a model granular glass former. Beyond the validation of very recent theoretical developments, it demonstrates their robustness against the many differences between a thermal numerical hard sphere glass former and a mechanically vibrated layer of granular disks ! On the methodological side, it illustrates the fruitfulness of our long-term close collaboration with our theoretician colleagues, now part of the Simons Collaboration on “Cracking the glass problem”.

Fig 7 : The Gardner transition revealed in a model granular glass former (artist view of the free energy landscape, bidisperse disks changing contacts, cage sizes vs. cage overlap)

A. Seguin, O. Dauchot, “Experimental Evidence of the Gardner Phase in a Granular Glass.”, PRL 117, 228001–5 (2016).
Editor’s suggestion & News and View.

First direct measurement of Pressure in an active liquid

Developing thermodynamics for out of equilibrium systems of dissipative particles has been a long-lasting effort driven by the impressive success of equilibrium thermodynamics. Recently, defining pressure in systems of active particles, the specificity of which is to turn the injected energy into directed motion, has attracted a lot of attention. While at equilibrium, mechanical, hydrodynamic and thermodynamic pressures are all equal quantities, which inherit their mutual properties, it is a priori not the case in active liquids. Probing experimentally the mechanical pressure exerted by a set of respectively passive isotropic and active polar disks onto two different flexible one-dimensional membranes, we have shown that the mechanical pressure, in the active case, strongly depends on the membrane in use, and is thus not a state variable. The underlying dynamical mechanisms are also responsible for a new form of instability.

G. Junot, G. Briand, R. Ledesma-Alonso, and O. Dauchot, “Active vs. Passive Hard Disks Against a Membrane : Mechanical Pressure and Instability”, PRL 119, 243459-2 (2017). Editor’s suggestions.

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Equipe

Olivier DAUCHOT, Directeur de Recherche au CNRS, est à la tête de l’équipe Effets Collectifs et Matière Molle. Chercheurs et enseignants-chercheurs (...) 

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Publications (depuis 2012)

G. Junot, G. Briand, R. Ledesma-Alonso, and O. Dauchot, Active versus Passive Hard Disks against a Membrane : Mechanical Pressure and Instability (...) 

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Informations Pratiques

Directeur
Elie Raphaël
elie.raphael (arobase) espci.fr

Directeur adjoint :
Olivier Dauchot
olivier.dauchot (arobase) espci.fr

Gestionnaire ESPCI :
Stéphanie Ledoux
stephanie.ledoux (arobase) espci.fr

Gestionnaire CNRS :
Solange Rogue
solange.rogue (arobase) espci.fr

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