A surface force apparatus for nanorheology under large shear strain

We describe a surface force apparatus designed to probe the rheology of a nanoconfined medium under large shear amplitudes (up to 500 $\mu$m). The instrument can be operated in closed-loop, controlling either the applied normal load or the thickness of the medium during shear experiments. Feedback control allows to greatly extend the range of confinement/shear strain attainable with the surface force apparatus. The performances of the instrument are illustrated using hexadecane as the confined medium

Non-linear rheology of a nanoconfined simple fluid

We probe the rheology of the model liquid octamethylcyclotetrasiloxane (OMCTS) confined into molecularly thin films, using a unique Surface Forces Apparatus allowing to explore a large range of shear rates and confinement. We thus show that OMCTS under increasing confinement exhibits the viscosity enhancement and the non-linear flow properties characteristic of a sheared supercooled liquid approaching its glass transition. Besides, we study the drainage of confined OMCTS via the propagation of "squeeze-out" fronts. The hydrodynamic model proposed by Becker and Mugele [Phys. Rev. Lett. {\bf 91}, 166104 (2003)] to describe such front dynamics leads to a conclusion in apparent contradiction with the dynamical slowdown evidenced by rheology measurements, which suggests that front propagation is not controlled by large scale flow in the confined films

Viscoelastic transient of confined Red Blood Cells

The unique ability of a red blood cell to flow through extremely small microcapillaries depends on the viscoelastic properties of its membrane. Here, we study in vitro the response time upon flow startup exhibited by red blood cells confined into microchannels. We show that the characteristic transient time depends on the imposed flow strength, and that such a dependence gives access to both the effective viscosity and the elastic modulus controlling the temporal response of red cells. A simple theoretical analysis of our experimental data, validated by numerical simulations, further allows us to compute an estimate for the two-dimensional membrane viscosity of red blood cells, $\eta_{mem}^{2D}\sim 10^{-7}$ N$\cdot$s$\cdot$m$^{-1}$. By comparing our results with those from previous studies, we discuss and clarify the origin of the discrepancies found in the literature regarding the determination of $\eta_{mem}^{2D}$, and reconcile seemingly conflicting conclusions from previous works

Frictional dissipation of polymeric solids vs interfacial glass transition

We present single contact friction experiments between a glassy polymer and smooth silica substrates grafted with alkylsilane layers of different coverage densities and morphologies. This allows us to adjust the polymer/substrate interaction strength. We find that, when going from weak to strong interaction, the response of the interfacial junction where shear localizes evolves from that of a highly viscous threshold fluid to that of a plastically deformed glassy solid. This we analyse as resulting from an interaction-induced ``interfacial glass transition'' helped by pressure

Energie et compétitivité

Dans un contexte de renchérissement prévisible de l’énergie au cours des vingt prochaines années, orienter l’effort d’innovation industrielle et l’offre de biens et services vers des technologies économes en énergie est une nécessité. Toutefois, une hausse des prix de l’énergie plus marquée en France que chez nos concurrents pénaliserait la compétitivité à court terme de l’industrie française. Cette Note expose les termes de l’arbitrage que doit affronter la France entre la préservation d’un élément significatif de sa compétitivité à court terme (le coût relativement faible de son énergie en particulier électrique) et la nécessaire transformation de ses avantages comparatifs à moyen-long terme (sous l’effet d’une vérité des prix énergétiques). À partir d’un travail économétrique original portant sur les exportations des entreprises françaises, nous estimons qu’une hausse de 10 % des prix de l’électricité en France réduirait la valeur des exportations en moyenne de 1,9 % et qu’une même augmentation du prix du gaz les réduirait de 1,1 %. La perte de compétitivité est sensiblement plus marquée pour les plus gros exportateurs, parti- culièrement dans les secteurs fortement dépendants de l’énergie. Cet effet négatif de court terme est à mettre en regard de l’effet de signal d’une hausse des prix de l’énergie sur les spécialisations à moyen-long terme, afin que la France ne reste pas en arrière dans la course à l’innovation « verte ». Nous tirons de cette analyse plusieurs enseignements. Tout d’abord, il convient d’annoncer la hausse des prix de l’énergie, de manière crédible, afin que les agents économiques l’intègrent dans leurs calculs et réorientent leurs choix de consommation et de production. Afin de limiter les effets négatifs d’un renchérissement de l’énergie sur la compétitivité à court terme, nous recommandons que la taxation supplémentaire de l’énergie soit utilisée pour réduire le coût du travail, une grande prudence quant au rythme de déclassement des équipements nucléaires historiques, dont le coût au kWh est particulièrement performant, une imputation différenciée de la charge de service public en fonction de l’intensité énergétique (comme en Allemagne) et une convergence des approches au niveau européen pour ce qui concerne les coûts de réseau.Les notes du Conseil d'analyse économie, n°6

Lift at low Reynolds number

Lift forces are widespread in hydrodynamics. These are typically observed for big and fast objects, and are often associated with a combination of fluid inertia (i.e. large Reynolds numbers) and some specific symmetry-breaking mechanism. In contrast, low-Reynolds-number flows are usually overdamped and do not exhibit such peculiar and interesting features. However, the inclusion of boundary effects qualitatively changes this picture. Indeed, in the context of soft and biological matter, recent studies have revealed the emergence of novel lift forces generated by boundary softness, flow gradients and/or surface charges. The aim of the present review is to gather and analyse this corpus of literature, in order to identify and unify the questioning within the associated communities, and pave the way towards future research around lift effects at low Reynolds numbers

Rate effects on layering of a confined linear alkane

We perform drainage experiments of a linear alkane fluid (n-hexadecane) down to molecular thicknesses, and focus on the role played by the confinement rate. We show that molecular layering is strongly influenced by the velocity at which the confining walls are approached: under high enough shear rates, the confined medium behaves as a structureless liquid of enhanced viscosity for film thickness below $\sim$10 nm. Our results also lead us to conclude that a rapidly confined film can be quenched in a metastable disordered state, which might be related with recent intriguing results on the shear properties of confined films produced at different rates [Zhu and Granick, Phys. Rev. Lett. {\bf 93}, 096101 (2004)]

Viscoelastic transient of confined Red Blood Cells

The unique ability of a red blood cell to flow through extremely small microcapillaries depends on the viscoelastic properties of its membrane. Here, we study in vitro the response time upon flow startup exhibited by red blood cells confined into microchannels. We show that the characteristic transient time depends on the imposed flow strength, and that such a dependence gives access to both the effective viscosity and the elastic modulus controlling the temporal response of red cells. A simple theoretical analysis of our experimental data, validated by numerical simulations, further allows us to compute an estimate for the two-dimensional membrane viscosity of red blood cells, $\eta_{mem}^{2D}\sim 10^{-7}$ N$\cdot$s$\cdot$m$^{-1}$. By comparing our results with those from previous studies, we discuss and clarify the origin of the discrepancies found in the literature regarding the determination of $\eta_{mem}^{2D}$, and reconcile seemingly conflicting conclusions from previous works