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Viscoelastic behavior of nanoconfined liquids between bare and decorated solids

Frédéric Restagno et Liliane Léger

à 11h en salle C. Brot

Mechanical properties of confined liquids down to molecular size are still poorly understood and continue to create a debate, despite their technological or fundamental importance. Some experiments with confined organic liquids show an abrupt liquid to solid transition for confinements higher than 6-7 molecular diameters. This means that a yield stress is required to shear the confined liquid. Some other experiments, on the contrary, present a continuous increase of the viscosity when the confinement increases. The case where a simple liquid is confined between two surfaces with end-grafted polymer chains swollen by the liquid is even more complicated.

Using a Surface forces apparatus (SFA) as a nanorheometer, we have investigated the mechanical behavior of different Newtonian liquids confined down to molecular sizes between bare surfaces or modified surfaces with end grafted chains. More precisely, in the dynamic mode of our SFA, a millimetric sphere of pyrex oscillates in the liquids at a frequency of some tens of hertz, with a small amplitude (fraction of nanometers) and continuously approaches a plane of pyrex (roughness <1 nm). The two components (in phase and in quadrature) of the force exerted by the liquid squeezed between the sphere and the plate is measured, allowing for a determination of the rheological properties of the fluids.

We will show that it is then possible to measure the penetration of the flow of a solvent in an Alexander-de Gennes brush. We will also show, comparing the observed viscoelastic behavior of a compressed polymer brush to available models, that the mechanical properties of the confining solid surfaces should never be neglected in nanorheology.


Fluides & Matériaux Complexes