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31 mars : séminaire "Viscous dissipation in confined liquid films"

Frieder Mugele –
University of Twente – Physics of Complex Fluids

- Jeudi 31 mars 2011 à 10h30 salle des thèses de l’ENSIACET.

Liquids behave differently on the nanometer scale as compared to
macroscopic scales. Molecular interaction forces give rise to novel
features that cannot be predicted on the basis of macroscopic concepts.
This lecture focuses the properties of confined liquid films studied by
high resolution atomic force microscopy (AFM) as well as molecular
dynamics simulations.

Thin liquid films confined between two solid surfaces self-assemble into
discrete molecular layers. This structure gives rise to oscillatory
solvation forces and produces pronounced maxima and minima in the
amplitude and phase signal of an AFM tip approaching a solid surface.
While these conservative forces are well-established, the consequences
for the dissipation are heavily debated. We use small amplitude AFM
spectroscopy to extract both conservative and dissipative forces from
amplitude-force-distance measurements obtained using both acoustic and
magnetic driving. Our measurements demonstrate the existence of peaks in
the damping that arise whenever the tip sample distance corresponds to a
half-integer number of liquid layers. The excess damping corresponds to
viscosity enhancement of two to three orders of magnitude compared to
the bulk.

To verify these observations we perform equilibrium molecular dynamics
simulations for a confined Lennard-Jones fluid. Making use of the
fluctuation-dissipation theorem, we extract the damping from the
time-autocorrelation of the force fluctuations on the solid surfaces.
The numerical calculations reproduce all the characteristic features of
the measurements and thereby yield a consistent of dissipation in
confined liquids. The methods presented here will be applicable for a
wide range of problems requiring quantitative force measurements in
ambient fluid.

- Contact : Olivier MASBERNAT


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