Physical aging of supercooled liquids

Collaborator : F. Ladieu

Left: Cryocooler. Right: Experimental setup allowing to quickly heat a liquid or to measure its dielectric response.
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Molecular dynamics of polar liquids

Collaborator : F. Ladieu, P-M. Déjardin

Left: Simulation box with 2160 glycerol molecules. Right: Distribution of dipole cross correlation for different ranks of the Legendre polynomial.
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Morphogenesis of glacial structures

Collaborators : N. Taberlet, N. Plihon, V. Langlois

Left: A glacier table. Right: Dirt cones on the Mer de glace, Alps.

We studied the formation of glacier tables and dirt cones observed on temperate zone glaciers. Glacier tables consist of a foot of ice supporting rocks measuring over a metre in size. Dirt cones form under layers of sand a few centimetres thick. The aim of this work was to use a combined laboratory and field approach to study the influence of parameters controlling ice melt on the formation dynamics of these structures.

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Slip of polymer liquids

Collaborators : F. Restagno, L. Léger, M. Grzelka, A. Chennevière

1. A step in the grafting of polymer chains. 2. Slip measurement setup 3. Polymer melt (PDMS) with fluorescent probes.

My PhD work, under the supervision of F. Restagno and L. Léger, involved an experimental study of the sliding properties of liquid polymers. Unlike simple liquids, liquid polymers do not necessarily respect the classical condition of non-slip at the wall.

A new method for measuring slip

The first part of my PhD involved developing a new method for measuring the slip of polymer liquids by tracking a photolysed pattern in a fluorescent fluid. This method, which is an evolution of the one previously used in the group, provides direct access to the fluid displacement field during flow [1].

Velocity profile of a PDMS melt sheared between two plates.

Using this new device, I experimentally investigated the effect of surface nature, liquid viscosity and temperature on the sliding of polymer melts. I was able to show that the slip effect is controlled by the viscosity of the melts and by a molecular friction coefficient that depends only on the chemical natures of the liquid and the surface. In particular, this coefficient of friction is the same for a melt as for an elastomer made from the same polymer [2].

Friction stress as a function of slip velocity, for a PDMS melt (red) and for a PDMS elastomer (blue) on the same solid surface.
Effect of temperature on the sliding of melts

By measuring the slip of melts and elastomers at different temperatures, we have shown that the temperature dependence of the coefficient of friction is an activated process whose activation energy depends on the surface. This can lead to increasing or constant sliding effects, depending on the system under consideration [4].

Interfacial friction coefficient, as a function of inverse temperature, for a PDMS melt and a PDMS elastomer on two types of solid surfaces.
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Glass transition in thin polymer films

Collaborators : F. Restagno, L. Léger

The glass transition in thin polymer films exhibits a surprising behavior: the glass transition temperature shows a significant decreases when the film thickness is less than a few tens of nm. During my M2 internship and at the begining of my PhD, I investigated the influence on this effect of the interaction between a polystyrene film and its substrate by grafting polymer chains on the surface [1].

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