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Elisabeth BOUCHAUD - ESPCI Paris

Mechanical response of confined gels

Résumé :

We have built up devices inspired from the classical Hele‐Shaw cell,
with which we can load both very soft solids and viscous liquids
in order to probe the mechanical response of gels, even quite close
to the sol‐gel transition. This response depends crucially on the
gliding (or no gliding) conditions at the glass plates which constitute
the cell.


For as-­‐received glass plates, polyacrylamide gels stick to the boundaries, we observe, beyond a critical strain which is
independent of the shear modulus, an elastic fingering
instability. This instability is shown to be sudden and hysteretic,
which suggests a sub-­critical character. We show that beyond
the adherence of the gel to the plates, key ingredients
are the incompressibility and the high deformability
of the material.


When surfaces are coated with polymeric brushes, and
we consider the mechanical response of an agar or gelatin
gel to an external tensile stress, there is perfect gliding
at the glass plates. In this case, we can grow quasi-­‐static
cracks at controlled velocities (1 micrometer per second to
1 centimeter per second). We analyze both the shape
of the observed propagating cracks and the displacement
fields around them.


For the largest velocities, we show that cracks exhibit a linear
apex, while they are well fitted by an elastci parabola only
at a large enough distance from the tip. We show that this
crack tip region is a zone of large deformations, which
is the result of a huge viscoelastic energy dissipation.


Biographie :

Elisabeth Bouchaud is a senior scientist working both in the Division of Solid State Physics at CEA-Saclay and in the Ecole de Physique et Chimie Industrielle de Paris. She has given pioneering contributions to the understanding of fracture in heterogeneous materials. She is now working mostly on the mechanics of soft matter. Her research interests include elastic instabilities in soft solids and viscoelastic materials, flow and fracture of colloidal suspensions and heterogeneities and mechanical response of glasses.

Elisabeth BOUCHAUD