Elisabeth Bouchaud's research interests focus on the fracture and the propagation of failure in both hard and soft solid materials.

Elisabeth is equally passionate about bridging the arts and the sciences. She is the Director of Théâtre de la Reine Blanche

Fracture of soft solids

G.Gimenes, M. Lefranc, E. Bouchaud

Extreme Mechanics

Soft Matter 2018

 

Soft solids are ubiquitous in everyday life - from the yogurt you eat to the cream you apply on your face, through mud, hair gel, fresh cement or bread dough -. It may be quite important to control their fracture properties: soft polymeric capsules containing a precious drug or cream should break only under specific circumstances, and artificial skin should not be torn at every movement of the body.

However, these fracture properties are ill-understood. The first difficulty arises from the fact that, in these materials, because of their softness, deformations become large even when the applied stress is moderate. Dealing with large deformations is indeed difficult because there is no theoretical framework comparable to Linear Elastic Fracture Mechanics.

These materials cannot even be gripped and pulled apart as it is done in conventional tests for hard solids. Hence, we had to resort to microfluidic devices in order to apply a displacement and trigger controlled fracture. Progressing cracks can be observed in conventional or confocal microscopy (Lefranc, Extreme Mechanics).

Another source of difficulty arises from the fact that these materials are often viscoelastic, and they may flow while they break.  In order to study this phenomenon, we have studied a colloidal suspension made of small silica beads in water (Ludox). Because the surfaces of the silica beads are all charged negatively, they repel each other, and the suspension reacts to stress as a liquid. If salt is added, the repulsive electrostatic interactions between beads are screened out, and when the salt concentration is raised, an increasingly cohesive paste is obtained. By studying both the shape of the observed cracks and the displacement field in the vicinity of the crack tip, we are able to characterise the region where non-linear processes occur, and to disclose the relevant time and length scales.

Another source of dissipation may arise from the disordered structure of theses materials: micro-cracks may form ahead of the crack tip, and create a damaged zone. Because Ludox colloids are invisible in optics (their diameter being ~20 nanometers), we could not study such mechanisms in this case. On the contrary, together with Peter Schall and his group at the University of Amsterdam, we used Casimir gels where colloids are really PNIPAM microgels spheres of about 1µm.  We could tune the cohesively of the suspension by varying the temperature, and study the extent of the damaged zone as a function of this control parameter.

On a chip mechanical test cell. Hele–Shaw chamber containing the sample of gel (in red) surrounded by fluorinated oil (in blue). Oil is sucked out of the chamber from points A, which results in a displacement of the gel boundaries in the y-direction. In this plane stress configuration, a mode I crack grows in the x-direction.

Crack edges are smooth and Williams linear elastic crack shape fits perfectly over the whole region of observation.

A mesoscopic study of plastic damage in amorphous materials 

C.M. Cejas, G.Gimenes, P. Tabeling, E. Bouchaud

Fracture mechanisms in amorphous systems are still not fully understood. Because of their disordered nature, these materials break involving dissipative processes such as secondary cracking ahead of the main crack tip and local structural rearrangements. The aim of our work is to make and fracture amorphous systems composed of big enough "atoms", so that the above mentioned mechanisms can be observed optically.


Our amorphous materials being soft gels, we use microfluidics both to fabricate concentrated emulsions using flow focusing technique. These emulsions are then polymerized and their controlled fracture is investigated . We synthesize soft emulsion gels based on a difunctional acrylic monomer through photopolymerization in order to
get an "atom" size of ~50µm. We vary the rheological properties of these emulsions by regulatingconditions for polymerization. Using conventional optical microscopy, we hydraulically fracture these soft gels at controlled ow rates and examine both micro-cracking and plastic events, i.e. the local irreversible displacements around the crack tip.

Random rearrangement of solidified emulsions