Quasi-static rheology of aqueous foams

Wednesday, September 16, 2009 - 9:00am - 9:45am
EE/CS 3-180
Georges Debrégeas (École Normale Supérieure)
Aqueous foams, like other macroscopically divided materials, display
intriguing rheological properties. The bubble-scale structure allows for
the existence of frozen stresses within the material which can not
spontaneously relax by thermal activation. Upon shearing, the system
undergoes a series of plastic events which irreversibly modify this
internal stress pattern. Reversely, the internal state of the material
controls to a large extent its mechanical response to shear.

To study this coupling, we have used a two-dimensionally confined aqueous
foam along with a numerical simulation. Through image analysis of the film
network, we can simultaneously probe the plastic flow and the frozen
stress field dynamics under quasi-static shearing. We show that under
oscillatory shear of moderate amplitude, the foam experiences a structural
relaxation that leads to a decrease of the shear modulus and the emergence
of normal stress differences. Upon continous shear, a shear-banding
instability is observed, which coincides with the emergence of spatial
heterogeneities in the internal stress field characteristics. The dynamics
of the internal stress field can be interpreted using a simple statistical

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