Berlin 2008 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 16: SYMPOSIUM Driven Soft Matter I

CPP 16.3: Invited Talk

Tuesday, February 26, 2008, 10:30–11:00, C 130

Spatial cooperativity in soft glassy flows — •Lydéric Bocquet — Lyon University and Technical University Munich

Amorphous glassy materials of diverse nature -- concentrated emulsions, granular materials, molecular glasses -- display complex flow properties, intermediate between solid and liquid, which are at the root of their use in many applications. A classical feature is the very non-linear nature of the flow rule relating stresses and strain rates. In this talk, I will present recent experimental results for the flow of thin layers of concentrated emulsions which, beyond the classical non-linearities of the rheological behaviour, demonstrate the existence of finite size effects in the flow behavior and the absence of an intrinsic local flow rule. In contrast, a rather simple non-local flow rule is shown to account for all the velocity profiles. This non-locality of the dynamics is quantified by a length, characteristic of the cooperativity of the flow at these scales, that is unobservable in the liquid phase (lower concentrations) and that increases with concentration in the jammed phase. These results will be discussed in the context of a generic elasto-plastic description for the flow dynamics, describing the non-local collective dynamics of the localized plastic events occuring during the flow. Beyond its practical importance for applications involving thin layers, e.g. coatings, our assessment of non-locality and cooperativity echoes observations on other glassy, jammed and granular systems, suggesting a possible fundamental universality.