1 Phd scholarship in Experimental Soft Matter Physics in France | Institut des Nanosciences de Paris

Title of the proposal: Dynamics of marginal liquid foams

Liquid foams are compact packings of gas bubbles in a surfactant solution. They belong to a broad class of disordered materials including colloidal matter, granular media or soft pastes that exhibit a jamming transition [1, 2]. Above a critical packing density (close to 2/3), bubbles are jammed in the packing so that foam behaves as an elastic solid. When the density is such that the bubbles are just in touch, the foam reaches a marginal state, of extreme fragility, where its mechanical properties become strongly non linear [3]. Under a strong enough applied load, the bubbles switch neighbours and the foam flows. Thus, at the local scale, the key-events of stationary flow are bubble rearrangements. At the macroscopic scale, the flow is characterized by a non-linear friction law, whose origin is not entirely understood [4]. Liquid foams have a strong impact on industrial applications. Indeed, due to their high ratio ofsurface to volume, their low density (compared to that of the liquid) and their unique mechanical properties, they are used in a wide variety of applications such as enhanced oil recovery, ore flotation, or as precursors of aerated solid materials for construction or food industry, for instance [1].

In this Phd thesis, we propose to study, using a multiscale experimental approach, the rearrangement dynamics in a foam close to the marginal state. Duration and spatial extent of rearrangement in a flowing foam will be studied for a packing density approaching its critical value. A major objective will consist in elucidating whether the events occur intermittently with no spatial correlation or if they involve collective avalanche phenomena. The results will allow to understand the quantitative link between local events and macroscopic friction law of a marginal foam. They will also allow to check the fundamental hypotheses of recent models inspired by the physics of glassy materials and that have been proposed for the rheology of foams and emulsions [5]. The spatial and temporal bubble dynamics will be probed using multiple scattering of coherent light “Diffusing-Wave Spectroscopy” [6]. This technique will be combined in situ with a rheometer to apply controlled strain or stress. Model foams will be produced using a microfluidic generator This thesis will be part of the C.N.E.S project “Foams of complex fluids” and the European Space Agency project « Soft Matter Dynamics » in which our team is involved. Their common objective is to study the dynamics of foams, emulsions, and granular matter under microgravity. They benefit from a dedicated instrument on board the International Space Station, for which our team will propose new experiments to probe the dynamics of foams subjected to a mechanical perturbation.

Application Deadline: 1 March 2014