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Bulk fingering instabilities in soft solids and in complex fluids

le 7 juin 2012
13H30

Elisabeth BOUCHAUD Directrice de Recherche au CEA, ESPCI, Gif-sur-Yvette

Elisabeth BOUCHAUD

Elisabeth BOUCHAUD

Instabilities in viscous liquids confined in Hele-Shaw cells have attracted considerable attention during the past. On the contrary, instabilities arising in soft elastic materials have started to be studied much more recently.

 In most cases,  fingering in confined elastomers arises at the interface with an elastic plate: Interfacial crack fronts lose their stability due to a competition between bulk and surface effects.

We present here a bulk fingering instability arising in a hyperelastic polyacrylamid gel.

Experiments are performed in two types of Hele-Shaw cells: One is the classical setup used for studying liquids, and the other one  has mobile sides which can be pulled at a prescribed velocity. In both cases, an instability is observed when the strain exceeds a critical value which is independent of the gel shear modulus. By colouring the material, we were able to show that fingers grow within the thickness of the cell, leaving behind layers of gels sticking to the upper and lower glass plates. It is shown that the wavelength (width of the fingers) increases non linearly with the spacing of the latter. Finally, at a later stage, when these layers are stretched beyond the fracture threshold, an interfacial crack nucleates and propagates, at a strain which depends on the shear modulus.
Similar instability is observed in a Maxwell liquid constituted by oil-in-water droplets microemulsion where the droplets are connected to each other by a telechelic polymer. In this case, the critical strain depends on the flow rate.

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Type :
Séminaires - conférences
Lieu(x) :
Campus de Cachan


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