I showed experimentally the existence of a purely elastic, nonviscous fingering instability which arises when air penetrates into an elastomer confined in a Hele-Shaw cell. This instability can be observed either when confined cavities are pressurized but also during debonding experiments. Fingers appear sequentially and propagate within the bulk of the material as soon as a critical strain, independent of the elastic modulus, is exceeded. Key elements in the driving force of the instability are the confinement of the gel and its adhesion to the plates of the cell, which result in a considerable expense of elastic energy during the growth of the air-gel interface. In the case of the linear geometry, studied in a debonding setting, we predict the wavelength and the critical vertical displacement – the onset – of the fingering.
I extended this study to viscoelastic materials where in a model Maxwell fluid, I showed that we can observe a continuous transition from this purely elastic fingering to a viscoelastic one at a speed that corresponds to the material characteristic time. As we show that bulk fingering can be of elastic origin additionally to its viscous counterpart, the Saffman-Taylor instability, we suggest that elasticity and viscous properties can either be stabilizing or destabilizing and lead to rich length scale selection mechanisms.
I also showed experimentaly that, under confinement, as its viscous counterpart, pushing a soft gel inside a stiffer one leads to a fingering instability of wavelength controlled by the shear modulus contrast.
A meniscus fingering instability in viscoelastic fluids. Baudouin Saintyves
Bulk elastic fingering instability in a Hele-Shaw cell. Baudouin Saintyves Physical Review Letters (2013).
. Proc. Natl. Acad. Sci. U.S.A (2013).
PhD Thesis (2013).