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Viscoelastic flows

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Viscoelastic fountain on a tubeless straw

Collaboration : Salima Rafaï

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The strong extensional viscosity of viscoelastic polymer solutions allows to suck fluid from a container through a nozzle elevated above the surface of the fluid without any connection between the pumping device and the pool other than the fluid column going up, a phenomenon referred to as tubeless siphon or Fano flow. The resistance to elongation is enough to balance gravity over distances much greater than the capillary length.

When the succion speed is increased, the shear stresses lead to entrainment of more fluid from the pool than is actually aspirated by the nozzle. This results in accumulation of the excess fluid and formation of a growing recirculating zone below the nozzle which periodically falls down to the pool. At even higher succion rates, the recirculation extends over the whole column height leading to the formation of a steady viscoelastic fountain with fluid going up to the nozzle at the center and flowing down outside. The transitions between the different regimes reveal a subtle interplay between the extensional and shear viscosities in viscoelastic flows

Fingering instabilities in a growing gel

Collaboration : Andrew Belmonte (Penn State University), Michael Sostarecz (Monmouth College), Sylvain Zorman (Yale)

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We performed an experimental study of the fingering patterns in a Hele-Shaw cell occurring when a gel-like material forms at the interface between aqueous solutions of a cationic surfactant (cetyltrimethylammonium bromide) and an organic salt (salicylic acid), two solutions known to form a highly elastic wormlike micellar fluid when mixed homogeneously. A variety of fingering instabilities are observed, depending on the velocity of the front (the injection rate), and on which fluid is injected into which. We have found a regime of nonconfined stationary or wavy fingers for which width selection seems to occur without the presence of bounding walls, unlike the Saffman-Taylor experiment. Qualitatively, some of our observations share common mechanisms with instabilities of cooling lava flows or growing biofilms.

Related publication : Fingering instabilities of a reactive micellar interface. T. Podgorski, M. Sostarecz, S. Zorman and A. Belmonte, Phys. Rev. E 76, 016202 (2007)