Blown liquid curtains and webbed jets (H. Lhuissier, MSC Paris)

I will discuss two experiments : the opening of a Newtonian liquid curtain and the impingement of a viscoelastic jet.
1. A liquid curtain, i.e., a vertical liquid sheet falling freely in the air, may be `fragile'. When it is punctured it usually either advects the hole, or entirely disrupts. We study, here, a controlled and localized perturbation induced by two facing air jets that squeeze the sheet symmetrically (a). For a sucient velocity in the jets, a steady hole forms in the wake. We present a simple unidimensional model for a narrow wake in which the dynamics is controlled by inertia and viscosity. When capillary waves are negligible, it quantitatively describes the thinning of the lm and the steady location of the hole.
2. A jet of a Newtonian liquid impinging on a wall at right angle spreads as a thin liquid sheet which preserves the radial symmetry of the jet. We observe that for a viscoelastic jet (solution of polyethylene glycol in water) this symmetry can break : Close to the wall, the jet develops radial wings (pointed out in (b), and seen from above, through a transparent wall, in (c)) which connect it to the sheet. We will discuss the mechanism of this surprising destabilization of the jet shape, which develops perpendicularly to the direction expected for bucking, and show how it organizes the flow in the sheet at a larger scale (a suspended Savart sheet is shown from a perpendicular view in (d)).