Magnetic instability between miscible fluids in a Hele--Shaw cell

C. Derec¹ - P. Boltenhagen² - S. Neveu³ - J.-C. Bacri¹

¹ Laboratoire Matière et Systèmes Complexes (MSC), UMR 7057 CNRS & Université Paris Diderot, 75205 Paris Cedex 13, France
² Hydrodynamique et Mécanique Physique, CNRS UMR 763 -- ESPCI, 75231 Paris Cedex 5, France
³ Laboratoire Liquides Ioniques et Interfaces Chargées, CNRS UMR 7612 -- Université Paris VI, 75252 Paris Cedex 05, France

Abstract
We show experimentally the existence of a magnetic instability between two miscible fluids. The sample is an aqueous ferrofluid. At time t = 0, the ferrofluid and water are put into contact in a Hele-Shaw cell and then submitted to a homogeneous magnetic field perpendicular to the cell. Above a threshold value of the magnetic field a fingering instability is observed at the diffuse interface, with fingers growing with time. A wavelength can be defined as the mean distance between fingers. This wavelength in the linear regime is approximatively equal to the thickness of the cell. Figs 7, Refs 19.