Magnetic instability between miscible fluids in a Hele-Shaw cell

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 axe 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.