Viscous fingering in a magnetic fluid. II. Linear Hele-Shaw flow

Viscous fingering phenomenon in a linear channel is studied for a magnetic fluid subjected to an external magnetic field. The competition between the hydrodynamic effects and the capillary effects leads to the formation of an interface between the air and the fluid which has a finger shape. It is th e so-called Saffman-Taylor instability (STI). The influence of the magnetic effects depends on the direction of the applied field: it is possible eith er to enhance or to reduce the destabilizing phenomena. We study the onset of the STI and compare the experimental results with the linear analysis in cluding the magnetic contribution. In the nonlinear regime, the measurement of the width of the finger as a function of the direction and the amplitud e of the magnetic field is understood using a phenomenological approach. (C ) 2001 American Institute of Physics.