INSTABILITIES OF THE UPSTREAM MENISCUS IN DIRECTIONAL VISCOUS FINGERING

New instabilities affecting the meniscus of a viscous fluid are presen ted. They occur in an experimental set-up introduced previously by Rab aud et al. (1990) in which a small quantity of a viscous fluid is plac ed in the narrow gap between two rotating cylinders. In this geometry the downstream meniscus located in the region where the two solid surf aces move away from each other is known to be unstable and to exhibit directional viscous fingering. In the present article it is shown that the upstream meniscus can also be unstable. Two types of instabilitie s are observed. In the first supercritical transition the front become s time-dependent with either standing or propagating waves. In a secon d transition, which is subcritical, parallel fingers of finite amplitu de are formed. The various types of spatio-temporal dynamical behaviou r are discussed.