About Confined States, Noise-Sustained Structures and Slugs

In the field of pattern formation in nonequilibrium systems it has turned out over the last couple of years to be a fruitful concept to study the onset of the first instability in systems such as binary fluid convection (Ahlers et al., 1987, Brand et al., 1984, Steinberg et al., 1987, 1989) or the electrohydrodynamic instability (EHD) in nematic liquid crystals (Joets and Ribotta, 1988). Before the emphasis in the physics community had been on the onset of thermal convection in simple fluids and on the Taylor vortex flow which arises as a first instability when the fluid in the gap between two concentric cylinders is subjected to a torque by rotating the inner cylinder. While one has learned a great deal from these studies, the onset of the first instability which is most easily accessible to theoretical treatment has always been stationary in nature. Phenomena such as oscillatory instabilities (Brand and Steinberg, 1983, Knobloch and Proctor, 1981), traveling waves, modulational instabilities etc. occurred in those systems only as higher instabilities which are harder to handle theoretically. By paying the prize of having to deal with a slightly more complex system, one has succeeded experimentally in bringing down to onset many exciting phenomena such as traveling waves in binary fluid convection (Walden et al., 1985), a co-dimension two point in binary fluid convection in a porous medium (Rehberg and Ahlers, 1985), Benjamin-Feir type behavior (Steinberg et al., 1987) etc.