Formation of spiral liquid curtains falling from a downward-facing free surface

The spontaneous formation of spiral patterns has been investigated intensively in excitable media, where the nonlinear coupling of excitation and recovery variables plays key roles in wave generation. Patterns rotate, in general, with spiral arms trailing the direction of rotation. We will report spiral patterns observed in a non- excitable fluid system. It consists of a downward-facing free surface of a horizontal liquid film maintained by continuous liquid feeding. The surface is unstable to the Rayleigh-Taylor (RT) instability. The resulting liquid discharge from the film occurs in different modes: in drops, in columns, and in curtains, depending on the liquid feeding rate, as reported by Pirat et al. (Phys. Rev. Lett., 92, 104501, 2004). Liquid curtains can exhibit spiral patterns with their arms leading the direction of rotation. Characterizing the formation of spiral curtains by an experiment and comparing the results with a phenomenologically developed theoretical model, we show that the patterns result from the synchronized development of the RT instability with the motion of curtains.