Fingering in a driven hele-shaw cell

A modified Hele-Shaw cell in which the plate gap can be modulated in time was constructed. Highly nonlinear fingers on the interface between air and water in the cell were observed as the plate gap was driven at a variety of frequencies, but typically near 60 Hz. Modified equations to describe the flow in a periodically driven cell were derived and the linear stability analysis of waves on a circular fluid-fluid interface was performed.