THE GENERATION OF 2-DIMENSIONAL VORTICES BY TRANSVERSE OSCILLATION OFA SOAP FILM

An experimental investigation of the dynamics of horizontal soap films stretched over circular or square boundaries undergoing periodic tran sverse oscillations at frequencies in the range 20-200 Hz is reported. Concomitant with modes of transverse flexural oscillations, it was ob served that two-dimensional vortices in the plane of the film an excit ed. The vortices map be either (i) large, scaling with the size of the cavity or (ii) small, localized at a wavelength or half-wavelength of the membrane modes. In the experiments a stable generation of one, tw o, ..., ten pairs of counter-rotating vortices were observed in finite regions of amplitude-frequency parameter space. The circulation stren gth of vortices in a given vortex pattern increases with increasing ex ternal forcing and with decreasing soap film thickness. A theoretical model based on the wave-boundary interaction of excited Marangoni wave s reveals a vorticity generation mechanism active in vibrating soap fi lms. This model shows that vorticity is generated throughout the entir e liquid volume by viscous diffusion, and qualitatively reproduces man y steady vortex patterns observed in the experiment. However, the mode l cannot explain the existence of the sometimes intense vortices obser ved far Gum the film boundary that do not appear to be generated by di ffusive processes. (C) 1998 American Institute of Physics.