Instability of the Kolmogorov flow in a soap film

We examine the instability of a soap film flow driven by a time-independent force that is spatially periodic in the direction perpendicular to the for cing (Kolmogorov flow). Linear stability analysis of an idealized model of this flow predicts a critical Reynolds number R-c approximate to root 2. in our soap film experiment, we find a critical value R-c approximate to 70. This discrepancy can be ascribed to frictional effects from viscous couplin g of gas to the film, which is neglected in the idealized model. The kinema tic viscosity of the surrounding gas and the thickness of gas layers on eac h side of the soap film are varied in the experiments to better understand these frictional effects. Our observations indicate that flow in the soap f ilm cannot be decoupled from flow in the surrounding gas. [S1063-651X(99)04 307-X].