The stabilizing effect of confinement on the Rayleigh-Taylor instability

A hydrodynamic instability can develop at the interface between a denser fluid placed atop a less dense one. We investigate this Rayleigh-Taylor instability between two miscible liquids in a confined Hele-Shaw geometry. Surprisingly, we find that the typically unstable interface between the dense and less-dense liquids is stabilized below a critical confinement, set by the gap spacing of the Hele-Shaw cell. This critical confinement shows power law scaling with the difference in densities between the liquids. We also discuss the dependence of the characteristic wavelength of the instability on fluid parameters and the gap spacing. Our measured wavelength in this confined geometry deviates strongly from theoretical predictions for unconfined systems, suggesting an important effect of geometry on the onset of the instability.