CROSSOVER BETWEEN SURFACE-TENSION AND GRAVITY-DRIVEN INSTABILITIES OFA THIN FLUID LAYER ON A HORIZONTAL CYLINDER

A thin annular layer of fluid coating a cylinder is subject to two dif ferent instabilities. One, driven by surface tension, is analogous to the Rayleigh instability of a liquid jet. The other is the Rayleigh-Ta ylor instability, which is driven by gravity. Measurements of the wave length and growth rate of periodic patterns of droplets which develop as a result of the instability of such a fluid layer are reported for cylinders with radius r in the range 0.0011 < r < 1.27 cm. For small r the wavelength and growth rate of the pattern are in agreement with t heoretical predictions for the surface-tension-driven instability. For large r, the Rayleigh-Taylor instability is observed. At intermediate r there is a region of crossover between the two limiting cases. (C) 1997 American Institute of Physics.