The linear stability of a drop of fluid during spin coating or subject to a jet of air

In this paper we investigate the linear stability of an initially axisymmetric thin drop of Newtonian fluid either on a uniformly rotating substrate (the simplest model for spin coating) or on a stationary substrate under the influence of an axisymmetric jet of air directed normally towards the substrate. Drops both with and without a dry patch at their center are considered. For each problem we examine both the special case of quasistatic motion (corresponding to zero capillary number) analytically and the general case of nonzero capillary number numerically. In all cases the drop is found to be unconditionally unstable, but the growth rate and wavenumber of the most unstable mode depend on the details of the specific problem considered. (C) 2002 American Institute of Physics.