The dynamics and stability of thin liquid films during spin coating. I. Films with constant rates of evaporation or absorption

A fixed volume of liquid is placed on a horizontal disk spinning at a constant angular speed. The liquid forms a film that thins continuously due to centrifugal drainage and evaporation or thins to a finite thickness when surface absorption counterbalances drainage. A nonlinear evolution equation describing the shape of the film interface as a function of space and time is derived, and its stability is examined using linear theory. When there is either no mass transfer or there is evaporation from the film surface, infinitesimal disturbances decay for small wave numbers and are transiently stable for larger wave numbers. When absorption is present at the free surface, the film exhibits three different domains of stability: disturbances of small wave numbers decay, disturbances of intermediate wave numbers grow transiently, and those of larger wave numbers grow exponentially.