Abstract
The investigation concerns the stability of an incompressible second-order fluid (visco-elastic) film flowing down an inclined plane under gravity with respect to two-dimensional disturbances. When the elastic parameter is negative as in the case of a solution of polyisobutylene in cetane, surface disturbances (‘soft’ waves) are found to be unstable. The analysis in this case also reveals the existence of growing shear waves (‘hard’ waves) which are highly damped in ordinary Newtonian fluids.