ONSET OF FOLDING IN PLANE LIQUID-FILMS

The onset of the folding effect characteristic of highly viscous liqui d films (plane jets) slowly impinging on a wall is studied. Nonlinear quasi-one-dimensional equations are derived to describe the how. In th e linear approximation they reduce to the eigenvalue problem, whose so lution predicts that instability (the onset of folding) sets in when t he length of the film exceeds a critical value. The critical folding h eights and the oscillation frequencies at the onset of instability are predicted as a function of how parameters. Theoretical results are co mpared with Cruickshank's (1988) experimental data. Agreement is quite good only in the range of parameters where the quasi-one-dimensional approximation is applicable (thin films at the onset of folding).