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The coating processes that are used to manufacture photographic film,
adhesive labels, and magnetic tapes are typically modeled as steady, two-dimensional
flows because the substrates are so wide and the coated films are so thin.
However, these steady, two-dimensional flow states may be unstable when
subjected to two- and three-dimensional disturbances. The flow instabilities
lead to defects that lower the coating quality or render the product useless
altogether. Therefore, the analysis of any coating process should also
include an evaluation of whether the flow is stable.
Determining the stability of coating flows is now possible using NEKTON
3.2, to be released this summer. The linear stability analysis method,
pioneered by K. Ruschak, D. Coyle, C. Macosko, L. E. Scriven, K. Christodoulou,
and others at the University of Minnesota, is used to compute the growth
rates of small 2D and 3D disturbances to the 2D steady base flow solution.
Stream function contours for the steady-state 2D base
flow (Ca=1, R/H=200)
Stability analysis of a forward roll coater. Since the
growth rates of some disturbances are positive, the flow is unstable
when perturbed. A "ribbing" defect is the result. NEKTON's predictions
are consistent with previously published results for this problem
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