The fluid dynamics of reverse roll coating

The flow in the metering gap of a reverse roll coater is examined by experiments and finite element solutions of the Navier‐Stokes equations. At high speed ratios and capillary numbers, the metered film flow deviates strongly from predictions of lubrication theory: the wetting line moves through the gap center and the metered film thickness passes through a minimum. The two flow instabilities found are ribbing, a sinusoidal cross‐web waviness extending smoothly down‐web; and cascade, an irregular V‐shaped cross‐web wave, repeated quasiperiodically down‐web. Experimental operability diagrams define parameter ranges where these instabilities and the steady two‐dimensional flow are encountered. Ribbing behavior is understood by consideration of the pressure gradient at the free surface. The mechanism of cascade is the intrusion through the gap of the wetting line, which causes the metered film to thicken and eventually reattach to the metering roll in a cyclical manner.