The fluid mechanics of narrow stripe coating for low viscosity Newtonian solutions was investigated. A narrow stripe could be produced either through a slot die with a shim to control the slot width or through a rectangular nozzle. A flow visualization technique was employed to observe the mechanism on how a narrow stripe breaks at high coating speeds. Two-dimensional and three-dimensional numerical simulations on coating flows were also carried out to examine the fluid motion. The coating solution was found to expand laterally after emanating from the slot die channel at low coating speeds. The stripe width contracts gradually with increasing coating speed until coating failures appear. The coating defect observed switches from ribbing to a periodic stable and break-up motion. The effects of various fluid properties, geometric and operating parameters on the variation of coating width and maximum coating speeds were examined. A universal correlation involving two dimensionless groups, Reynolds and Bond number, was established for the prediction of coating width.

The pinning behavior of the downstream contact line between the coating bead and coating die corner for slot die coating was studied. For uniform coating, the downstream contact line must not go beyond the die corner; otherwise, an undesirable vortex may appear near the downstream meniscus. The locations of the upstream and the downstream menisci were computed by the commercial software package FLOW‐3D®, and the theoretical predictions were verified by flow visualization experiments. Three possible approaches in die design were investigated to ensure the downstream meniscus remains in constant contact with the die surface. These were (1) reducing the angle between the slot die shoulder and the die lip, (2) increasing the lyophobicity of the die surface, and (3) raising the coating gap between the die lip and the substrate. The first method was found to be the most effective, as it also increases the maximum coating speed. The second method was feasible only if proper surface modification is applied. The third method potentially caused the creation of a new vortex under the die lip. © 2012 Wiley Periodicals, Inc. Adv Polym Techn 32: E249–E257, 2013; View this article online at http://wileyonlinelibrary.com. DOI 10.1002/adv.21271