Abstract
A flow visualization technique was applied to observe the liquid emptying from scaled-up gravure cells of different shapes. Dilute Newtonian poly(vinyl)alcohol solutions were used as tested fluid. The emptying process depends on the stability of the liquid bridge that connects the moving roller and the gravure cell. It was found that the downstream free surface of the liquid bridge first extends in the coating direction after the moving roller contacts the coating solution and then the position of the free surface remains unchanged until the upstream free surface position is adjusted and fixed, then the liquid bridge shrinks as coating starts until the bridge breaks up. The adjustments of the upstream free surface depends strongly on the fluid viscosity. Two limiting cases can demonstrate how the upstream free surface varies; for a low-viscosity solution with a smaller coating gap, the position of the upstream free surface is fixed until the downstream free surface reaches a steady position before coating starts. However, for a high-viscosity solution with a larger coating gap, the upstream free surface will even pass the gravure cell and the liquid bridge is out of the gravure cell. Variation of the upstream free surfaces are between these two limiting cases. Other factors such as the departing angle of the gravure cell, the gap between the moving roller and the gravure cell, and capillary number will also influence the fluid motion of the gravure cell.