Flow visualization of the liquid emptying process in scaled-up gravure grooves and cells

The liquid-emptying process in scaled-up gravure grooves and cells is studied using flow visualization in order to better understand gravure coating and printing processes. Water and two different glycerin/water mixtures serve as the test liquids, and the emptying process is initiated by moving over the groove or cell a rotating roller or a glass top with a curved surface. For the scaled-up groove, a region of recirculating flow is observed to attach to the moving glass top. When the glass top is used to drive flow in the scaled-up cell, an air bubble may appear inside the cell when the gap between the bottom of the curved surface and the top of the cell is zero. When this gap is positive, a liquid bridge is formed, dragged across the cell, and then broken, leaving some liquid inside the cell. The amount of liquid remaining in the cell, VrVr, is measured for different liquids, surface speeds, and gap distances for both the glass top and the rotating roller. The effect of using a soft elastomeric covering on the glass top and roller is also explored. For each liquid, VrVr increases as the speed of the glass top or roller increases. The data are correlated by multiplying VrVr by a liquid-dependent shift factor, which leads to a power-law relationship between the shifted VrVr and the capillary number. These experimental observations and measurements can be used to benchmark theoretical calculations, which can then be applied to design gravure grooves and cells that empty in a controlled way.