In curtain coating, liquid flows out of a slit and falls under gravity onto a horizontally moving web. The speed of the web with respect to the speed of flow from the slit determines the thickness of the coating on the web. A key issue is the stability of the contact line on the web, that is, the locations where the web surface first encounters liquid. An unstable contact line can result in air entrainment under the coating the produces a non-uniform coating thickness and other defects. Simulations, such as those shown here, can be used to investigate the stability of a liquid curtain with respect to changes in the liquid and web speeds, and changes in liquid properties like viscosity, surface tension, and adhesion force with the web.
On the left is a CFD simulation of the start-up of curtain coating with Newtonian fluid viscosity 25.13 cpoise, web speed of 2.1 m/s and coating thickness of 61 microns. The detail of edge retraction is clearly visible. The simulation required only 375,000 computational cells and was completed in less than 6 hours on a 2 GHz PC.