Stability of viscoelastic liquid curtain

Abstract

Thin liquid sheets occur in several practical applications. In curtain coating, a thin liquid sheet is formed and falls freely over a considerable height before it impinges onto a moving substrate. Precision curtain coating was developed for multi layer photographic films but its use has expanded to many different applications. One of the important limits of this process is the stability of the liquid curtain, which may define the minimal thickness that can be coated at a given web speed. The condition at which a low viscosity Newtonian liquid curtain breaks was first derived based on a simple balance between inertial and capillary forces. There is a critical flow rate below which the curtain becomes unstable. For viscoelastic liquids, the normal stress related to the stretching of polymer molecules as the liquid accelerates down the curtain changes the force balance. Here, the critical condition at which a viscoelastic liquid curtain breaks was determined as a function of the rheological properties of the liquid. The results show that high extensional viscosity liquids create more stable curtains. Liquid additives could be used to push the limits of curtain flow rates to smaller values and consequently to widen the operability window of the process.