Wetting phenomena during processing of high-viscosity shear-thinning fluid

Slot-die coating windows have been previously obtained by several researchers for low-viscosity solutions. However, practical applications necessitate the use of relatively high-viscosity (>> 1 Pa s) non-Newtonian fluids for coating films. Also, earlier theoretical studies predicting coating windows are mainly limited to 2D single-phase studies, and based the prediction of the coating window on the upstream contact angle made by the coating fluid with the substrate. The numerical predictions varied significantly from the experimental findings in most cases due to oversimplification of the model. In the current study, experimental and theoretical methods are used for obtaining the coating window for blackstrap molasses, which is classified as a relatively high-viscosity, shear thinning solution. The coating process is monitored by a microscope connected to a camera placed under the slot-die. Air entrainment is found to occur in a three-step or four-step cycle. In addition, a comprehensive theoretical study is performed by simulating the coating process using a 3D, multiphase, volume of fluid (VOF) method. The simulation is found to predict the coating window within 10% accuracy of experimental results. Also, the model does an excellent job of predicting the formation of air bubbles at high coating speeds. (c) 2011 Elsevier B.V. All rights reserved.