Wetting behavior of the shear thinning power law fluids

Abstract

The knowledge of the wetting characteristics of a coating solution is a prerequisite of assuring the final quality of thin films manufactured by the slot die coating process. Because the maximum coating speed is limited by defects, such as air entrainment, which are directly related to film quality, an understanding of the coating limits is critical. Despite the existence of a vast body of literature to understand the occurrence of defects and the coating limits to produce defect-free films, the mechanism of air entrainment is still not well understood, especially for various classes of solutions. In this study, the shape of the upstream meniscus of a mildly shear thinning non-Newtonian coating bead has been studied both numerically and experimentally and subsequently linked to the air entrainment threshold. It has been found that the dynamic contact angle reaches its maximum value at the air entrainment threshold, primarily as a function of the capillary number. In addition, this work suggests that the onset of the dripping and air entrainment boundaries of solutions that follow the power law can be predicted based on the dynamic contact angle as a function of the consistency and power law indices.