Contact line dynamics in curtain coating of non-Newtonian liquids
Alireza Mohammad Karim, Wieslaw J. Suszynski, Saswati Pujari, Lorraine F. Francis, Márcio S. Carvalho
Abstract
The rheological characteristics of liquids play an important role in the flow near dynamic contact lines, where the deformation rates are extremely large. The liquid contact line dynamics and the free surface configuration in the curtain coating of polymer solutions were experimentally studied using visualization to determine the effect of two rheological characteristics: shear thinning and extensional thickening. The critical web speeds for heel formation, where the contact line on the moving substrate shifts upstream of the falling curtain, and air entrainment, where the contact line shifts downstream and air bubbles appear, were determined over a range of flow rates. The critical conditions were compared to the behavior observed for a Newtonian liquid. Moreover, the contact line dynamics were described by three dimensionless parameters: the Deborah number, the Ohnesorge number, and the ratio of the web speed to the liquid curtain velocity at the contact line.