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.
Highlights
Curtain coating is used for high-speed precision coating of singlelayer and multi-layer films for a variety of applications
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
At a constant flow rate, the dynamic contact line is stable between a range of roll speed
Summary
Curtain coating is used for high-speed precision coating of singlelayer and multi-layer films for a variety of applications. Understanding the relationship between rheological properties of the non-Newtonian liquid and the onset of air entrainment and heel formation is important to define the optimal operating conditions for uniform film production using curtain coating. This relationship has received little attention in the literature and is not well understood. We experimentally study the effect of shear thinning and extensional thickening (viscoelastic) behavior on the onset of heel formation and air entrainment, which defines the velocity range at a specified flow rate at which a uniform liquid film can be produced. This work builds on our past studies of the effect of rheology on curtain breakup. Aqueous solutions of xanthan gum (shear thinning liquid films) as well as high-molecular weight polyethylene oxide solutions (extensional thickening liquid films) were used as model liquids in the experiments
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