Experimental Study on Air Entrainment in Slot Die Coating of High-Viscosity, Shear-Thinning Fluids

The effects of slot die geometry, substrate geometry, and fluid properties on air entrainment in coatings have been extensively investigated by several researchers in the past. However, no studies exist to explain the bubble breakup mechanisms during slot die coating of highly viscous, shear- thinning solutions. An experimental study is performed to understand the bubble breakup mechanisms and the qualitative impact of geometric conditions and viscosity of the coating solution on the sizes of the air bubbles. Blackstrap molasses and dilute blackstrap molasses are used as the primary test solutions in this study. Two bubble breakup mechanisms were identified based on the shape of the sawteeth formed prior to air entrainment. In addition, smaller slot and coating gaps and high viscosity solutions were found to produce smaller bubble sizes. For example, reducing slot gap by 41% resulted in a 58% reduction in the effective bubble diameter. It was also found that as the dynamic contact angle approaches 180 with increasing velocity, the behavior follows that of the interface formation model. (C) 2012 Elsevier Ltd. All rights reserved.