Dynamic Surface Tension Measurement in Liquid Curtain Coating Systems
Dynamic surface tension at millisecond surface ages dictates liquid curtain stability and dynamic wetting limits in paperboard coating operations.
Marangoni Elasticity
Gradient Restoration
Surface phenomena driven by differences in surface tension along an interface create localized fluid movements known as marangoni flows. In a drying coating or an ink film, marangoni elasticity acts as a force that attempts to heal thin spots or defects. This occurs because liquid naturally flows from areas of low surface tension to areas of high surface tension.
Solute Transport
Variations in the concentration of surfactants or solvents across the surface of the liquid cause these tension gradients to form. As a solvent evaporates more quickly from one area, the surface tension there rises, pulling nearby fluid toward that point. This transport of material is a factor in the formation of orange peel patterns or Benard cells in the finished coating.
If the elasticity is too strong, it can lead to an uneven surface as the fluid migrates away from certain zones.
Coating Defect
Management of these forces is required to ensure a smooth and glossy finish on high-end packaging. Chemists use leveling agents to minimize the gradients and promote a more uniform drying process. This control prevents the appearance of craters and streaks that would compromise the visual quality of the print.
The balance of these interfacial forces determines the final topography of the coated board.