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.
Gibbs Elasticity
Interface Resilience
Physical properties governing the stability of thin liquid films against mechanical disturbances are categorized as interfacial rheology. In a coating or ink, gibbs elasticity describes the ability of a liquid lamella to resist thinning when it is stretched. This property arises from the change in surface tension that occurs when the surface area of a film increases.
Concentration Gradient
When a film is pulled, the local concentration of surfactant molecules on the surface decreases, which causes a temporary rise in surface tension. This tension difference acts as a restoring force that pulls liquid back into the stretched area to prevent a rupture. The magnitude of this effect depends on the concentration of surfactants in the bulk solution and their rate of migration to the interface.
In high-speed coating, this elasticity is a factor in preventing the break-up of the liquid curtain.
Foam Stability
High levels of this elasticity are often associated with the formation of persistent foams that can interfere with the application of the coating. If the bubbles are too stable, they will not pop when the coating is leveled, leading to voids in the final film. Formulators must balance the need for film stability with the requirement for quick air release.
Controlling this property is a necessary step in the development of defect-free coatings.