Meaning
Cohesive energy per unit area that causes the surface of a liquid to behave like a stretched elastic membrane. Scientists measure surface tension force to determine how a liquid will spread across or bead up on a solid substrate. The magnitude of this energy dictates the stability of the interface between the coating material and the air.
Wetting Equilibrium
Balance between cohesive and adhesive forces determines the contact angle of a droplet. In systems where the surface tension force of the liquid is much higher than the surface energy of the substrate, the fluid will fail to wet the surface. This mismatch results in a beaded coating that does not form a continuous film.
Pre-treating the substrate can modify these interactions to ensure a uniform application.
Defect Formation
Spontaneous movement of fluid away from areas of low surface energy creates thin spots or holes in a wet film. When the surface tension force varies across a drying layer due to temperature or concentration gradients, the Marangoni effect pulls material toward regions of higher tension. This migration produces a wavy surface known as orange peel.
Controlling the evaporation rate and the chemical composition of the mixture limits these unwanted movements.
Interface Stability
Mechanical integrity of a liquid bridge or a meniscus is maintained by the tension at the boundary. High-speed coating operations rely on a stable meniscus to draw a thin layer of fluid onto a moving web. If the surface tension force is too low, the liquid curtain may break and cause a shutdown of the production line.
Maintaining the correct tension ensures that the coating remains intact at high velocities.
