On air entrainment in coatings

Area of Science:

• Fluid Dynamics
• Materials Science
• Surface Science

Background:

• Air entrainment in coatings is a common issue affecting product quality.
• Understanding the dynamics of the air-liquid interface is crucial for controlling coating processes.

Purpose of the Study:

• To investigate the mechanisms of air bubble entrainment into coatings.
• To characterize contact line dynamics at the air-liquid interface.
• To explore methods for preventing air entrainment during coating.

Main Methods:

• Experimental characterization of contact line dynamics around a fiber in a coating die.
• Utilizing viscous liquids like glycerin to observe air entrainment.
• Applying pressurized liquid supply to control the free surface interface.

Main Results:

• A critical fiber speed was identified, above which air entrainment occurs.
• The critical capillary number (Ca(c)) was found to depend on the Morton number of the coating material.
• Pressurized liquid supply effectively stabilized the meniscus, preventing air entrainment.
• Stable wetting was achieved at high capillary numbers (Ca ≈ 50) by controlling meniscus location.

Conclusions:

• Air entrainment is governed by fiber speed and fluid properties.
• Pressurized liquid supply offers a method to control meniscus position and eliminate air entrainment.
• This technique allows for stable coating at high operational speeds.