How to control air bubble content or defoam using high shear mixers

A high-shear mixer is a device that achieves mixing, dispersion, emulsification, and homogenization by generating intense shear forces and turbulence between a high-speed rotating rotor and stator. The intensity of the shear force is closely related to the speed, impeller structure, and material viscosity. The generation or disappearance of bubbles depends largely on the combined effects of local pressure changes, fluid dynamics, and shear intensity during the mixing process. In industrial production, excessive bubble content can affect product uniformity, gloss, texture, and stability. Therefore, bubble control and defoaming are crucial aspects of high-shear mixer applications.

Analysis of Bubble Generation Mechanisms
During high-shear mixing, bubbles typically arise from dissolved gases in the material itself, agitation introduced during raw material addition, and unstable gas-liquid interfaces caused by low-pressure areas. High-speed rotation in a high-shear mixer creates localized low-pressure areas, which facilitate the release of gas from the liquid, forming tiny bubbles. Furthermore, the shear and turbulence caused by the high-speed impeller can break large bubbles into smaller microbubbles. If these microbubbles are not released or ruptured in a timely manner, they can remain in the product, affecting the final quality.

Defoaming Principles of High-Shear Mixers
High-shear mixers primarily rely on the following methods to defoam:
Shear Bubble Breaking
The shear force between the high-speed rotor and stator effectively disrupts the bubble membrane structure, breaking large bubbles into smaller ones that ultimately escape through the liquid surface or exhaust system. Shear bubble breaking is the core method for controlling microfoam content.
Local Negative Pressure Release
High-shear mixers create a localized negative pressure area at the stator outlet or mixing chamber. The negative pressure compresses and ruptures bubbles, increasing gas solubility and promoting their dissolution into the liquid, thereby reducing visible bubbles.
High-speed Turbulence Promotes Bubble Ascent
Turbulence increases the convection velocity in the liquid microregion, accelerating bubble migration to the liquid surface and facilitating their escape from the system, thereby reducing trapped bubbles in the product.
Synergistic Effects of Shear and Heating
For high-viscosity liquids or emulsions, properly controlling the material temperature can reduce the viscosity and enhance bubble bursting efficiency. High-shear mixers can locally transfer heat energy to the liquid surrounding the bubbles through high shear, enhancing the defoaming effect.

Key Factors Affecting Bubble Control
The defoaming effect of a high-shear mixer is influenced by several process parameters:
Speed: Higher speeds increase shear force and enhance bubble breaking, but excessively high speeds may introduce new bubbles.
Impeller structure: Different impeller geometries significantly influence shear intensity and fluid shear uniformity. Conical or multi-layer impellers generally offer better defoaming performance.
Material viscosity: Bubbles migrate slowly in high-viscosity liquids, requiring appropriate adjustment of shear intensity and temperature to improve defoaming efficiency.
Mixer chamber design: Appropriate stator aperture, outlet geometry, and flow path design can accelerate bubble breakage and escape.
Material temperature and density: Moderate temperature reduces viscosity and surface tension, facilitating bubble breakage; density differences affect bubble bubbling speed.

Application Examples of High-Shear Mixers in Real-World Industries
High-shear mixers are widely used for bubble control and defoaming in industries such as food, pharmaceuticals, cosmetics, and coatings. For example, bubbles are easily introduced during the preparation of cosmetic emulsions. Using a high-shear mixer can eliminate microbubbles, ensuring product surface gloss and stability. The uniformity of suspensions in pharmaceutical preparations is crucial for dosage accuracy. High-shear mixers can be used to defoam and prevent air entrapment that can cause dosage deviation. In coating production, high-shear mixers can eliminate bubbles, improving coating uniformity and adhesion.

Industrial Operation Recommendations

Defoaming operations typically employ a staged control strategy: initially mixing at a low shear rate, then gradually increasing the shear force to disrupt microbubbles. Furthermore, a combination of feed method, exhaust design, and temperature regulation can maximize defoaming efficiency. Equipment maintenance and regular cleaning are also crucial to maintaining long-term defoaming performance.