Mixing Process and Fluid Dynamics Terms

Successful mixing depends not only on equipment design but also on fluid behaviour, energy input, and hygienic process control. Parameters such as viscosity, shear rate, and residence time determine whether dispersion is complete and repeatable at production scale.

Understanding these process fundamentals supports reliable scale-up, stable product quality, and efficient long-term operation in hygienic manufacturing environments.

Key Mixing Process and Fluid Dynamics Definitions:

What is viscosity?
Viscosity is a fluid’s resistance to flow and a defining parameter in mixer selection. High viscosity products require greater energy input and specialized impeller design to achieve uniform mixing.

What is shear rate?
Shear rate describes the velocity gradient generated in a fluid during mixing. It directly influences dispersion efficiency, droplet breakup, and texture development.

What is residence / hydration time?
Hydration time is the duration required for a powder particle to fully absorb liquid and reach its final functional state (such as full viscosity or solubility). In inline mixing, “residence time” is the brief window the product spends inside the mixer. High-shear inline mixers are designed to maximize this interaction in a split second, often reducing the need for long “aging” or “holding” times in a tank.

What is air entrainment?
Air entrainment is the unwanted incorporation of air bubbles during mixing, potentially causing foam, oxidation, and reduced product density.

What is cavitation in shear mixers?
Cavitation occurs when vapor bubbles form and then rapidly collapse within a pump or mixer, usually due to a sudden drop in pressure. In powder induction, this can happen if the vacuum is too high or if the liquid flow is restricted. Cavitation can “pockmark” metal surfaces (like impellers) and drop mixing efficiency, making “NPSH” (Net Positive Suction Head) a critical calculation for any inline installation.

What is degassing?
Degassing removes dissolved or entrained gases to improve product stability, appearance, and filling accuracy.

What is de-aeration?
De-aeration is the active removal of entrained air or dissolved gases from a liquid. A dedicated de-aeration process ensures that the final product has a higher density, better shelf stability, and a more “premium” visual appearance without bubbles.

What is NPSH (net positive suction head)?
NPSH is the measure of the pressure at the suction side of a pump or mixer. In powder induction systems, maintaining a proper NPSH available is vital to prevent cavitation. If the pressure drops too low, the liquid will “boil” at room temperature, creating vapor bubbles that damage the equipment and ruin the vacuum needed to pull in powder.

What are “dead zones”?
Dead zones are areas within a mixing tank or pipeline where fluid velocity is near zero, meaning the product is not being actively mixed. Inline mixers are designed to eliminate dead zones by forcing the entire product stream through the high-shear zone, ensuring 100% of the material is processed uniformly.

What is phase separation?
Phase separation occurs when mixed components separate over time due to insufficient droplet size control or stabilization.

What is an Exothermic Heat Reaction?
An exothermic heat of solution occurs when certain powders are added to a liquid and the dissolution process releases thermal energy, causing the temperature of the mixture to rise spontaneously.

What is an Endothermic Dissolution?
The opposite of exothermic. Some powders (like Urea or certain Sugars/Polyols) absorb heat from the liquid as they dissolve, causing the temperature to drop significantly. A sudden drop in temperature can drastically increase the viscosity of the liquid, making it harder for the induction pump to maintain a vacuum and pull in more powder.

What is shear-thinning (Pseudoplastic) behavior?
Many common ingredients, such as gums, starches, and sauces, exhibit shear-thinning behavior, where the fluid’s viscosity decreases as the shear rate increases. This is a major advantage for inline high-shear mixers; as the product enters the high-shear zone, it becomes “thinner” and easier to process, only regaining its full thickness once it reaches the target vessel or container.

What is Shear-Thickening (Dilatant) Behavior?
The opposite of shear-thinning; these fluids become more viscous (thicker) when force is applied. Examples include high-concentration starch slurries.

What is Yield Stress?
Yield stress is the minimum amount of force (shear stress) required to make a fluid start to move. Products like ketchup or mayonnaise have high yield stress – they won’t flow under gravity alone. In a mixing system, the pump and mixer must be powerful enough to overcome this initial resistance to ensure the entire batch stays in motion and avoids “dead zones.”

What is scale up in mixing processes?
Scale-up is the transition from laboratory or pilot production to full industrial manufacturing while maintaining product quality, droplet size, and processing efficiency.

What is clean in place (CIP)?
CIP is the automated cleaning of internal equipment surfaces without dismantling, ensuring hygienic compliance, reduced downtime, and repeatable sanitation.

What is sterilize-in-place (SIP)?
SIP uses heat or steam to sterilize closed processing systems, essential in pharmaceutical and aseptic food production.

What is process repeatability?
Process repeatability is the ability to consistently replicate precise product specifications—such as viscosity, particle size, and color—across multiple production cycles. In industrial mixing, this is achieved by automating critical parameters like powder induction rates, tip speeds, and residence times.