An Overview of Low Shear Mixers

Shear is the difference of velocity between two fluid vectors. When you apply shear to a non-newtonian fluid (fluid whose viscosity changes with shear), its viscosity will change:

• It decreases: pseudoplastic
• It increases: dilatant
• It becomes as solid above a mini shear: Bingham
• It decreases with time under shearing: thixotrope
• It increases with time under shearing: rheopectic

Shear depends on the velocity gradient in the fluid. Specific power (W/m3) can also give information on the shear stress.

What is the purpose of low shear mixing?

Some mixing processes require very low shear such as flocculation or polyelectrolyte. Shear will typically destroy flocs in the flocculation process. A gentle and low shear mixing is required. Low shear mixing can also be used for solid suspension, where high flow is required.

It depends on the impeller type and rotating speed. It is high for a turbine, low for a propeller. Some processes require very low shear such as flocculation, for which shear would destroy the flocs. 

Heat transfer, solid suspension, homogenization (of miscible liquids) require flow, and therefore low shear.

On the other hand, high shear will be required for emulsion, immiscible liquid homogenization, gas or liquid dispersion.

In water treatment, unlike flocculation, high shear is required for coagulation, to rapidly mix the coagulant with the water.

How does blade design impact the level of shear?

Impellers are designed to produce more or less flow, and more or less shear. For the same power rating, the lower the flow, the higher the shear.

A propeller such as the SABRE will deliver high flow but low shear. A turbine (e.g. Rushton) that is designed for radial flow, will deliver low flow but high shear.

At what speeds do both low shear and high shear mixers operate?

Very low speeds are required to limit the tip speed and shear. Typical flocculation mixers will use an axial flow propeller with tip speeds lower than 1.5m/s.