Homogenizer vs Overhead Stirrer (for High Speed Mixing)
Intro
Choosing the proper equipment for your high speed, high shear mixing application is a must. Applications as diverse as homogenization, emulsification, and dispersion require both high shear and high speed mixing. Laboratories specializing in these (and other) applications often question whether a homogenizer or overhead stirrer is most adept for their purpose.
Choosing the right equipment isn't easy: there are many models and brand available, and the differences aren't always clear. And as with most lab equipment purchases, it's not easy to CTRL + Z away an incorrect buy.
While no article can encompass every mixing application and scenario, the below serves as a valuable starting point.
Definitions
- Homogenization refers to mixing a substance so that it's components are uniformly distributed.
- Dispersion refers to a system in which distributed particles of a material are dispersed into a continuous phase of a second material. Most commonly, this involves either solid particles in a liquid or liquid particles in another liquid; the latter is commonly called emulsion.
Summary
Below is a summary table to help distinguish the differences between homogenizers and overhead stirrers. Note that the values below are merely typical; many models have unique specifications.
| Homogenizer | Overhead Stirrer | |
|---|---|---|
| Top speed (rpm) | 30,000 | 3,000 |
| Max viscosity (cP) | 15,000 | 50,000 |
| Max solid content (w/w) | 15% | 65% |
| Common applications | Micro- and nano-emulsion, tissue & cell homogenization | Paint & ink dispersion, adhesive & ceramic emulsion |
| Example unit | Scilogex D160 | Caframo BDC2002 |
Homogenizer
The homogenizer is excellent at generating both high shear and high speed. Often toothed or slotted, a homogenizer's rotor can reach speeds of 30,000 rpm.
Because the stator restricts flow and pump, the maximum effective viscosity is typically between 10,000 to 20,000 cP. For solid-in-liquid dispersions, the solid content should be less than 15% w/w.
The homogenizer is favored for emulsions, both micro- and nano-. It's also effective for biological dispersion (tissue and cell homogenization), and for breaking down gums or gels.
Overhead Stirrer
When an overhead stirrer is equipped with a high shear impeller, it's often called a disperser. The mixture is pulled into the churning impeller blade and its high hydraulic shear.
This style of high-shear impeller goes by a few names, including Cowles, disperser, dissolver, sawblade, and sawtooth. The vortex created by overhead stirrers is helpful for dispersing solids. Overhead stirrers often create a "donut hole" or "hurricane" shape within the mixture as the blade created as a depressed eye around which the elevated mixture rotates.
Overhead stirrers aren't as speedy as homogenizers, with top speeds that typically reach 3,000 rpm (although 6,000 rpm models exist). But the higher flow can accommodate a wider range of viscosities, from water to 50,000 cP. In addition, formulations above 15% w/w -- and as high as 65% w/w -- can be readily mixed by an overhead stirrer.
Applications favoring overhead stirrers include paint and ink mixing, emulsions for adhesives and ceramics, and essential oil microencapsulation.
Caution
Keep in mind that high-speed + high-shear mixing generates heat. This heat can impact temperature-sensitive mixtures.
Likewise, high-shear can be too extreme for some mixtures. For example, carbomers have a high molecular weight, and could be degraded by high-shear mixing.
(Information provided courtesy of Caframo.)