Rotor-stator homogenization uses a rotating metal shaft (the rotor) inside a stationary metal casing (the stator). The rotation of the rotor creates a suction effect which draws the sample into the space between the rotor and stator, in which it is subject to very high shear forces due to the an extreme change in velocity in the small space between the rotor and stator. (The laws of fluid mechanics state that the velocity in the fluid immediately adjacent to the rotor is the same as the velocity of the rotor, while the velocity of the fluid immediately adjacent to the stationary stator is zero.) Centrifugal forces then push the material out through slots in the stator, and the rapid motion of the fluid caused by the rotor-stator ensures that the process is repeated as the liquid and sample repeatedly cycle through it.
Below is a close-up video of a PRO Scientific Rotor-Stator being used.
There are a number of benefits and drawbacks to any instrument which uses a probe to homogenize samples, including rotor-stators. Because you can switch between probes, the volume range which can be processed is greater than with other methods. There are rotor-stator homogenizers that, using different probes, can homogenize volumes between 30 microliters and 30 liters. Additionally, there is effectively no maximum volume - rotor-stator homogenizers exist for laboratory, pilot, and industrial scale applications.
Rotor-stator homogenizers are very fast and efficient for single samples. Because of the use of probes, however, rotor-stator homogenizers are not as well suited for multi-sample, high-throughput applications. If cross-contamination is a concern, the probe must be washed between each use. Some manufacturers who provide packs of lower-cost probes or disposable, limited-use probes which are intended to allow you to process a number of samples using a different, clean probe each time (such as the PRO Multi-Gen Generator Probes). There are also a number of automated, higher-throughput rotor-stator homogenizers. These are generally more expensive than a bead mill of equivalent throughput, but allow for processing larger samples. There are also a number of rotor-stator homogenizers which allow semi-continuous in-line processing, and can therefore handle very large volumes. Along with high-pressure homogenizers, these are the only kinds of homogenizers where true industrial-scale units exist.
Rotor-stator homogenizers are very well suited for liquid applications, such as mixing or creating emulsions. They are also very good for breaking open cells and homogenizing relatively soft tissue. If homogenizing solids, keep in mind that the particles need to fit between the rotor and stator in order to become homogenized. While for soft solids (such as most soft tissue) the suction effect can partially overcome the shape of the tissue, for harder tissue (for example tablets or fibrous tissue) the sample may need to be pre-processed such that the particle size is sufficiently small. Probes with saw-toothed heads can help tear apart break down fibrous samples and many other solids.
For best results with a rotor-stator, the probe should be moved around inside the sample during use. This helps ensure that the sample is uniformly and completely homogenized. It can also help reduce the necessary run time, especially when operating near the maximum operating volume for the instrument.
Rotor-Stators impart a moderate amount of heat into the sample during use, mostly due to frictional forces. If your application is heat-sensitive, consider methods to cool your sample. For most laboratory scale applications, attaching the sample container to a clamp and placing it in an ice bath is appropriate.
To help maximize the useful life of your probes, ensure they are cleaned after each use. Cleaning the probes in a volatile cleaner, such as 70% ethanol, will help them dry faster.
The most important thing to consider when purchasing a rotor-stator homogenizer is the volume range. Additionally, take a close look at the various probes available for it, as the probes are just as important, and often about as expensive, as the instrument itself.
A common mistake when evaluating rotor-stator units is to take the RPM as an indicator of power. What is important is the velocity of the rotor, which you can calculate as the RPM multiplied by the circumference of the rotor (C = π*d). A large probe may have a much lower RPM than a small one but still have more processing power due to the higher rotor velocity.
When considering maximum volume ratings, keep in mind that the rating is for aqueous samples. If processing viscous liquids or aqueous solutions with a sufficient amount of solids in them such as to make them act as a more viscous liquid, give yourself plenty of leeway. If you are near the maximum volume range for a particular instrument, choose a model capable of processing larger volumes. Depending on the viscosity of the sample, the maximum volume range could be reduced by up to a factor of 10. If you’re unsure if an instrument would be able to process your sample, just give us a call or send us an email.
Can you run a rotor-stator homogenizer continuously?
Generally not. Rotor-stator homogenizers operate at extremely high speeds and can wear or burn out if the usage duration is unusually long. Furthermore, if used within normal operating conditions, a rotor-stator should be able to fully homogenize a batch within a few minutes. If long-term or continuous mixing are necessary, an overhead stirrer should be used.
The exception to this are some rotor-stators which are intended for use in flow-through or in-line processes. Some of these rotor-stator homogenizers are capable of processing for extended periods of time due to differences in their design.
Are probes for rotor-stator homogenizers cross-compatible?
Generally not. Rotor-stators do not operate on a simple chuck system like overhead stirrers (or common household drills) and instead have special connections which are designed to hold the probe securely in place while spinning the rotor at extremely high speeds. Some brands have probes which are cross-compatible with other models from that brand - for instance, PRO Scientific rotor-stators have probes which can be used across all their models - however most brands have different probes for each different model of motor unit.
What is the most viscous substance that can be processed with a rotor-stator homogenizer?
As with any probe-based homogenizer, if the homogenizer can't create flow, the substance will not be processed. For a rotor-stator, that point is usually around 10,000 centipoise (cP), which is approximately the viscosity of honey or molasses. Keep in mind, however, that at this high of a viscosity you will likely need to operate at a volume far lower than the rated maximum volume of your system, as those ratings assume you are working with something that has the viscosity of water (1 cP).