If you’ve decided that rotor-stator homogenization is the right method for your application, you still need to choose the right probe for the task at hand. Probes (also called generators) vary in size and shape, and the one you need will depend on several factors including the volume of the sample and the type of material you’re homogenizing.
Choosing the correct probe can help ensure improved process efficiency, enabling you to obtain your desired result in the shortest period of time. Aside from the obvious benefits of speeding up your process, improved efficiency also means you can lower heat generation (an inherent side effect of rotor-stator homogenization), which is particularly important for heat-sensitive samples.
The main factors to consider when selecting a probe are:
- Probe type and design
- Probe diameter
- Probe length
- Throughput and cross-contamination
Let’s look at each of these in detail:
1. Probe Type and Design
One of the most important factors is choosing what type of probe to use. The two most common types are saw-tooth and flat-bottom.
- Saw tooth: This type of probe has a jagged edge around its base, which looks similar to a set of sharp teeth. Saw tooth probes are recommended for fibrous material such as tissue samples. Although these probes can often make their way through large pieces of tissue, homogenization time can be reduced if samples are pre-processed by cutting them into smaller pieces or pulverizing them.
- Flat bottom: As the name suggests, this type of probe has a flat bottom. These generators are more suitable for applications requiring gentler homogenization. For example, flat bottom probes may be used in emulsification or standard mixing applications.
A flat bottom and saw tooth probe.
Another factor is the design of the stator. For instance, the IKA probes sometimes have ratings for "coarse," "medium," and "fine." The Scilogex D500 has coarse and fine generators and "standard" and "emulsion" rotors.
Left to right: Coarse, medium, and fine dispersing elements for the Ultra-Turrax® T 65.
The difference between these is how enclosed the design of the stator head is and how full the rotor is. This is essentially a matter of how much material is moving in close proximity to other material, which determines the amount of shear force created. Greater shear force leads to smaller particle sizes. However, the more enclosed design of high-shear probes means they are less suitable for use with solids. They also create slightly less flow, so they often cannot be used with as large a volume as lower-shear probes.
While flat-bottom and saw-tooth are the most common types of probes, there are also some specialty probes available. For example, a cryo generator can break frozen samples and cryogenically mill sediment and soil. You may be able to homogenize these samples with other probes, but the specially-designed cryo generator offers more efficient, complete processing.
IKA offers an extensive range of specialty generators. A high-speed dissolver agitator shaft (below left) is designed for intensive mixing and dissipating agglomerates. The stirring shaft (below right) turns the Ultra-Turrax® T 50 into a high-speed stirrer for quick dissolving, mixing, and deagglomeration.
A cutting head (below left) is designed to crush larger pieces of fibrous material, for example, vegetables, fruit, and vegetation. The jet mixer head (below right) will speed up mixing and dissolving. It is suitable for suspension of powders that are typically difficult to dissolve or for processing substances that have sedimented or hardened.
2. Probe Diameter
The most suitable probe dimensions will depend on your vessel size and sample volume.
The first dimension to consider is diameter. As a rough guide, to achieve optimal efficiency, the diameter of the generator probe should be roughly ten times smaller than the diameter of the vessel. Bear in mind though that this simply optimizes the process through creation of the most efficient flow patterns and doesn’t need to be strictly adhered to.
Another factor to consider if you are processing a solid sample is the initial particle size. The probe diameter should be at least twice as large as the particles you’re homogenizing.
3. Probe Length
The other important dimension is the length of the probe. One way to gauge the correct length is to consider that the aeration hole near the top of the probe should not be submerged in the sample, but rather completely exposed above the surface. Meanwhile, for optimal efficiency, the end of the probe should be fairly close to the bottom of the vessel, roughly two-thirds below the sample surface.
Note that some probes contain a mid-bearing that needs to be lubricated during processing. To ensure the sample lubricates this bearing, the volume should be high enough that at least half the length of the generator is covered.
In case you’re unsure about dimensions, the specifications of most probes will tell you what volume range they are suitable for, which can act as a good guide.
The chart below shows some of the different types and dimensions you can expect to see. These probes are for the PRO Scientific range of homogenizers (click to enlarge the image).
4. Throughput and cross-contamination
Another consideration when purchasing a probe is the throughput of your application. If you want to process multiple samples in quick succession, it can be time-consuming to clean a generator between every sample, particularly if cross-contamination is a concern.
You can make the process more efficient (and reduce the chance of cross-contamination) by purchasing a multi-pack of suitable probes (if available). For some models, you’ll find disposable plastic probes such as those designed for the IKA Ultra-Turrax® T 18:
If you want to avoid disposables due to environmental or cost concerns, multi-packs of stainless steel probes offer a compromise. For example, Multi-Gen Generator Probes for PRO Scientific Rotor-Stators come in a pack of 12 so you can process a dozen samples without the need for cleaning in between.
Aside from selecting the right type and size of probe, you might consider using accessories designed to optimize your process. For example, the IKA Silentstream acts as a flow breaker to avoid vortexing and minimize the induction or air into the sample. It helps ensure the bearing system doesn’t run dry and enables the processing of small volumes at high speed.
The IKA Silentstream.
You can also purchase deflector heads such as these offered by PRO Scientific:
An acetyl resin and stainless steel deflector head (these come in various sizes).
These help homogenize larger volume and high-viscosity materials, for example, lotions, creams, and gels. The head promotes vertical movement of heavy samples, and improves the flow in larger volumes.
The material is forced up the tapered walls of the deflector then cycled down to the base of the vessel.