A lot of people ask us about homogenizing solids with a rotor-stator homogenizer. In many instances, rotor-stator homogenizers can homogenize samples with solids in it, as well as many samples where the primary material is solid and you are looking to create a suspension or perform an extraction. There is one key rule of thumb to follow, however:
When homogenizing solids, the largest diameter of any solid particles should be no larger than one half the diameter of the probe. For example, if you are using a 20 mm probe, any solid particles in the sample should be no more than 10 mm at its widest.
Why is this? The high-shear area that samples must be exposed to in order to process is actually in the small gap between the rotor and the stator. The solids need to be small enough to be at least partially sucked into that space or simply blugeoned apart by the force of the spinning rotor. However, if the solid particles are too large they will simply bounce off (or get stuck on) the non-moving outer stator and will not homogenize.
As with all rules of thumb, it does not hold true in all situations. There are many things which are simply too hard or tough to homogenize, as the shear forces are insufficient to tear the particles apart. For instance, pieces of rubber, bone, or small rocks would be largely impervious to the forces of a rotor-stator homogenizer.
Likewise, there are things that are so soft and pliable that their size is almost irrelevant. Adipose tissue or any soft, spongy material would be readily sucked into the shaft and torn apart. Something readily flexible with a very high aspect ratio, like a long and wide but very flat leaf, is also not bound by this rule as it will easily crumple into a much smaller size than its original form.
Additionally, when homogenizing larger solid particles, use of a saw-tooth probe will help. Saw-tooth probes have sharp, jagged edges which will catch solids and help cut them apart. This is especially recommended when homogenizing anything fibrous, such as most animal or plant tissues.
Keep in mind that larger diameter probes accommodate higher volumes than smaller diameter probes. This means that if you have a low-volume application which necessitates a smaller probe, you'll need to ensure that any solid matter in your sample is smaller as well. Conversely, large-volume applications with larger probes can usually accommodate larger solid particles.
If you're ever uncertain what rotor-stator and probe are right for your application, just give us a call or send us an email and we'll be happy to help you find the right homogenizer for your needs.