All homogenizers which operate in batch, as opposed to those which process a flow-through or continuous stream, have volume limitations. Some of these are impossible to get around. For instance, bead mill homogenizers utilize closed vessels, usually tubes, in which the homogenization occurs. The size of the tubes dictates how large your sample can be, and that's all there is to it.
Other homogenizers, primarily homogenizers that utilize a probe, can process substances in open containers, such as beakers or flasks. Rotor-stator and ultrasonic homogenizers operate in this manner. So the question arises: why can't we process a larger volume?
In many situations, you can, but not a lot of people realize this. It's never ideal, but doing so can help in a pinch.
The D500 Homogenizer Package, which is a rotor-stator style homogenizer (left) and the Q700 Sonicator, which is an ultrasonic homogenizer (right) both utilize probes.
Understanding Why Volume Limitations Exist
A probe-based homogenizer's maximum volume limitation isn't based on the amount of material that the homogenizer can process. Theoretically speaking, any homogenizer could process an infinite amount of material presuming that a) the material entered the area where the homogenizing forces (shear, cavitation, etc.) were being applied and b) we had sufficient time to process it to achieve the desired result (% cell lysis, desired particle size, etc.). The volume limitation is only because of flow.
In order to process a sample, the homogenizer needs the material to enter the area where the homogenizing forces are generated. For a rotor-stator, it needs to pull the sample into the area between the rotor and stator where the high shear forces are created. For an ultrasonic homogenizer, the sample needs to pass in front of the probe (also called a "horn" for ultrasonic systems). This requires the entirety of the sample to be well mixed. In a normal homogenization setup, the homogenizer also serves as the mixer. While they can mix, they are designed for homogenization, not mixing. Large volumes are not well mixed by a homogenizer, and may result in "dead zones" which do not flow and, therefore, are not homogenized.
Overcoming Volume Limitations
The use of a secondary mixer in the vessel, such as an overhead stirrer or magnetic stirrer, can create far more mixing than the homogenizer can on its own. By coupling one of these with a homogenizer, you can sometimes greatly surpass the ordinary maximum volume restrictions of a homogenizer.
Some small magnetic stirrers, like this FlatSpin stirrer, can fit between the legs of an H-stand which are available for some homogenizers.
If you are going to use a stirrer with a homogenizer, there are a few things you should be aware of:
- Always ensure that the homogenizer probe tip is in liquid, not air. Stirring at a high speed may cause a vortex, and the probe should be away from this vortex. Ideally, you will stir at a speed which does not cause a vortex. Operating an ultrasonic horn in air will damage it and potentially break it. Many rotor-stator probes have a bearing which require lubrication by the medium being homogenized or else they will cease up, requiring replacement of the bearing and potentially the entire probe.
- The stirring element, in other words the stir bar or the propeller of the overhead stirrer, should never contact the probe of the homogenizer, as it could irreparably damage both instruments.
- Having a stirrer will help get around volume limitations, not viscosity limitations. While viscosity does reduce the maximum volume that can be processed with a probe-based homogenizer, the homogenizer still cannot process any volume above a specified viscosity. If you are unsure what that viscosity is for a particular instrument, ask us!
- Ensure that your homogenizer is suitable for extended use. Most are, but if you're going to be running the homogenizer frequently and for long periods of time, you want to ensure you have a good quality system that isn't going to burn out. Again, when in doubt, ask us!
Similar to using a stirrer to create secondary mixing, you can also move the location of the probe throughout the substance you are homogenizing. This is most readily done if you are using a small, handheld homogenizer, but it can even be done in larger containers by raising / lowering the homogenizer on its stand, rotating the vessel which contains your mixture, or just about anything else you can think of. Just be very certain to never contact the probe to the sides or bottom of the container. With an ultrasonic homogenizer, you shouldn't even come close to the sides or bottom of the vessel. The amount of space required is unique to each instrument. Consult the user manual for the spacing that a given ultrasonic homogenizer requires.
Operating a homogenizer above its volume limitation is not an ideal situation. If you have a larger volume than a given homogenizer can process, look for a unit that has a larger volume rating. That will provide the best performance and most efficient homogenization. However, if you have a homogenizer and need to temporarily go past its volume limitations, operating it in conjunction with a stirrer can potentially allow you to process volumes that you would not otherwise be able to.
