High-pressure homogenizers are a fairly broad catch-all term for any homogenizer that forces a stream of primarily liquid sample through a system which subjects it to any one of a number of forces which is intended to homogenize the sample and / or reduce the particle sizes of any components within it. In practice, depending on the setup of a particular system, a high pressure homogenizer could operate on any combination of shear forces, impact, and cavitation.
One of the most common setups consists of a tank to which high pressure is applied in order to force the liquid sample contained therein through a valve or membrane with very narrow slits. This act causes high shear, a large pressure drop, and cavitation, all of which act to homogenize the sample. Sometimes the high pressure stream is directed at a blade, ring, or plate, upon which the sample collides at a high speed, to aid in homogenization.
High-pressure homogenizers are most commonly used for creating emulsions and for cell lysis when relatively large volumes are being processed. These are also the kinds of homogenizers used in the dairy industry, albeit on a larger scale.
High-pressure homogenizers can effectively process large volumes of liquid sample thoroughly and reproducibly. For this reason, and because high-pressure homogenizers do not deposit any material into the sample, high-pressure homogenizers are the type used in the dairy industry.
Some high pressure homogenizers also allow a very high degree of flexibility in the process due to the ability to modulate the process stream. Furthermore, with potentially vigorous processing and the capability to recycle the effluent stream, high-pressure homogenizers can often achieve very small, sub-micron particle sizes.
Due to cost, large-volume processing is often the only time that high-pressure homogenizers make sense. They are very expensive; a relatively inexpensive unit costs over $10,000. Also, to prevent cross-contamination, the entire unit needs to be cleaned every time it is used.
High-pressure homogenizers tend to be large and extremely heavy.
High-pressure homogenizers should not be used if your sample contains too much particulate or solid matter, as this could clog the openings in the valves. Be sure to clean the unit thoroughly after each use, both to prevent cross-contamination and to clean any particulate away from the valves. Failure to do so will result in higher operating pressures which may decrease instrument life and / or decrease the efficiency of homogenization.
If one pass does not provide satisfactory homogenization, you can run the sample through the homogenizer multiple times. Some homogenizers have configurations allowing for recycling of the effluent for easy multiple-pass homogenization.
As with most homogenizers, the most important thing to consider is sample size! Different high-pressure homogenizers exist to accommodate a very wide range of sample sizes. Also, consider whether your homogenization process will be batch or continuous, as not all homogenizers are capable of both.
If you have reason to believe that your homogenization will be particularly difficult and may require multiple passes, look for units that can recycle the effluent feed for automated multi-pass homogenization.
Consider the difficulty of cleaning the homogenizer, as for most applications it will need to be cleaned after every use.