Particle Size Reduction

Homogenizers are commonly used to reduce the size of solid particles. The solid particles may be dry, in a precipitated solid state with a liquid buffer, or suspended in solution. The milling of pharmaceutical preparations is a common application.

Particle sizes may be sufficiently reduced and the resulting suspension made sufficiently homogenous such that otherwise insoluble particle remain in suspension for an extended period of time. In the image below, vial a shows coagulated carbon nanotubes at a concentration of 2500 mg/l, which are insoluble in a water and SDS solution. A Sonicator is then used to homogenize the sample for three minutes (vial c). The nanotubes remain in solution after four months at room temperature (vial d).

Choosing a Homogenizer for Particle Size Reduction

Bead mills, ultrasonic homogenizers, and rotor-stator homogenizers are appropriate for particle size reduction, although bead mills and ultrasonic homogenizers are often capable of achieving smaller particle sizes than rotor-stators.

The most important consideration is whether the particles will be processed with a liquid or if you will be performing dry milling. Ultrasonic homogenizers, as well as most rotor-stators, require liquid in order to operate. If you will be performing dry milling, a bead mill is most appropriate.

Bead mills are suitable for wet or dry milling, but laboratory bead mills are only capable of homogenizing small samples (on the order of microliters to single-digit milliliters). When selecting a bead mill, you will want a highly powerful unit. (We ordinarily try to be brand-neutral in the Application Center, but the Precellys 24 is an extremely powerful unit and has been used to reduce pharmaceutical preparations to particle sizes in the 100s of nanometers.)

If using a rotor-stator or ultrasonic homogenizer, you want to ensure the most vigorous homogenization possible. To do that, you want to use the largest probe which is compatible with your sample volume.

While it is possible to use a high-pressure homogenizer to reduce particle sizes, we generally advise against it due to differences in cost and effectiveness (unless you already have access to one, in which case go ahead and give it a try!) Additionally, particles larger than the pore size through which the sample is forced may result in clogging of the instrument.