Methods Of Particle Size Measurement
When it comes to milling, ultimate particle size is a key parameter that affects quality. Of course, the point of milling is most often to achieve particle size reduction. Typically, process engineers hope to achieve a carefully selected target particle size range. In general, the tighter the particle size distribution, the better the outcome.
Tighter Is Better
Tight particle size distribution means the majority of particles in a given batch fall within a specific, narrow size range. Smaller or larger particles tend to be exceptionally rare when distributions are tight, as documented on a steep curve plotted on a graph.
Particle size affects material properties, such as reactivity or dissolution rate, stability in suspension, efficacy of delivery (e.g., pharmaceutical applications), packing density and porosity, appearance, texture, flowability and viscosity. Any of these properties can impact the suitability, functionality, performance and/or aesthetic appeal of a final product. Thus, particle size is of crucial importance.
The best milling equipment yields the tightest particle size distribution curves, with the least amount of effort expended (i.e., time, energy and money). If you have to mill more than once, for example, you will be roughly doubling the time, energy and effort expended on achieving a particular target particle size range.
The Quadro® Comil® Conical Mill is an excellent example of an expertly engineered machine capable of achieving uniform particle size distributions. Indispensable to manufacturers in the food, pharmaceutical and fine chemical industries, the Comil® excels at deagglomeration and dispersion, too.
So, how do you measure particle sizes to be sure you are reaching designated particle size targets?
Methods of Measurement
Various methods of particle size assessment exist. They include microscopy (minute physical inspection of sample particles), simple physical analytical sieving, sedimentation techniques, electrical sensing zone method (i.e., Coulter Counter), laser diffraction and the permeametry technique.
Microcopy comprises everything from simple manual optical microscopy to more difficult (and expensive) methods, such as transmission and scanning electron microscopy, and automatic and image-analysis microscopy. Manual microscopy is relatively inexpensive, but time consuming and subject to human error. More sophisticated microscopy techniques eliminate potential errors introduced by human operators, but they can be prohibitively expensive. Emulsions may be unsuitable for analysis using the latter two techniques.
Sieving, or gradation, is the simplest, and most common, method of particle sizing. Progressively smaller-mesh sieves can be used to determine average particle size in a given sample of materials. Mesh pores typically range from as small as 37 micrometers (400 mesh) to about 3 millimeters. Percentages of different particles sizes are thus determined fairly readily.
This method harnesses Stokes’ Law to determine particle size based on the observation that the terminal velocity of a particle in fluid increases with size. In other words, smaller particles take longer to settle out of solution, providing a handy way to estimate average particle sizes.
Of course, this relatively inexpensive method has certain disadvantages, and may not be suitable for all uses. For instance, particles to be measured must be completely insoluble in the suspending fluid, and of sufficiently great size to overcome counteracting phenomena, such as Brownian motion. Re-agglomeration is another potential pitfall.
The electrical sensing zone method uses changes in impedance (electrical resistance) generated as particles in electrolyte solution pass through an orifice, as measured by two electrodes. The amplitude of voltage pulses is proportional to the volume of particles.
The process is simplified by the use of a machine (e.g., Coulter Counter) specifically designed for this purpose. This method is primarily used for counting and sizing cells, bacteria, viruses, etc. The Coulter Counter has essentially transformed — and vastly simplified — the practice of hematology, for example.
Also known as laser light scattering techniques, these approaches comprise two separate categories: amplitude dependent and amplitude independent. Both are predicated on the fact that particles in solution diffract laser light directed through the solution at varying angles. In general, angles of diffraction (light scattering) are inversely related to particle size.
A photosensitive detector in combination with a computer is used to calculate particle size distribution values. While relatively fast and accurate, these methods are expensive, and dependent on an assumption of differing refractive indices between the particles under investigation and the suspending liquid.
This refers to a method that assesses particle size by passing the substance in question (which may be a gas or liquid) through a powder bed featuring particles of known dimensions. Pressure drop and flow rate through the bed correlates with particle size of the sample substance.