Particle Size and Shape Analysis
Posted by Pat Moran on Sep 20, 2016
The Importance of Particle Size and Shape
Characterization of particle morphology (particle size and shape) is a foundational element of nearly all of our particle engineering pursuits. This is because nearly every important bulk powder property that we engineer and design towards derives from, and is inherently correlated to, particle morphology. We must keep in mind that the particle morphology that is observed on the micro-scale translates into the macro-scale bulk powder properties. Particle morphology dictates how particles associate and pack together, thus determining the bulk void-volume structure which in-turn dictates bulk density, compressibility, and other powder flow properties.
Further, particle morphology, and very specifically surface-to-volume ratio will effect chemical and physical reaction rates as well as mass transfer rates between a particle and its surroundings. This includes reactions like oxidation, as well as physical phenomena like adsorption, absorption, permeation and desorption. Particles with a higher surface-to-volume ratio are more easily accessible to molecular species in the surrounding environment as there are more sites to bind to or react with, and the permeation pathways into the powder cores are shorter for smaller volume particles.
Ultimately, in designing our products and ingredients, we engineer our particle morphology to obtain the properties that will yield a high-performance powder that is functional in processing, handling, and end-use applications.
Quantitative Evaluation of Particle Size and Shape
There are many routes by which one can obtain particle size and shape distributions. Methods include: sieving, microscopy, dynamic image analysis, laser diffraction, aerodynamic analysis and electrozone or Coulter counting. There are advantages and disadvantages to each particle sizing methodology. However, for identification of particle shape, the image-based methods, microscopy or dynamic image analysis are preferred.
Image-based analyses generally consist of taking many high-resolution microscopic images of the powder sample, with the goal of non-destructively gathering a representative sampling of images. In microscopy, the powder is generally stationary, resting on a flat slide or stage while images are obtained. In dynamic image analysis, the powder is often observed while cascading or flowing in carrier liquid or air stream while high-speed images are captured. Regardless of the method for capturing the images, once they are obtained they must be transformed, often “thresholded” and converted to black and white in order to isolate the particles. If the particles are not easily isolated by simple thresholding, more sophisticated techniques may be used, such as edge-finding or water-shedding. Once individual particles are isolated, computer software can calculate various shape and size parameters on a particle-by-particle basis. After the parameterization, the data can be scaled from pixels to units of length and area and subjected to a variety of statistical analysis techniques in order to maximize value in the overall development process.
In conclusion, it is critical to understand the link between particle morphology and powder performance, and how to quantitatively measure particle morphology and use this data to guide the product development process.