Now You Can Characterize your Particles in All Three Dimensions
By Terry Stauffer
Dynamic Image Analysis (DIA) is a particle characterization technique rapidly growing in popularity, because it measures so much more, and so much better, than older conventional methods. With the rare exception, conventional methods measure and report only one particle parameter, the Equivalent Spherical Diameter (ESD). Three dimensional (3-D) DIA can report all the many size and shape parameters that 2-D DIA can, with one major addition, Thickness (T), or the smallest dimension of the particles. To get more information and specifications about the only dynamic image analyzer which measures all three major dimensions of every individual particle, please click here.
Most methods of particle characterization are ensemble measurement techniques, which means that they measure some type of signal given off by a large ensemble of particles in the sensing zone at the same time. Laser Diffraction (LD), Sedimentation, and Sieving are three important examples. Electrical Sensing Zone (ESZ) is an exception, in that it measures individual particles, one at a time, but still reports only the ESD parameter, along with a particle population count. Just as importantly, it must be noted that in these ensemble methods, the particles tumble through the sensing zone in all of their different random orientations.
Therefore, unless the particles are spheres, the data will not represent accurate information about the particles different sizes and shapes. A rod-shaped particle seen from the end, would be reported as a sphere with a diameter the size of the smallest dimension of the rod. Seen in it’s longest dimension, it would be reported as a sphere with that diameter. So, for example, if one of the rods were 3 x 1 microns in size (an aspect ratio not out of the ordinary for many particulate materials) the volume of the 3-micron sphere would be 27 times the volume of the 1-micron sphere. And the ESD is almost always reported as a volume distribution. So this same particle could generate both of these answers for size, a difference of a factor of 27, depending on its orientation as it tumbled through the sensing zone.
Dynamic image analysis is commonly considered the most recent and advanced, and also becoming widely used, commercial particle characterization technique. It has considerably higher resolution than the most used conventional techniques, because it measures each particle individually, giving, arguably, infinite resolution. And it can report upwards of some 26 different size and shape parameters for each particle. This technique is also uncomplicated and easy to use. Quite simply, photos of each particle are digitized, stored in a viewable image file, and then measured by counting the pixels, whose size is known. No other particle characterization method could be so straight-forward. It became commercially viable with the advent of inexpensive, high-speed, high-resolution CCD (and now CMOS) cameras, and affordable high-speed, large mass- storage computers. The only thing left to do was to include software that quickly calculates all the traditional particle size and shape parameters which microscopists had been reporting for years. But now, unlike with manual microscopy, large statistically valid samples of particles can be automatically measured in only a few minutes.
Most DIA products suffer from the same random orientation of particles that the conventional single sizing methods do. This only adds noise to the data, making the information of little value for particles not shaped like spheres. A few DIA products do employ techniques that keep the particles oriented with their largest two dimensions (or largest projected area) toward the camera, and give noise-free data. They cannot, however, provide the smallest particle dimension, Thickness (T), as this dimension lies along the focal axis and can’t be seen by the camera. These products are therefore only two-dimensional (2-D) DIA analyzers.
This is a series of digital photos of one particle (fused glass bead) tumbling through the field of view of the 3-D DIA. The progression is left to right in the sequence. The first photo, on the left, shows the largest projected area of the particle. This is the photo that will be used to assign to the Length of the particle and also the maximum distance perpendicular to the Length, Width. The fourth photo in the series shows the minimum projection and thus the Thickness can be reported thus giving the particle the correct quantitative dimensions of all three of its major axes and all the correct shapes calculated from sizes. It’s very important to the glass bead industry (and all others) to use 3-D image analysis. They need to produce batches which are single spherical beads, and not elongated, fused or agglomerated particles. If two fused particles, or agglomerated particles were viewed only once in a 2-D analysis, they could be viewed showing the smallest projected orientation. The image would then appear to be a near sphere and be considered to be in spec.
To summarize, the dimensions of all three of the major axes of a 3-dimensional particle can be measured correctly only by the method of particle tracking performed by the Microtrac 3-D DIA (Dynamic Image Analyzer). Any 2-D analysis averages all the different orientations of randomly tumbling particles, biasing the largest dimensions to incorrect smaller values, and the smallest dimension to incorrect larger values. And image analyzers using largest projected area orientation only cannot measure the Thickness dimension, because it extends along the focal axis behind the particle and out of view of the camera. Shape parameters reported by image analysis are ratios of the different measured size dimensions of the particles. Correct shape parameters can also therefore only be calculated by the 3-D analysis employed by the Microtrac DIA.
Terry Stauffer has over 40 years of particle characterization experience. Terry has worked for all the major players in the particle analysis industry at some point in his career. To connect with Terry on LinkedIn, please follow this link. www.linkedin.com/pub/terry-stauffer/54/227/8a3