Particle and Surface Sciences (PSS) is offering no obligation technical advice on particle characterisation applications, and has the latest brochures and information on particle size measurement.
Accurately determining particle size has become essential in many industries, as it is a fundamental physical characteristic that must be selected, monitored and controlled from the raw material source to the finished product.
There is an optimum particle size, or at least a smallest and largest acceptable size, for most items involving particles.
The key to accurate particle size determination is selecting the most appropriate sizing instrument for a particular application.
If all particles were spheres, their size would be defined explicitly by their diameter or radius; if cubical, the length along one edge would be characteristic; and if of another regular shape, another equally appropriate dimension could be chosen.
Unfortunately, the majority of particles are quite irregular and an arbitrary definition of size is the only simple solution.
Moreover, typical collections of particles include many different sizes and shapes.
Thus, a definition is required that accommodates this diversity, and the 'equivalent spherical diameter (or radius)' satisfies this requirement.
Equivalence of size means that a specific, experimentally measured attribute of the particle is the same as that of a sphere of a certain size.
In other words, when the particle under test and a sphere of a specific size are exposed to the same conditions, the identical reaction occurs.
Examples of reactions that may occur are the light scattering characteristics (scattering equivalency), the attainment of a certain terminal settling velocity (settling equivalency), or the displacement of fluid (volume equivalency).
Therefore, obtaining particle size information about a material may be accomplished by a number of techniques, each of which test for a specific equivalency.
Micromeritics offers instruments that use three different techniques.
This allows one to select the best technique for a material and application rather than trying to adapt one method to all situations.
The sizing techniques used by Micromeritics are light scattering, sedimentation and electric sensing zone.
As is always the design objective with Micromeritics instruments, each instrument provides the user with high data quality in terms of resolution and accuracy, and with a dependable measuring tool in terms of reliability, repeatability and reproducibility.
A particle illuminated with monochromatic, collimated light scatters the light at specific angles relative to the incident beam and at specific intensities at these angles as described by the Mie and Fraunhofer theory.
The intensity-angle relationship is a function of particle size, the wavelength of incident light, and the relative refractive index of the suspension fluid and particle.
If multiple particles of various sizes are illuminated simultaneously, the resulting pattern is the summation of all the contributions of intensity by each particle at each angle.
There are various embodiments of Mie and Fraunhofer scattering theories in particle sizing instrumentation.
These are generally referred to as 'laser particle size analysers', but are more appropriately described as 'laser diffraction' or 'low angle light scattering' (LALS) techniques.
These instruments are generally applied to either the sizing of liquid or solid particles suspended in a gaseous medium (aerosols), or to solids suspended in liquid.
Other applications are also found in laboratories and in production environments such as in sizing freely falling (cascading) particles or particles forcefully carried by fluid streams.
By far, the widest application of LALS is to solid particles suspended in liquid media.
Light scattered from the assemblage of particles under test is detected at various angles and the intensity at each of these angles recorded.
This set of ordered pairs (angle, intensity) describes the experimentally collected scattering function.
Extracting information about the quantity of particles in each size class from this set of data requires that the composite information be deconvoluted into individual scattering patterns from each size class according to the Mie or Fraunhofer theory for spherical particles.
Deconvolution is based on fitting a summation of 'reference' scattering patterns from spherical particles to the experimentally collected light scattering signature of the particle assemblage.
The particle sizes and quantities reported, therefore, represent the collection of spherical particles whose composite (summed) scattering pattern best reproduce the experimental data.
Micromeritics' Saturn Digisizer 5200 particle size analyser uses light scattering to determine particle size.
The hallmarks of this instrument are the high resolution of the scattering angle and the wide dynamic range of light intensity accommodation.
These capabilities are made possible by the use of a 1.3 million element charge-coupled device (CCD).
Fine-tuning of the optical alignment is accomplished in software by dynamically remapping the array around the central beam (optical axis).
Reproducible, pinpoint alignment assures that the instrument will provide the same data for the same sample from analysis to analysis and from laboratory to laboratory.
High-resolution capabilities mean that a slight change in size or quantity distribution from sample to sample will be distinguishable.
Sedimentation is nature's primary method of size separation as evidenced by the deposition of waterborne and airborne materials.
It has been used by man to separate particles by size longer than any other method with the exception of sieving.
Sizing by sedimentation is not difficult and the process is described rigorously by Stokes' equation.
The difficulty arises in determining the quantity of particles in each size class.
This problem was solved by the use of soft X-rays to detect mass.
Micromeritics introduced 'X-ray sedimentation' as a measuring tool more than 30 years ago.
For many industrial applications and scientific studies, Micromeritics' Sedigraph remains the de facto standard technique.
The Sedigraph employs sedimentation from a homogeneous liquid-solid suspension to separate a sample by size.
The absorption of X-rays directly detects the mass concentration in the spatially separated collection of particles.
Measuring the rate at which particles of a certain density fall under the influence of gravity through a liquid of known density and viscosity provides all of the necessary parameters to apply Stokes' equation and determine the equivalent spherical sizes of the particles.
In this case, the size reported is the size of a sphere that has the same settling rate as the test particle.
The electric sensing zone method detects the volume of liquid displaced by a particle as individual particles of a sample suspended in an electrolyte are transported through a sensing zone.
The sensing zone is a short capillary that separates two volumes of electrolyte liquid, each volume being maintained at different electrical polarities.
When a particle is being swept through the sensing zone, it displaces some volume of the conducting liquid, thus decreasing the conductance of current through the capillary.
The momentary interruption of current flow results in an electrical pulse that is detected by sensing circuitry.
The amplitude of the pulse is proportional to the volume of electrolyte displaced and, thus, to the volume of the particle.
The instrument sums the number of particles (pulses) occurring in each volume (amplitude) class.
From these data, a frequency distribution of particle volume versus particle size is obtained.
For particles of uniform density, volume is directly proportional to mass.
Although volume is measured directly, size is reported as the equivalent spherical size and equals the size of a sphere that displaces the same volume of liquid, as does the particle under test.
Micromeritics' Elzone particle sizing and particle counting analysers use the electric sensing zone technique.
The Elzone is immune to almost all physical properties of samples.
It can measure true concentration of particles to give number of particles per ml, per gram, PPM, and so on.
Particles can be dispersed in a liquid that is different from the electrolyte.
Particle and Surface Sciences are the sole distributors throughout Australia and New Zealand for particle characterisation instrumentation from Ankersmid, Dataphysics, DOP Solutions, Micromeritics, Lighthouse Worldwide Solutions, Nanosight Technology, Clemex, Impact Test equipment, Mipoy, as well as its own products, the Pola 2000 and Pola 3000 particle counters.
Particle testing solutions are also available at the Particle and Surface Sciences Analytical Services Laboratory.
Laboratory services include the characterisation of particle size, particle shape, particle counting, surface area, nanoparticle testing, micropore analysis, density, sieving, pore size by gas adsorption or mercury porosimetry.
Visit the company's website for a copy of the Analytical Laboratory Services catalogue.
