ParticlesA particle is an individual item without size limitations, the only constraint being that it is small relative to its surroundings. A group of interacting particles is called a particulate system (Harr, 1925).Civilization is built on particulate systems: cement, soil, brick, mortar, paint, glass and metals are a few examples. The overwhelming majority of the materials handled by industry is in particle form. Tens of billions of tons of material are handled by the industry: for example, Mining: iron, copper, aluminum, coal, limestone, sand, gravel, phosphate; Chemical: cement, plastics, fertilizer, paint, metals, foods, glass, ceramics, brick, cinder block; Construction: concrete, soils, sediments; Farming: grains, produce, soils; Waste Products: garbage, refuse, sewage, sludge, fly ash, mine tailings, stream sediment. Some 70 % of the nation’s pollution problems are due to particulates (Meloy, 1980).
WHAT IS PARTICLE SCIENCE AND TECHNOLOGY?
IT BEGINS WITH PARTICLE FORMATION PROCESSES PARTICLE PROCESSES
1-POWDERS, 2-GRANULES, 3-CRYSTALS, 4-FLAKES, 5-PELLETS, 6-PASTES, 7-EMULSIONS, 8-DISPERSIONS
Particles and particles technologies have a profound impact on everyday lives. It is safe to say that everyone has dealt with particles in someway, at the same time, in someplace in his or her everyday life. In the US alone, the industrial output impacted by particulate systems was almost one trillion dollars in 1993 for ten major industries alone [Xu, Renliang, 2000].
Particle property: 1- Particle size 2- Particle shape 3- Particle morphology 4- Particle surface and their distributions.
Particle Characterization Particle shape is an important parameter to identify for a clearer understanding of process optimization (
http://www.malvern.co.uk/). Particle characterization for industry: Biotechnology, food and drink, pharmaceutical, cosmetics, chemicals, mining and minerals, power generation, cement, metal powders, plastics and polymers, surface coatings, electronics, ceramics.
It is well known that particle shape as well as size distribution affects the physical characteristics and behavior of particulate materials. In the pharmaceutical, toner and power coating industries for example, quantifying particle shape as well as size is already highly desirable. Many other industries are assessing the impact of shape on process robustness and product quality, but are hampered by the lack of a universal shape analysis technique.
Characterization of particle shape has always been difficult to achieve in a routine way. Traditionally, particle shape measurements have involved microscopy because of the large amount of information it provides. However, for statistical significance large numbers of particles need to be characterized and this, combined with sample preparation is very time consuming. By being able to monitor the shape of particles in a substance it will be easier to predict how the material will process (
http://www.malvern.co.uk/).
What is particle shape?
A Corel Draw program view of a mineral particle taken by SEM
Particle shape
It is not possible to discuss rationally the size of a particle or any distribution associated with the sizes of an ensemble of particles without first considering the three-dimensional characteristics of the particle itself. This is because the size of a particle is expressed either in terms of linear dimension characteristics derived from its shape or in terms of its projected surface or volume. As will be shown, some methods of expressing particle size discard any concept of particle shape and instead express the size in terms of some type of equivalent spherical size. An appropriate starting place for a discussion of particle shape can be found in USP General Test 776. In the shape performance aspect of this particular test procedure, USP requires that “for irregularly shaped particles, characterization of particle size must also include information on particle shape.” The general method defines several descriptors of particle shape. The USP definitions of these shape parameters are● acicular: slender, needle-like particle of similar width and thickness● columnar: long, thin particle with a width and thickness that are greater than those of an acicular particle● flake: thin, flat particle of similar length and width● plate: flat particle of similar length and width but with greater thickness than flakes● lath: long, thin, blade-like particle● equant: particles of similar length, width, and thickness; both cubical and spherical particles are included.
In ordinary practice, one rarely observes discrete particles but typically is confronted with particles that have aggregated or agglomerated into more-complex structures. USP provides several terms that describe any degree of association:● lamellar: stacked plates● aggregate: mass of adhered particles● agglomerate: fused or cemented particles● conglomerate: mixture of two or more types of particles● spherulite: radial cluster● drusy: particle covered with tiny particles.The particle condition also can be described by another se- ries of terms:● edges: angular, rounded, smooth, sharp, fractured● optical: color, transparent, translucent, opaque● defects: occlusions, inclusions.Furthermore, surface characteristics can be described as●cracked: partial split, break, or fissure●smooth: free of irregularities, roughness, or projections● porous: having openings or passageways● rough: bumpy, uneven, not smooth●pitted: small indentations [Brittain, H.G., 2001, Pharm. Technol. 25 (7) 96-98]
Size mean everything!It really is not possible to continue a discussion of particle shape or size without first developing definitions of particle diameter. This step is, of course, rather trivial for a spherical particle because its size is uniquely determined by its diameter. For irregular particles, however, the concept of size requires definition by one or more parameters. It often is most convenient to discuss particle size in terms of derived diameters such as a spherical diameter that is in some way equivalent to some size property of the particle. These latter properties are calculated by measuring a size-dependent property of the particle and relating it to a linear dimension.Certainly the most commonly used measurements of particle sizes are the length (the longest dimension from edge to edge of a particle oriented parallel to the ocular scale) and the width (the longest dimension of the particle measured at right angles to the length). Closely related to these properties are two other descriptors of particle size: Feret’s diameter, which is the distance between imaginary parallel lines tangent to a randomly oriented particle and perpendicular to the ocular scale, and Martin’s diameter, which is the diameter of the particle at the point that divides a randomly oriented particle into two equal projected areas The coordinate system associated with the measurement is implicit in the definitions of length, width, Feret’s diameter, and Martin’s diameter because the magnitude of these quantities requires some reference point. As such, these descriptors are most useful when discussing particle size as measured by microscopy because the particles are immobile. Defining spatial descriptors for freely tumbling particles is considerably more difficult and hence requires the definition of a series of derived particle descriptors. However, given the popularity of techniques such as electrozone sensing or laser light scattering, derived statements of particle diameter are extremely useful.All of the derived descriptors for particle size begin with the homogenization of the length and width descriptors into either a circular or spherical equivalent and make use of the ordinary geometrical equations associated with the derived equivalent. For instance, the perimeter diameter is defined as the diameter of a circle having the same perimeter as the projected outline of the particle. The surface diameter is the diameter of a sphere having the same surface area as the particle, and the volume diameter is defined as the diameter of a sphere having the same volume as the particle. One of the most widely used derived descriptors is the projected area diameter, which is the diameter of a circle having the same area as the projected area of the particle resting in a stable position. Several other derived descriptors of particle diameter have been used for various applications. For instance, the sieve diameter is the width of the minimum square aperture through which the particle will pass. Other descriptors that have been used are the drag diameter, which is the diameter of a sphere having the same resistance to motion as the particle in a fluid of the same viscosity and at the same velocity; the free-falling diameter, which is the diameter of a sphere having the same density and the same free-falling speed as the particle in a fluid of the same density and viscosity; and the Stokes diameter, which is the free- falling diameter of a particle in the laminar-flow region. [Brittain, H.G., 2001, Pharm. Technol. 25 (7) 96-98] Distribution of particle sizesAll analysts know that the particles that constitute real samples of powdered substances do not consist of any single type but instead will generally exhibit a range of shapes and sizes. Particle-size determinations therefore are undertaken to obtain information about the size characteristics of an ensemble of particles. Furthermore, because the particles being studied are not the exact same size, information is required about the average particle size and the distribution of sizes about that average.One could imagine the situation in which a bell-shaped curve is found to describe the distribution of particle sizes in a hypothetical sample; this type of system is known as the normal distribution. Samples that conform to the characteristics of a normal distribution are described fully by a mean particle size and the standard deviation. Table I shows an example of a sample exhibiting a normal distribution in which 3000 particles have been sorted according to an undefined determiner of their size. In the usual data representation, the number of particles in each size fraction is identified, and then one calculates the percentage of particles in each size fraction. This calculation yields the particle size histogram. The number frequency ordinarily is used to construct a cumulative distribution, which can be ascending or descending depending on the nature of the study and what information is required. [Brittain, H.G., 2001, Pharm. Technol. 25 (7) 96-98]
Particle size distribution of a mineral ground by a mill.
