Particle size (general)
Encyclopedia
Particle size is a notion introduced for comparings dimension
s of solid
particles (flecks), liquid
particles (droplets), or gas
eous particles (bubbles).
The notion of particle size applies to
The particle size of a spherical object can be unambiguously and quantitatively defined by its diameter
.
However, a typical material object is likely to be irregular in shape
and non-spherical. The above quantitative definition of particle size cannot be applied to non-spherical particles. There are several ways of extending the above quantitative definition, so that a definition is obtained that also applies to non-spherical particles. Existing definitions are based on replacing a given particle with an imaginary sphere
that has one of the properties identical with the particle.
Another complexity in defining particle size appears for particles with sizes below a micrometre
. When particle becomes that small, thickness of interface
layer becomes comparable with the particle size. As a result, position of the particle surface becomes uncertain. There is convention for placing this imaginary surface at certain position suggested by Gibbs and presented in many books on Interface and Colloid Science
,,,,,.
Definition of the particle size for an ensemble (collection) of particles presents another problem. Real systems are practically always polydisperse
, which means that the particles in an ensemble have different sizes. The notion of particle size distribution
reflects this polydispersity. There is often a need of a certain average particle size for the ensemble of particles. There are several different ways of defining such a particle size.
There are several methods for measuring particle size. Some of them are based on light
, other on ultrasound
, or electric field
, or gravity, or centrifugation
. They are briefly described in the section particle size distribution
.
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...
s of solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
particles (flecks), liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
particles (droplets), or gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
eous particles (bubbles).
The notion of particle size applies to
- Colloidal particleColloidal particleColloidal particle is a small amount of matter having size typical for colloids and with a clear phase boundary . A group of such particles or being a macromolecule or a molecular aggregate .A colloidal sized particle is defined in diameter from 1-1000 nanometers...
s; - ParticlesParticle (ecology)In marine and freshwater ecology, a particle is a small object. Particles can remain in suspension in the ocean or freshwater, however they eventually settle and accumulate as sediment. Some can enter the atmosphere through wave action where they can act as cloud condensation nuclei...
in ecology; - Particles present in particulate matter;
- Particles that form a granular materialGranular materialA granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact . The constituents that compose granular material must be large enough such that they are not subject to thermal motion fluctuations...
.
The particle size of a spherical object can be unambiguously and quantitatively defined by its diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
.
However, a typical material object is likely to be irregular in shape
Shape
The shape of an object located in some space is a geometrical description of the part of that space occupied by the object, as determined by its external boundary – abstracting from location and orientation in space, size, and other properties such as colour, content, and material...
and non-spherical. The above quantitative definition of particle size cannot be applied to non-spherical particles. There are several ways of extending the above quantitative definition, so that a definition is obtained that also applies to non-spherical particles. Existing definitions are based on replacing a given particle with an imaginary sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
that has one of the properties identical with the particle.
- Volume based particle size equals the diameter of the sphere that has same volume as a given particle. Which equals 2*(3*volume(particle)/4/pi)^(1/3).
- Weight based particle size equals the diameter of the sphere that has same weight as a given particle. Which equals 2*(3*weight(particle)/4/pi/density(particle)/g)^(1/3).
- Area based particle size equals the diameter of the sphere that has the same surfaceSurfaceIn mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
area as a given particle. Which equals 2*(area(particle)/4/pi)^(1/2). - Hydrodynamic or aerodynamic particle size equals the diameter of the sphere that has the same dragDrag (physics)In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
coefficient as a given particle.
Another complexity in defining particle size appears for particles with sizes below a micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
. When particle becomes that small, thickness of interface
Interface (chemistry)
An interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...
layer becomes comparable with the particle size. As a result, position of the particle surface becomes uncertain. There is convention for placing this imaginary surface at certain position suggested by Gibbs and presented in many books on Interface and Colloid Science
Interface and colloid science
Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm and 1000 nm dispersed in a continuous...
,,,,,.
Definition of the particle size for an ensemble (collection) of particles presents another problem. Real systems are practically always polydisperse
Polydisperse
A collection of objects is called polydisperse or polysized if they have a broad range of size, shape and mass characteristics. A sample of objects that have a uniform size, shape and mass distribution are called monodisperse. In practice, polydisperse collections are common because it is...
, which means that the particles in an ensemble have different sizes. The notion of particle size distribution
Particle size distribution
The particle-size distribution of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amounts of particles present, sorted according to size...
reflects this polydispersity. There is often a need of a certain average particle size for the ensemble of particles. There are several different ways of defining such a particle size.
- There is an International Standard on presenting various characteristic particle sizes. This set of various average sizes includes median size, geometric mean size, average size.
There are several methods for measuring particle size. Some of them are based on light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
, other on ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
, or electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
, or gravity, or centrifugation
Centrifugation
Centrifugation is a process that involves the use of the centrifugal force for the sedimentation of mixtures with a centrifuge, used in industry and in laboratory settings. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture...
. They are briefly described in the section particle size distribution
Particle size distribution
The particle-size distribution of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amounts of particles present, sorted according to size...
.