The particle density or true density of a particulate solid or powder, is the density of the particles that make up the powder, in contrast to the bulk density, which measures the average density of a large volume of the powder in a specific medium (usually air).
A 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 are large enough such that they are not subject to thermal motion fluctuations. Thus, the lower size limit for grains in granular material is about 1 µm. On the upper size limit, the physics of granular materials may be applied to ice floes where the individual grains are icebergs and to asteroid belts of the Solar System with individual grains being asteroids.
Bulk density is a property of powders, granules, and other "divided" solids, especially used in reference to mineral components, chemical substances, (pharmaceutical) ingredients, foodstuff, or any other masses of corpuscular or particulate matter. It is defined as the mass of many particles of the material divided by the total volume they occupy. The total volume includes particle volume, inter-particle void volume, and internal pore volume.
The particle density is a relatively well-defined quantity, as it is not dependent on the degree of compaction of the solid, whereas the bulk density has different values depending on whether it is measured in the freely settled or compacted state (tap density). However, a variety of definitions of particle density are available, which differ in terms of whether pores are included in the particle volume, and whether voids are included.
Measurement
The measurement of particle density can be done in a number of ways:
Archimedes' principle
The powder is placed inside a pycnometer of known volume, and weighed. The Pycnometer is then filled with a fluid of known density, in which the powder is not soluble. The volume of the powder is determined by the difference between the volume as shown by the pycnometer, and the volume of liquid added (i.e. the volume of air displaced). A similar method, which does not include pore volume, is to suspend a known mass of particles in molten wax of known density, allow any bubbles to escape, allow the wax to solidify, and then measure the volume and mass of the wax/particulate brick.
A slurry of the powder in a liquid of known density can also be used with a hydrometer to measure particle density by buoyancy.
A hydrometer is an instrument used for measuring the relative density of liquids based on the concept of buoyancy. They are typically calibrated and graduated with one or more scales such as specific gravity.
Buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.
Another method based on buoyancy is to measure the weight of the sample in air, and also in a liquid of known density.
A column of liquid with a density gradient can also be prepared: The column should contain a liquid of continuously varying composition, so that the maximum density (at the bottom) is higher than that of the solid, and the minimum density is lower. If a small sample of powder is allowed to settle in this column, it will come to rest at the point where the liquid density is equal to the particle density.
Volumetric measurement
A gas pycnometer can be used to measure the volume of a powder sample. A sample of known mass is loaded into a chamber of known volume that is connected by a closed valve to a gas reservoir, also of known volume, at a higher pressure than the chamber. After the valve is opened, the final pressure in the system allows the total gas volume to be determined by application of Boyle's law.
A gas pycnometer is a laboratory device used for measuring the density—or, more accurately, the volume—of solids, be they regularly shaped, porous or non-porous, monolithic, powdered, granular or in some way comminuted, employing some method of gas displacement and the volume:pressure relationship known as Boyle's Law. A gas pycnometer is also sometimes referred to as a helium pycnometer.
Boyle's law, most often referred to as the Boyle–Mariotte law, or Mariotte's law, is an experimental gas law that describes how the pressure of a gas tends to increase as the volume of the container decreases. A modern statement of Boyle's law is
The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.
A mercury porosimeter is an instrument that allows the total volume of a powder to be determined, as well as the volume of pores of different sizes: A known mass of powder is submerged in mercury. At ambient pressure, the mercury does not invade the interparticle spaces or the pores of the sample. At increasing pressure, the mercury invades smaller and smaller pores, with the relationship between pore diameter and pressure being known. A continuous trace of pressure versus volume can then be generated, which allows for a complete characterization of the sample's porosity.
Archimedes' principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid. Archimedes' principle is a law of physics fundamental to fluid mechanics. It was formulated by Archimedes of Syracuse.
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure the "accessible void", the total amount of void space accessible from the surface. There are many ways to test porosity in a substance or part, such as industrial CT scanning. The term porosity is used in multiple fields including pharmaceutics, ceramics, metallurgy, materials, manufacturing, hydrology, earth sciences, soil mechanics and engineering.
In physics, astronomy, chemistry, biology and geography, number density is an intensive quantity used to describe the degree of concentration of countable objects in physical space: three-dimensional volumetric number density, two-dimensional areal number density, or one-dimensional linear number density. Population density is an example of areal number density. The term number concentration is sometimes used in chemistry for the same quantity, particularly when comparing with other concentrations.
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Related Research Articles
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A fluidised bed is a physical phenomenon occurring when a quantity of a solid particulate substance is placed under appropriate conditions to cause a solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium. This results in the medium then having many properties and characteristics of normal fluids, such as the ability to free-flow under gravity, or to be pumped using fluid type technologies.
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Inverse gas chromatography is a physical characterization analytical technique that is used in the analysis of the surfaces of solids.
A measuring instrument is a device for measuring a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item under study and the referenced unit of measurement. Measuring instruments, and formal test methods which define the instrument's use, are the means by which these relations of numbers are obtained. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty.
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A density meter, also known as a densimeter, is a device that measures the density. Density is usually abbreviated as either
or
. Typically, density either has the units of or . The most basic principle of how density is calculated is by the formula:
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