Particle Properties
Surface and Bulk Forces
Learning Objectives:
- Describe the types of surface forces, including van der Waals forces, electrostatic forces, and capillary forces.
- Define bulk forces, including gravitational forces and buoyancy forces, and their role in macroscopic systems.
Surface Forces Examples:
- Surface Tension: Surface tension is the force that acts along the surface of a liquid or a gas-liquid interface, causing it to behave like a stretched elastic membrane. Surface tension arises due to the cohesive forces between liquid molecules, which tend to minimize the surface area.
- Buoyancy: Buoyancy is the upward force exerted on an object submerged in a fluid, such as a liquid or gas. It is a result of the pressure difference between the top and bottom of the object.
Particle Size Distribution
Learning Objectives:
- Describe common techniques used to measure particle size distribution, including microscopy, laser diffraction, sedimentation, and sieving.
- Discuss the influence of particle size on the physical and chemical properties of materials, including surface area, reactivity, flowability, and mechanical properties.
Particle Size Distribution
- The size of a spherical homogeneous particle is uniquely defined by its diameter.
- For regular, compact particles such as cubes or regular tetrahedra, a single dimension can be used to define size.
- With some regular particles it may be necessary to specify more than one dimension: For a cone the base diameter and height are required whilst for a cuboid three dimensions are needed
- For irregular particles, it is desirable to quote the size of a particle in terms of a single quantity, and the expression most often used is the "equivalent diameter". This refers to the diameter of a sphere that would behave in the same manner as the particle when submitted to some specified operation.
- The assigned equivalent diameter usually depends on the method of measurement, hence the particle-sizing technique should, wherever possible, duplicate the process one wishes to control.
- Several equivalent diameters are commonly encountered. For example, the Stokes' diameter is measured by sedimentation and elutriation techniques; the projected area diameter is measured microscopically and the sieve- aperture diameter is measured by means of sieving.
- The PSD of a material can be important in understanding its physical and chemical properties.
- It affects the strength and load-bearing properties of rocks and soils.
- It affects the reactivity of solids participating in chemical reactions, and needs to be tightly controlled in many industrial products such as the manufacture of printer toner and cosmetics.
Krauss, CC0, via Wikimedia Commons
Solved Example: 9105-01
Particle size analysis can be done using:
A. Pycnometer
B. Hydrometer
C. One point method
D. Sedimentometer
Correct Answer: B
Solved Example: 9105-02
The sedimentation analysis for particle size distribution is adopted for the soil particle that are finer than:
A. 75 micron
B. 625 micron
C. 425 micron
D. 47.5 micron
Correct Answer: A