Angle of Repose
|
(or critical angle of repose) The minimum angle made by the inclined plane with the horizontal surface such that the body lying on the inclined plane is just at the verge of sliding down along the inclined plane. When bulk granular materials are poured onto a horizontal surface, a conical pile will form. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material.
|
In the case of a pile of granular material.
|
Centrifugal Force
|
An outward force associated with rotation. Centrifugal force is one of several so-called pseudo-forces (or inertial forces), so named because, unlike real forces, they do not originate in interactions with other bodies situated in the environment of the particle upon which they act. Instead, centrifugal force originates in the rotation of the frame of reference within which observations are made.
|
Cheerio Effect
|
The tendency for small wettable floating objects to attract one another as a result of a combination of surface tension and buoyancy.
|
Chemical Bonding
|
The physical process responsible for the attractive interactions between atoms and molecules, and which confers stability to diatomic and polyatomic chemical compounds. In general, chemical bonding is associated with the sharing of electrons between the participating atoms. Molecules, crystals, and diatomic gases and most of the physical environment around us are held together by chemical bonds.
|
Coffee Ring Effect
|
A pattern left by a puddle of particle-laden liquid after it evaporates. Named after the characteristic ring-like deposit along the perimeter of a spill of coffee. The pattern is due to capillary flow induced by the differential evaporation rates across the drop: liquid evaporating from the edge is replenished by liquid from the interior. The resulting edgeward flow can carry nearly all the dispersed material to the edge.
|
Cohesion
|
The action or property of like molecules sticking together, being mutually attractive. This is an intrinsic property of a substance that is caused by the shape and structure of its molecules which makes the distribution of orbiting electrons irregular when molecules get close to one another, creating electrical attraction that can maintain a macroscopic structure such as a water drop.
|
Compression
|
The result of the subjection of a material to compressive stress, resulting in reduction of volume. The opposite of compression is tension. In simple terms, compression is a pushing force.
|
Crystallisation
|
The (natural or artificial) process of formation of solid crystals precipitating from a identical solution or melt, or more rarely deposited directly from a gas. Crystallisation is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.
|
Deposition (physical)
|
A process in which gas transforms into solid (also known as desublimation). The reverse of deposition is sublimation. One example of deposition is the process by which, in sub-freezing air, water vapour changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground.
|
Electret
|
An Electret is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet.
|
Solid particles can be attracted by the charge of an electret.
|
Electrolysis
|
A method of separating chemically bonded elements and compounds by passing an electric current through them. Electrolysis involves the passage of an electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes.
|
Faraday Wave
|
(or Faraday Ripples) Nonlinear standing waves that appear on liquids enclosed by a vibrating receptacle. When the vibration frequency exceeds a critical value, the flat hydrostatic surface becomes unstable. The waves can take the form of stripes, close-packed hexagons, or even squares or quasiperiodic patterns.
|
Faraday waves are used as a liquid-based template for directed assembly of microscale materials including soft matter, rigid bodies and biological entities.
|
Flocculation
|
A process of contact and adhesion whereby the particles of a dispersion form larger-size clusters.
|
Fluidisation
|
A process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state. This process occurs when a fluid (liquid or gas) is passed up through the granular material. When fluidized, a bed of solid particles will behave as a fluid, like a liquid or gas.
|
When fluidised, a bed of solid particles will behave as a fluid, like a liquid or gas. Like water in a bucket: the bed will conform to the volume of the chamber, its surface remaining perpendicular to gravity; objects with a lower density than the bed density will float on its surface, bobbing up and down if pushed downwards, while objects with a higher density sink to the bottom of the bed.
|
Gravitation
|
A natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass.
|
Hydraulic Press
|
A hydraulic mechanism for applying a large lifting or compressive force. It is the hydraulic equivalent of a mechanical lever.
|
Impact Force
|
A high force or shock applied over a short time period. Such a force or acceleration can sometimes have a greater effect than a lower force applied over a proportionally longer time period.
|
Magnetic Field
|
A vector field which surrounds magnets and electric currents, and is detected by the force it exerts on moving electric charges and on magnetic materials. When placed in a magnetic field, magnetic dipoles tend to align their axes parallel to the magnetic field. Magnetic fields also have their own energy with an energy density proportional to the square of the field intensity.
|
For example, in the case of iron filings.
|
Magnetism
|
One of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron, cobalt, and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field.
|
Mechanical Force
|
A mechanical influence that causes a free body to undergo an acceleration.
|
Melting
|
A process that results in the phase change of a substance from a solid to a liquid. The internal energy of a solid substance is increased (typically by the absorbtion of heat) to a specific temperature (called the melting point) at which it changes to the liquid phase.
|
Voids or holes in a solid material are inherantly removed by melting.
|
Nucleation
|
The extremely localised budding of a distinct thermodynamic phase. Some examples of phases that may form via nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation. Most nucleation processes are physical, rather than chemical, but a few exceptions do exist (e.g. electrochemical nucleation).
|
Ostwald Ripening
|
An observed phenomenon in solid solutions or liquid sols that describes the change of an inhomogeneous structure over time, i.e., small crystals or sol particles dissolve, and redeposit onto larger crystals or sol particles. Occurs because larger particles are more energetically favored than smaller particles. This stems from the fact that molecules on the surface of a particle are energetically less stable than the ones in the interior. Ostwald ripening is also observed in liquid-liquid systems, causing diffusion of monomers (i.e. individual molecules or atoms) from smaller droplets to larger droplets due to greater solubility of the single monomer molecules in the larger monomer droplets.
|
Example: the re-crystallization of water within ice cream which gives old ice cream a gritty, crunchy texture. Larger ice crystals grow at the expense of smaller ones within the ice cream, creating a coarser texture.
|
Photophoresis
|
The phenomenon that small particles suspended in gas (aerosols) or liquids (hydrocolloids) starts to migrate when illuminated by a sufficiently intense beam of light. The existence of this phenomenon is owed to a non-uniform distribution of temperature of an illuminated particle in a fluid medium.
|
Physical Containment
|
Partial or complete enclosure of an object or substance by some physical medium, typically for the purpose of protection or restriction of movement.
|
Physisorption
|
(or physical adsorption) A type of adsorption in which the adsorbate adheres to the surface only through Van der Waals (weak intermolecular) interactions, which are also responsible for the non-ideal behaviour of real gases.
|
Sedimentation
|
The motion of molecules in solutions or particles in suspensions in response to an external force such as gravity, centrifugal acceleration or electromagnetism. Sedimentation may pertain to objects of various sizes, ranging from suspensions of dust and pollen particles to cellular suspensions to solutions of single molecules such as proteins and peptides.
|
Settling
|
The process by which particulates settle to the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction exerted by that force. For gravity settling, this means that the particles will tend to fall to the bottom of the vessel, forming a slurry at the vessel base.
|
Solvation
|
(commonly called dissolution) The process of attraction and association of molecules of a solvent with molecules or ions of a solute. As ions dissolve in a solvent they spread out and become surrounded by solvent molecules. The bigger the ion, the more solvent molecules are able to surround it and the more it becomes solvated.
|
Supersaturation
|
A solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapour of a compound that has a higher (partial) pressure than the vapour pressure of that compound.
|
Thermal Contraction
|
The tendency of matter to reduce in volume (contract) in response to a change in temperature or when a substance is cooled.
|
