Acoustic Radiation Pressure
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A force on a particle suspended in the field of a sound wave arising from the scattering of the acoustic waves on the particle.
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Adsorption
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A process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms (the adsorbate). Most industrial adsorbents fall into one of three classes: 1. Oxygen-containing compounds (e.g.silica gel and zeolites 2. Carbon-based compounds (e.g. activated carbon and graphite) 3. Polymer-based compounds
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Amphiphiles
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A chemical compound possessing both hydrophilic and hydrophobic properties. Common amphiphilic substances are soap and detergent.
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Bingham Plastic
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A viscoplastic material that behaves as a rigid body at low stresses but flows as a viscous fluid at high stress. A common example is toothpaste, which will not be extruded until a certain pressure is applied to the tube. It then is pushed out as a solid plug.
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Centrifugal Force
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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.
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Colloid
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A type of chemical mixture where one substance is dispersed evenly throughout another. The particles of the dispersed substance are only suspended in the mixture, unlike a solution, where they are completely dissolved within. This occurs because the particles in a colloid are larger than in a solution - small enough to be dispersed evenly and maintain a homogenous appearance, but large enough to scatter light and not dissolve.
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Density Gradient
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Variation of density across an area or through a volume.
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Diamagnetism
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The property of an object which causes it to create a magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect. It is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field. Diamagnetism is generally a quite weak effect in most materials, although superconductors exhibit a strong effect.
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Drag
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A drag (sometimes called fluid resistance) is the force that resists the movement of a solid object through a fluid (a liquid or gas). The most familiar form of drag is made up of friction forces, which act parallel to the object's surface, plus pressure forces, which act in a direction perpendicular to the object's surface.
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Eddy Currents
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An electrical phenomenon caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause eddys of circulating current within the conductor which create electromagnets with magnetic fields that opposes the change of the magnetic field.
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Ekman layer
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The layer in a fluid where there is a force balance between pressure gradient force, Coriolis force and turbulent drag. In addition to enforcing the zero velocity condition at the wall, these Ekman layers can also control long-range properties of the flow. A classical illustration is given by the everyday experience of how a cup of tea returns to rest after stirring.
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Elastic Recovery
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The tendancy of a material that has been stretched to return to its original shape.
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Elasticity
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The physical property of a material when it deforms under stress (e.g. external forces), but returns to its original shape when the stress is removed. The amount of deformation is called the strain. The elastic regime is characterized by a linear relationship between stress and strain, denoted linear elasticity.
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Electret
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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.
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Electric Field
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The space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field (that can also be equated to electric flux density). This electric field exerts a force on other electrically charged objects.
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Electrodeposition
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The process of using electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal. Primarily used for depositing a layer of material to bestow a desired property (e.g., abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.) to a surface that otherwise lacks that property. Another application uses electroplating to build up thickness on undersized parts.
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Electrohydrodynamics
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Includes the following types of particle and fluid transport mechanisms:Electrophoresis, electrokinesis, dielectrophoresis, electro-osmosis, and electrorotation. In general, the phenomena relate to the direct conversion of electrical energy into kinetic energy, and vice versa.
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Electromagnetic Induction
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The generation of electromotive force (EMF) in a current-carrying conductor exposed to a changing magnetic field
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Electrorheological Effect
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Electrorheological (ER) fluids are suspensions of extremely fine non-conducting particles (up to 50 micrometres diameter) in an electrically insulating fluid. The apparent viscosity of ER fluids changes reversibly by an order of up to 100,000 in response to an electric field. A typical ER fluid can go from the consistency of a liquid to that of a gel, and back, with response times of the order of ms. Sometimes called the Winslow Effect.
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Electrostatic Induction
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A redistribution of electrical charge in an object, caused by the influence of nearby charges. Electrostatic generators, such as the Wimshurst machine, the Van de Graaff generator and the electrophorus, use this principle. Electrostatic induction should not be confused with electromagnetic induction; both are often referred to as 'induction'.
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Electrostatics
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The phenomena arising from stationary or slowly moving electric charges
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Electroviscous Effect
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Change in a liquid's viscosity induced by a strong electrostatic field.
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In the case of a divided solid suspended in a liquid.
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Faraday Wave
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(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.
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Faraday waves are used as a liquid-based template for directed assembly of microscale materials including soft matter, rigid bodies and biological entities.
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Ferromagnetism
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The mechanism by which certain materials (such as iron) form permanent magnets and/or exhibit strong interactions with magnets. Responsible for commonly observed magnetism phenomena, e.g. 'fridge magnets. A material is 'ferromagnetic' only if all its magnetic ions add a positive contribution to the net magnetisation. If some of them subtract from the net magnetisation (i.e. are partially anti-aligned), then the material is 'ferrimagnetic'.
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Flocculation
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A process of contact and adhesion whereby the particles of a dispersion form larger-size clusters.
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Force
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That which can cause an object with mass to accelerate. Force has both magnitude & direction, i.e. it's a vector quantity. An object with constant mass will accelerate in proportion to the net force acting upon it and in inverse proportion to its mass (or the net force on an object is equal to the rate of change of momentum it experiences). Forces acting on objects may cause them to rotate or deform, or result in a change in pressure.
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Forced Convection
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Heat advection by a fluid which is not due to the natural forces of buoyancy induced by heating. In forced heat convection, transfer of heat is due to movement in the fluid which results from many other forces, such as (for example) a fan or pump.
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Fractal Forms
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A fractal is generally 'a rough or fragmented geometric shape that can be split into parts, each of which is (at least approximately) a reduced-size copy of the whole,' a property called self-similarity. A mathematical fractal is based on an equation that undergoes iteration, a form of feedback based on recursion.
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Free Convection
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Movement of molecules of fluids (or gases) dues to density differences in the fluid (or gas) occurring due to temperature gradients.
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Freezing
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A phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. Colloquially it is applied to water, but technically it applies to any liquid. All known liquids, except liquid helium, freeze when the temperature is lowered enough.
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Gel
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A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute crosslinked system, which exhibits no flow when in the steady-state. By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional crosslinked network within the liquid. It is the crosslinks within the fluid that give a gel its structure (hardness) and contribute to stickiness (tack).
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Gravitation
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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.
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Groove
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A long and narrow indentation built into a material, generally for the purpose of allowing another material or part to move within the groove and be guided by it.
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Holes
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A hole is a confined lack of structure in some part of an object.
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Inertia
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The resistance of any physical object to a change in its state of motion or rest. It is proportional to an object's mass.
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Johnsen-Rahbek Effect
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An attractive force that occurs when an electric potential is applied across the boundary between a metallic surface and the surface of a semiconducting material. The magnitude of the force depends on the voltage and the specific materials involved.
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London Dispersion Force
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A weak intermolecular force acting between atoms and molecules arising from quantum induced instantaneous polarisation multipoles in molecules. Can therefore act between molecules without permanent multipole moments.
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Lorentz Force
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The force on a point charge due to electromagnetic fields. When a wire carrying an electrical current is placed in a magnetic field, each of the moving charges, which comprise the current, experiences the Lorentz force, and together they can create a macroscopic force on the wire (sometimes called the Laplace force).
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Magnetic Field
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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.
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Magnetism
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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.
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Metastability
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A general scientific concept which describes states of delicate equilibrium. A system is in a metastable state when it is in equilibrium (not changing with time) but is susceptible to fall into lower-energy states with only slight interaction. It is analogous to being at the bottom of a small valley when there is a deeper valley close by
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Photophoresis
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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.
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Physical Containment
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Partial or complete enclosure of an object or substance by some physical medium, typically for the purpose of protection or restriction of movement.
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Physisorption
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(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.
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Porosity
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The quality of being porous - i.e. having voids or spaces within a solid substance within or through which fluids can be present.
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Potential Well
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The region surrounding a local minimum of potential energy. Energy held in a potential well is unable to convert to another type of energy (kinetic energy in the case of a gravitational potential well) because it is captured in the local minimum of a potential well and so may not proceed to the global minimum of potential energy, as it would naturally tend to due to entropy.
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Pressure Increase
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Pressure is an effect which occurs when a force is applied on a surface. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point.
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Reaction (physics)
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In classical mechanics, Newton's third law states that forces occur in pairs: the Action and the Reaction. Both forces are equal in magnitude and opposite in direction. The distinction between action and reaction is purely arbitrary: any one of the two forces can be considered an action, in which case the other (corresponding) force automatically becomes its associated reaction.
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Rheopecty
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(or rheopexy) The rare property of some non-Newtonian fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shearing force, the higher its viscosity. Rheopectic fluids, such as some lubricants, thicken or solidify when shaken (the opposite type of behaviour, in which fluids become less viscous the longer they undergo shear, is called thixotropy and is much more common).
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Sedimentation
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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.
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Settling
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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.
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Solvation
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(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.
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Sorption
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The action of both absorption and adsorption taking place simultaneously, i.e. the effect of gases or liquids being incorporated into a material of a different state and adhering to the surface of another molecule. Absorption is the incorporation of a substance in one state into another of a different state (e.g., liquids absorbed by solid or gas absorbed by liquid). Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another molecule.
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Static Friction
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(or Stiction) Two solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Static Friction is a threshold, not a continuous force.
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Suction
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The flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area.
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Suspension
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A heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase (solid) is dispersed throughout the external phase (fluid - which may be liquid or gas) through mechanical agitation, with the use of certain excipients or suspending agents.
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Thixotropy
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The property of some non-Newtonian pseudoplastic fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite amount of time to attain equilibrium viscosity when introduced to a step change in shear rate. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.
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Some thixotrophic materials have a divided solid as one of their constituents, e.g. some clays.
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Triboelectric Effect
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A type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated (such as through rubbing). The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties.
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Van der Waals Force
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The sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules. Includes Keesom Force, Debye Force and London dispersion force.
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Vortex Ring
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A torus shaped vortex in a fluid i.e. a region where the fluid mostly spins around an imaginary axis line that forms a closed loop. The dominant flow in a vortex ring is said to be toroidal, more precisely poloidal. Within a stationary body of fluid, a vortex ring can travel for relatively long distance, carrying the spinning fluid with it.
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In the case of a divided solid dispersed in a fluid, e.g. a smoke ring.
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Wetting
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The ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces. Adhesive forces between a liquid and solid cause a liquid drop to spread across the surface. Cohesive forces within the liquid cause the drop to ball up and avoid contact with the surface.
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