![]() ![]() When two fluids of equal volume are placed in identical capillary viscometers and allowed to flow under the influence of gravity, the more viscous fluid takes longer than the less viscous fluid to flow through the tube. It is frequently measured using a device called a capillary viscometer - basically a graduated can with a narrow tube at the bottom. Kinematic viscosity is a measure of the resistive flow of a fluid under the influence of gravity. The other quantity called kinematic viscosity (represented by the Greek letter ν "nu") is the ratio of the viscosity of a fluid to its density. The quantity defined above is sometimes called dynamic viscosity, absolute viscosity, or simple viscosity to distinguish it from the other quantity, but is usually just called viscosity. There are actually two quantities that are called viscosity. ![]() Ten poise equal one pascal second making the centipoise and millipascal second identical. The most common unit of viscosity is the dyne second per square centimeter, which is given the name poise after the French physiologist Jean Poiseuille (1799–1869). The pascal second is more rare than it should be in scientific and technical writing today. Despite its self-proclaimed title as an international system, the International System of Units has had little international impact on viscosity. The SI unit of viscosity is the pascal second, which has no special name. Or if you prefer calculus symbols (and who doesn't)… F The similarity to Newton's second law of motion ( F = ma) should be apparent. The more usual form of this relationship, called Newton's equation, states that the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity. (dynamic) viscosityįormally, viscosity (represented by the symbol η "eta") is the ratio of the shearing stress ( F/ A) to the velocity gradient ( ∆ v x/∆ y or dv x/ dy) in a fluid. Fluids resist the relative motion of immersed objects through them as well as to the motion of layers with differing velocities within them. It is suggested that viscosity is roughly measured by how long it takes the treacle to flow from one end of a boiling tube to the other.Informally, viscosity is the quantity that describes a fluid's resistance to flow. The "Treacle Test" experiment is designed to enable students to investigate how temperature, volatiles and crystals may affect viscosity. A “crystal mush” will clearly have increased viscosity. Again, this applies particularly to the cooler, more viscous magmas typical of destructive plate margins. Some magmas have already begun to crystallise by the time they reach the surface. The release of gas during eruption is particularly likely to be explosive if the magma is both viscous (as gas is released, so viscosity is immediately increased) and volatile rich. A high volatile content decreases viscosity (like adding water to treacle), and is probably the main factor in enabling some highly viscous (but also volatile-rich) melts to reach the surface at all. Magmas formed by melting of mantle rocks have generally low volatile contents, but those formed by partial melting of crustal rocks are often volatile-rich. Magma invariably contains small amounts of dissolved gas (water, CO 2 etc) which is released as pressure is removed. Rhyolites may reach the surface at temperatures of less than 900 oC, and so have much higher viscosity. Not surprisingly, magmas formed by partial melting of mantle rocks are much hotter – well over 1200 oC for some Hawaiian basalts – than is the case for crustally derived melts. ![]() Magma temperatures reflect the melting points of their mineral components. ![]() These magmas erupt as basalts or intrude as gabbro, and are far less viscous. Low-silica magmas are typically formed by partial melting of mantle rocks beneath mid-ccean ridges or at “hot spots” like Hawaii. The more extensive silicate chain molecules render these magmas highly viscous, so when eruption occurs it is usually explosive (e.g. Such magmas erupt as andesites and rhyolites or are intruded as granite masses. Silicate-rich magmas are typically formed at destructive plate boundaries, by partial melting and/or assimilation of crustal rocks (which are richer in silica than the rock of the mantle). The nature of volcanic eruptions is highly dependent on magma viscosity and also on dissolved gas content. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |