![]() ![]() Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. In materials science and engineering, one is often interested in understanding the forces or stresses involved in the deformation of a material. In a general parallel flow, the shear stress is proportional to the gradient of the velocity. The relative strength of this force is a measure of the fluid's viscosity. Since the shearing flow is opposed by friction between adjacent layers of fluid (which are in relative motion), a force is required to sustain the motion of the upper plate. Definition Dynamic viscosity Illustration of a planar Couette flow. Viscum also referred to a viscous glue derived from mistletoe berries. The word "viscosity" is derived from the Latin viscum (" mistletoe"). A fluid that has zero viscosity is called ideal or inviscid. Zero viscosity (no resistance to shear stress) is observed only at very low temperatures in superfluids otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. For example, the viscosity of a Newtonian fluid does not vary significantly with the rate of deformation. However, the dependence on some of these properties is negligible in certain cases. In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. For a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. Experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow. ![]() For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. Viscosity quantifies the internal frictional force between adjacent layers of fluid that are in relative motion. Thus its SI units are newton-seconds per square metre, or pascal-seconds. Viscosity is defined scientifically as a force multiplied by a time divided by an area. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. You would not then need any further tests.The viscosity of a fluid is a measure of its resistance to deformation at a given rate. If it has been raised by a recent infection (a very common cause) then it is likely to return to normal when your infection improves. However, before you have further tests, your doctor may suggest that you have the ESR, CRP, PCT or PV test repeated after a period of several weeks or months. It is not usually possible to make a diagnosis of a certain condition just from a raised ESR, CRP, PCT or PV level. PCT is more specific for pneumonia and sepsis and is used to support a possible diagnosis to enable treatment to start quickly. This may prompt a doctor to do further tests to find the cause. A raised ESR, CRP, PCT and PV may indicate that some inflammatory condition is likely to be the cause. For example, you may be unwell but the cause may not be clear. In other words, a raised level means that 'something is going on' but further tests will be needed to clarify exactly what that is. When are these blood tests used? To help diagnose diseasesĮSR, CRP, PCT and PV are nonspecific tests. PCT is more sensitive than CRP so is a better marker for sepsis. Procalcitonin levels are only used for pneumonia and sepsis. It is also lowered in conditions where your body has lower protein levels - for example, in some liver or kidney diseases. Some conditions lower the ESR - for example, heart failure, polycythaemia and sickle-cell anaemia.
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