For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared rather, they depend on how quickly the shearing occurs. In other materials, stresses are present which can be attributed to the deformation rate over time. 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. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Caption by Erik Klemetti (Denison University) and Adam Voiland (Earth Observatory).The viscosity of a fluid is a measure of its resistance to deformation at a given rate. NASA Earth Observatory images by Robert Simmon, using Landsat 8 data from the USGS Earth Explorer. Like Chao, the flows shown above have pressure ridges caused by the compression of the cooling top of the lava as the flow advanced. They have smaller flow fronts (10 to 30 meters tall) in comparison to the sheer 400-meter cliffs at Chao, as well as more prominent lava levees along the edges. In comparison to the Chao dacite in Chile (the product of viscous lava), the flows at Zhupanovsky and Dzenzursky are much narrower and longer. These features form as lava cools and hardens along the edges or top of a flow while the center of a flow still advances. Distinctive lava levees are visible along the edges of many of the younger flows. The exact ages of the flows are unclear, but the eruptions that produced them likely occurred during the past few thousand years. In the image, younger lava flows appear grey, while older flows are covered by green vegetation. The image was acquired by the Operational Land Imager (OLI) on the Landsat 8 satellite on September 9, 2013. Many characteristics of a low-viscosity lava flow are visible in this image of Zhupanovsky and Dzenzursky volcanoes on Russia’s Kamchatka Peninsula. They also tend to have smaller flow fronts and levee-like structure along their edges. While viscous lava flows are defined by steep flow fronts and pressure ridges, low-viscosity lavas tend to move faster and create longer, narrower shapes. These rivers of rock can take many shapes and move at very different rates depending on the viscosity of the magma, the slope of the land, and the rate of an eruption. Streams of molten rock that ooze from gaps or vents in the Earth’s surface are called lava flows, and they can pose a hazard to everything in their paths. Read part 1 of this story to see an example of a lava dome created by highly viscous lava.
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