Viscosity is the resistance to flow of a fluid, affected by factors such as molecule size and shape, interactions, and temperature. It can be measured with viscometers and is important in industries such as oil transportation and engine lubrication. Viscosity also plays a role in volcanism, with thicker magma leading to more explosive eruptions.
Viscosity is a scientific term that describes the resistance to flow of a fluid. The fluid can be a liquid or a gas, but the term is more commonly associated with liquids. To take a simple example, syrup has a much higher viscosity than water: It takes more force to move a spoon through a jar of syrup than in a jar of water because the syrup is more resistant to flowing around the spoon. This resistance is due to the friction produced by the molecules of the fluid and affects both the extent to which a fluid will oppose the movement of an object through it and the pressure required to move a fluid through a tube or pipe. Viscosity is influenced by a number of factors, including the size and shape of the molecules, the interactions between them, and temperature.
Measurement
The viscosity of a liquid can be measured in several ways by devices called viscometers. These can measure the time it takes for a fluid to move a particular distance through a pipe or the time it takes for an object with a given size and density to fall through the liquid of interest. The SI unit for this is the pascal-second, where the pascal is the unit of pressure. This quality is then measured in terms of pressure and time, so that, under a given pressure, a viscous liquid will take longer to travel a given distance than a less viscous one.
Factors affecting viscosity
As a rule, fluids with larger, more complex molecules will have higher viscosities. This is especially true for the long chain molecules found in polymers and heavier hydrocarbon compounds. These molecules tend to get entangled with each other, preventing their movement.
Another important factor is how molecules interact with each other. Polar compounds can form hydrogen bonds that connect separate molecules, increasing overall resistance to flow and movement. Although water is a polar molecule, it has low viscosity due to the fact that its molecules are small. The most viscous liquids tend to be those with long molecules that have obvious polarity, such as glycerin and propylene glycol.
Temperature has a major effect on viscosity, so viscosity measurements for fluids are always given with temperatures. In liquids it decreases with temperature, as can be seen if syrup or honey is heated. This is because the molecules move more and therefore spend less time in contact with each other. Conversely, resistance to motion in gases increases with temperature. This is because, as the molecules move faster, there are more collisions with each other, which reduces the ability to flow.
Importance to industry
Crude oil is often transported long distances through regions with varying temperatures, and the rate of flow in response to pressure varies accordingly. The oil flowing through Alaska is more viscous than the oil in pipelines in the Persian Gulf, due to differing ground temperatures, and consequently more pressure must be applied to keep it moving. To address the force required to deliver oil through piping, sensors in some pipes measure the viscosity of the fluid and determine whether more or less pressure needs to be added to keep the oil flowing steady and stable.
Of course, engine oil is also subject to viscosity changes when heated by an engine. Oil that becomes too thin from the heat of the engine will not work properly. To solve this problem, polymers are added to the oil to keep friction rates constant at higher temperatures.
Relevance to volcanism
The viscosity of magma, or hot, molten rock beneath the Earth’s surface, is an important factor in the study of volcanoes. Liquid lava tends to cause more frequent but less violent eruptions, as it flows easily from magma chambers and out of the volcano. It also allows the dissolved gas to escape more easily. Thicker magma tends to trap this gas under high pressure, and more force is required to eject the lava from the volcano, allowing great pressure to build up over time. When this type of volcano erupts, it does so explosively, often with catastrophic consequences.
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