But when stirred, mayonnaise yields very easily to the force. Mayonnaise is a good example: It has a high viscosity when subjected to the relatively low shear of gravity (i.e., an open jar of mayonnaise does not readily spill out if the jar is turned on its side). With some fluids, known as non-Newtonian fluids, the viscosity varies with the rate at which the fluid is forced to move. Most lubricant oils are considered Newtonian. For Newtonian fluids, absolute viscosity is constant, as is kinematic viscosity, and the two measurements are relative to one another. Those fluids are known as Newtonian fluids (because they adhere to Sir Isaac Newton’s law of fluid mechanics). Most fluids maintain a constant viscosity regardless of the pace at which the fluid is forced to move. Q: Does a fluid’s viscosity change in relation to the rate of shear stress (i.e., how rapidly or slowly the fluid is stirred, pumped or otherwise forced to move), or is the viscosity constant?Ī: The answer depends on the specific fluid. For example, if you were to tip over open containers of honey and water, the honey would pour out more slowly because its kinematic viscosity is higher. The slower the fluid’s flow in relation to gravity, the higher its viscosity. Kinematic viscosity, in simple terms, measures a fluid’s resistance to flow under the force of gravity. For example, it takes more force to mix yogurt in a blender than to mix fruit juice because yogurt has a higher absolute viscosity. The more force required to move the fluid, the greater the fluid’s absolute viscosity. Q: What are absolute viscosity and kinematic viscosity, and what is the difference between them?Ī: Absolute and kinematic represent the two methods for measuring and reporting viscosity.Ībsolute (also known as dynamic) viscosity measures a fluid’s resistance to flow when the fluid is subjected to a controlled force, such as a moving machine part, pressurized air or a pump. The greater the resistance to flow, the higher the fluid’s viscosity. Below are answers to some common questions about viscosity that may shed some light on this complex yet vital aspect of lubrication.Ī: Viscosity is a fluid’s resistance to flow at a given temperature. Understanding viscosity is important for proper lubricant selection and management, and ultimately for maximizing the performance, efficiency and life span of your equipment. This may lead to significant damage and possibly equipment failure. A lubricant’s viscosity is generally considered one of its most important properties, and with good reason: If the viscosity of a lubricant is even modestly different from what is needed for a particular component and application, the lubricant will not be able to lubricate the component effectively.
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