Viscosity of Liquids
Although liquids and gases both have viscosity, it is liquids that are most commonly analyzed for their viscous properties. By understanding the viscosity of a liquid, what affects that viscosity, and whether the viscosity is considered low or high, it is possible to choose a liquid that is appropriate for a specific application. To apply low viscosity liquids in engineering, knowing how various liquids are used in specific applications, and how the conditions of use affect the viscosity of liquids are important things to consider.
Low Viscosity Liquids
Low viscosity liquids are liquids that flow freely. Depending on the specific situation, how viscous a liquid is can play an important role in its application. Some examples of liquids with low viscosities are shown in the following table:
What does low viscosity mean?
Generally, the viscosity of a liquid can be associated with its “thickness”, with low viscosity liquids being less “thick”. Often, the difference between water and honey is used to illustrate how the viscosity of a liquid can be visualized. In terms of a more technical definition, low viscosity liquids have less resistance to flow when they are subject to shear stress.
The viscosity of a liquid is related to how well its layers move in relation to each other. The layers of a low viscosity liquid move easily because there is less friction between them. The viscous force of a liquid is expressed by the following equation:
where μ is the coefficient of viscosity, A is the area over which the force is acting, and du/dy is the velocity gradient in the direction perpendicular to the flow. The SI unit of viscosity is Newton-second per square meter, and the equivalent English unit is pound-seconds per square foot.
What is considered low viscosity?
The Society of Automotive Engineers developed a viscosity index (VI), which is most often used for describing the viscosity of motor oils or other lubricants. While the VI is only one method of evaluating viscosity, on this scale, a VI less than 35 is considered low.
In more general terms, low viscosity can be defined as a viscosity less than about 100, although various fields are likely to specify their own range for what constitutes a low viscosity liquid. Depending on the application of a liquid, including the environment in which it is used, viscosity can be very dynamic. As a result, a liquid may be considered to have high viscosity in one application, but low viscosity in another. Based on the application, an engineer can determine just what is considered “low viscosity” when choosing a liquid.
What causes low viscosity?
Low viscosity in liquids is the result of temperature, with higher temperatures decreasing viscosity. Many liquids exhibit low viscosity at room temperature, such as water or alcohol, and other liquids demonstrate low viscosity under increasing heat. For example, cooking oil, although rather inviscid at room temperature, becomes even less viscous when it is heated in a pan.
Many engineering applications involve heat, and in these applications the viscosity of a liquid is very important to understand. As the heat increases during operation, the viscosity of the liquid will decrease, which will lead to it functioning differently.
How to Measure Low Viscosity
Viscosity is measured using a device called a viscometer. Although there are many types of viscometers, the basic principle of all types measures the flow rate of a liquid. Some viscometers measure how long it takes for a known volume of a liquid to flow from one point to another, while others measure how quickly an object moves through a liquid.
Through refinement and new technologies, the accuracy of viscometers has been improved. While some viscometers are used in laboratory settings for precise measurement and analysis, many devices are employed in practical engineering to determine the viscosity of liquids in specific applications.
Using a viscometer, it is possible to establish the VI of a liquid, thereby determining if it has low viscosity with the following equation:
where U is the viscosity of the liquid being measured at 40°C, L is the viscosity of a hypothetical liquid with a VI of 0 at 40°C, and H is the viscosity of a hypothetical liquid with a VI of 100 at 40°C, both of which have the same viscosity as the liquid being tested at 100°C. The values for L and H are found in lookup tables.
Benefits of Low Viscosity
Because low viscosity liquids flow more freely, they have a wide range of applications. In the realm of lubrication, low viscosity lubricants can improve the performance and lifetime of parts. When used with pumps or other devices that produce a flow, low viscosity liquids require less force to move. In other words, less energy will be expended to pump a liquid with a low viscosity, which has a number of benefits depending on the application.
Low viscosity also increases Reynold’s number for a flowing fluid. In turn, this reduces the friction factor and the overall pressure drop for the flow.
Disadvantages of Low Viscosity
While there are advantages to using liquids with low viscosity, sometimes it is not the best option. Taking lubrication as an example once more, lower viscosity means less lubrication. Because viscosity decreases with increasing temperature, high-heat applications are more likely to require higher viscosity liquids during operation. Without an appropriate understanding of the viscosity of a liquid and the effect of heat on that viscosity, the viscosity can become too low for the particular application.