What Is Viscosity in Physics?

How viscous is your stew?

Technician testing viscosity of fluid in laboratory
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Viscosity is a measurement of how resistant a fluid is to attempts to move through it. A fluid with a low viscosity is said to be "thin," while a high viscosity fluid is said to be "thick." It is easier to move through a low viscosity fluid (like water) than a high viscosity fluid (like honey).

Newtonian and Non-Newtonian Fluid Viscosity

Most common fluids, called Newtonian fluids (yes, another thing named after that Newton), have a constant viscosity. There is a greater resistance as you increase the force, but it's a constant proportional increase. In short, a Newtonian fluid keeps acting like a fluid, no matter how much force is put into it.

In contrast, the viscosity of non-Newtonian fluids is not constant, but rather varies greatly depending on the force applied. A classic example of a non-Newtonian is Oobleck (sometimes called "slime," and often made in elementary school science classes) which exhibits solid-like behavior when a large amount of force is used on it. Another set of non-Newtonian fluids are known as magnetorheological fluids. These respond to magnetic fields by becoming nearly solid but reverting to their fluid state when removed from the magnetic field

Why Viscosity Is Important in Daily Life

While viscosity may seem of minor importance in daily life, it can actually be very important in many different fields. For example:

Lubrication in vehicles. When you put oil into your car or truck, you should be aware of its viscosity. That's because viscosity affects friction and friction, in turn, affects heat. In addition, viscosity also affects the rate of oil consumption and the ease with which your vehicle will start in hot or cold conditions. Some oils have a more stable viscosity, while others react to heat or cold; if your oil's viscosity index is low, it may become thinner as it heats. This can cause problems as you operate your car on a hot summer's day!

Cooking. Viscosity plays a very large role in the preparation and serving of food. Cooking oils may or may not change viscosity as they heat, while many become much more viscous as they cool. Fats, which are moderately viscous when heated, become solid when chilled. Different cuisines also rely on viscosity of sauces, soups, and stews. A thick potato and leek soup, for example, when it is less viscous, becomes French vichyssoise. Some viscous fluids add texture to foods; honey, for example, is quite viscous and can change the "mouth feel" of a dish.

Manufacturing. Manufacturing equipment requires appropriate lubrication to run smoothly. Lubricants that are too viscous can jam and clog pipelines. Lubricants that are too thin provide too little protection for moving parts.

Medicine. Viscosity can be of critical importance in medicine as fluids are introduced into the body intravenously. Blood viscosity is a major issue: blood that is too viscous can form dangerous internal clots, while blood that is too thin will not clot; this can lead to dangerous blood loss and even death.