What Is Viscosity in Physics?

Technician testing viscosity

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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).

Key Takeaways: Importance of Viscosity

  • Viscosity, the "thickness" of fluid, refers to how resistant a fluid is to movement within it.
  • Water has a low or "thin" viscosity, for example, while honey has a "thick" or high viscosity.
  • The law of viscosity has important uses in such areas as inkjet printing, protein formulations and injections, and even food and beverage manufacturing.

Viscosity Definition

Viscosity refers to the thickness of fluid. By that definition, water has a low viscosity, or its viscosity is "thin," while honey has a high viscosity, or its viscosity is "thick." At a molecular level, viscosity is caused by the interaction of molecules in fluid. Think of viscosity as the friction between molecules in fluid.

In a fluid with high viscosity such as honey, there is a great deal of friction between molecules. And this friction slows down the movement of molecules. With water, by contrast, there is far less friction, and molecules can move about more quickly and freely, which is why water's viscosity is thin.

Viscosity results from the interaction, or friction, between molecules in a fluid. Similar to friction between moving solids, viscosity will determine the energy required to make a fluid flow. In physics, viscosity is often expressed using Isaac Newton's equation for fluids, which is similar to Newton's second law of motion. This law states that when a force acts on an object, it will cause the object to accelerate. The larger the mass of the object, the greater the force will need to be to cause it to accelerate.

Viscosity Formula

The viscosity formula is often expressed using Newton's equation for fluids:

F / A = n (dv / dr)

where F represents force and A represents area. So, F/A, or force divided by area, is another way of defining viscosity. Dv divided the dr represents the "sheer rate," or the speed the liquid is moving. The n is a constant unit equal to 0.00089 Pa s (Pascal-second), which is a dynamic viscosity measurement unit. This law has some important practical applications such as inkjet printing, protein formulations/injections, and food/beverage manufacturing.

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.