Scientists Say: Viscosity

This is a measure of how much a fluid resists stress

This drop of honey flows downward slowly because it is viscous. 

Dino Giordano/Flickr/(CC BY 2.0)

Viscosity (noun, “Vis-KOS-ih-tee”, adjective, viscous, “VIS-kuhs”)

A measure of how much a fluid can resist pressure or tension. It’s also used to describe how thick a liquid is.  Gooey liquids such as honey, maple syrup and ketchup have high viscosity. They pour very slowly. Water or acetone (a liquid used in paint thinner and nail polish remover) have very low viscosity. You can see that because these liquids pour very quickly.

In a sentence

Water has reduced viscosity when it’s full of bacteria all swimming in the same direction.

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Power Words

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acetone  A chemical produced by the body that is detectable in people’s breath. It’s also an extremely flammable liquid solvent used, for example, in nail polish remover.

bacterium (plural bacteria)  A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

stress  (in biology) A factor, such as unusual temperatures, moisture or pollution, that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional, or behavioral reaction to an event or circumstance, or stressor, that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative. (in physics) Pressure or tension exerted on a material object.

viscosity  The measure of a fluid’s resistance to stress. Viscosity corresponds to the idea of how “thick” a liquid is. Honey is very viscous, for instance, while water has relatively low viscosity.

viscous   The property of being thick, sticky and hard to pour. Molasses and maple syrup are two examples of viscous liquids.

Bethany Brookshire is the staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.

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