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Viscosity is a measure of a fluid's resistance to flow.

Do not confuse the term viscous with vicious (mean and nasty!).

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Additional Info

Liquids such as molasses or motor oil are examples of highly viscous substances. The viscosity of a liquid generally decreases as the temperature is increased. For example, warm molasses is easier to pour and stir than cold molasses because it has a lower viscosity when warm.

Two common units of viscosity are the poise which is 1 dyne per second per square centimeter and the centipoise (100 centipoise = 1 poise). These can also be expressed as (Newton)(second) per square meter.

Gases also have a resistance to flow, although these viscosities are much lower than for liquids.

When selecting a stirring motor for use with viscous liquids, one generally needs to purchase a unit that has a geared motor that provides the necessary torque. Safety Emporium has a link to a handy selection chart on its HST20 Series Stir Tester page.

SDS Relevance

Safety Data Sheets are required to list physical properties such as viscosity (if known and/or relevant) in Section 9 (physical and chemical properties).

Viscous materials pour slowly and may be difficult to stir. In general, the viscosity of a syrupy material tends to decrease as the temperature increases. Your workplace procedures for the handling and transfer of viscous materials may require you to heat them, producing and entirely new set of thermal hazards.

Further Reading

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See also: area units.

Additional definitions from Google and OneLook.

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