why does viscosity decrease as temperature increases?

Question

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Answer 1

All fluids (gases included) resist flow. Absolute viscosity is a measure of this resistance to flow. The absolute viscosity of a gas for given conditions may be calculated from the following formula:



Note: The term "absolute" is added to "viscosity" to differentiate it from "kinematic viscosity" (discussed later). When "viscosity" alone is used in these modules, assume it refers to absolute viscosity.

As this equation indicates, the viscosity of a gas increases as the temperature increases. It's harder to push something (e.g. particles) through a hot gas stream than a cooler one due to increased molecular activity as temperature rises, which results in increased momentum transfer between the molecules. For liquids, the opposite relationship between viscosity and temperature holds. The viscosity of a liquid decreases as temperature increases. It's harder to push something through a cold liquid than a hot one because in liquids, hydrogen bonding increases with colder temperatures.

Gas viscosity actually increases very slightly with pressure, but this variation is very small in most air pollution-related engineering calculations.

One unit commonly used to describe viscosity is the centipoise (cp).



The viscosity of air and other gases at various temperatures and at a pressure of one atmosphere may be found in engineering tables. Table 1 provides the viscosity of air at various temperatures. The viscosity data in Table 1 is based on Equation 1.





Kinematic Viscosity

The absolute viscosity and density of a gas are occasionally combined into a single parameter since both of these parameters are found in many common equations describing gas flow characteristics. The combined parameter is termed the kinematic viscosity. It is defined in Equation 2.



The ratio of the absolute viscosity to the density of a fluid often appears in dimensionless numbers such as the Reynolds number, which is discussed later in this Module. The kinematic viscosity can also be used in equations describing particle motion through gas streams. The expression for kinematic viscosity is used to simplify these calculations.

Answer 2

As the temperature of a fluid or gas increases the stickyness or viscosity of the substance decreases. This means that the molecules of the substance have a greater mean free path or distance between each other before they colide due to the thermal energy received while heated. There are various emperical equations which are used to describe this phenomena.