# Relationship between viscosity and temperature in gases

The viscosity of liquids increases as temperature decreases, whereas the viscosity of gases increases as temperature increases. Viscosity is a fluid's resistance. From the basis of the kinetic theory of gases, viscosity in gases increases with temperature because the increase in kinetic energy will increase. Increase in temperature increases the velocity of the molecules of a liquid. This implies that the forces of attraction between the molecules are reduced thus.

House paint is a shear-thinning fluid and it's a good thing, too.

## Viscosity II: Gas Viscosity

Brushing, rolling, or spraying are means of temporarily applying shear stress. This reduces the paint's viscosity to the point where it can now flow out of the applicator and onto the wall or ceiling. Once this shear stress is removed the paint returns to its resting viscosity, which is so large that an appropriately thin layer behaves more like a solid than a liquid and the paint does not run or drip.

Think about what it would be like to paint with water or honey for comparison. The former is always too runny and the latter is always too sticky. Toothpaste is another example of a material whose viscosity decreases under stress. Toothpaste behaves like a solid while it sits at rest inside the tube. It will not flow out spontaneously when the cap is removed, but it will flow out when you put the squeeze on it.

### Viscosity of Gases

Now it ceases to behave like a solid and starts to act like a thick liquid. You don't have to worry about it flowing off the brush as you raise it to your mouth. Shear-thinning fluids can be classified into one of three general groups. A material that has a viscosity that decreases under shear stress but stays constant over time is said to be pseudoplastic. A material that has a viscosity that decreases under shear stress and then continues to decrease with time is said to be thixotropic.

If the transition from high viscosity nearly semisolid to low viscosity essentially liquid takes place only after the shear stress exceeds some minimum value, the material is said to be a bingham plastic. Materials that thicken when worked or agitated are called shear-thickening fluids.

An example that is often shown in science classrooms is a paste made of cornstarch and water mixed in the correct proportions. The resulting bizarre goo behaves like a liquid when squeezed slowly and an elastic solid when squeezed rapidly. Ambitious science demonstrators have filled tanks with the stuff and then run across it.

As long as they move quickly the surface acts like a block of solid rubber, but the instant they stop moving the paste behaves like a liquid and the demonstrator winds up taking a cornstarch bath.

Viscosity of fluids is the key physical property that dictates the design of pipelines to transport material. Thus, an understanding of liquid and gas viscosity is essential for engineering a chemical process. The viscosity of gases played an important role in the historical development of the kinetic theory of gases see Keith J.

James Clerk Maxwell, who is famous for the Maxwell-Boltzmann distribution of molecular velocities and Maxwell's Equations of electromagnetic radiation, proposed in that gases possess a distribution of velocities.

Although now fully accepted, this proposal went against the conventional theory of the time that a range of velocities would be equalized by molecular collisions.

## VISCOSITY OF GASES

He was troubled with his own proposal, however, because it had "the curious result" that viscosity is independent of pressure which was "certainly very unexpected. These measurements were made using an apparatus in attic of their house, and the temperature was controlled through appropriate stoking of the fireplace. The results were reported inreconciling his kinetic theory of gases with observed gas viscosities. A diatomic molecule has three translational, three rotational, and one vibrational degree of freedom.

As a result, Maxwell proclaimed that the kinetic theory "could not possible satisfy the known relation between the two specific heats of a gas" and "the result of the dynamical theory, being at variance with experiment, overturns the whole hypothesis, however satisfactory the other results might be.

This required the advent of quantum mechanics, which explained that the degrees of freedom for molecular vibration and rotation around the axis of a linear molecule were to be neglected because the excited quantum states for these motions were too high in energy to be accessed at room temperatures. Both gas effusion and gas viscosity experiments validate the kinetic theory of gases and provide access to microscopic information from macroscopic measurements. The rate of gas effusion provides a means of determining the average molecular velocity, as faster molecules will strike the pinhole area more frequently and therefore effuse more rapidly.

The viscosity of a gas provides a means for determining molecular diameters, as viscosity arises from collisions among molecules.

### Viscosity – The Physics Hypertextbook

Apparatus A gas sample is drawn through a thin capillary tube. The change in pressure of the system is measured with a manometer as a function of time. A calibration flask of known volume is provided to determine the volume of the system. Laminar flow In order to measure gas viscosities, laminar flow is assumed in the capillary. Laminar flow implies that the gas flows in "layers" such that each layer moves at a velocity infinitesimally different than the layers adjacent to it.

Since the wall is stationary, the layer along the wall has a velocity of zero. The fluid flows more quickly the further away it is from the stationary wall. Laminar flow is commonly experienced in smooth streams and rivers, where water flows slowly along the banks and rapidly in the center. Viscosity coefficient Adjacent laminar sheets experience friction as they slide past one another.