# Factors that affect viscosity of incompressible newtonian fluid?

1. Dec 21, 2008

### v_pino

What are the factors that affect viscosity of incompressible newtonian fluid?

Here is what I think:

Temperature:
When we increase the temperature of a fluid (controlled volume) the frequency ofintermoleculer collisions increases. Does this mean viscosity decreases? And if so, does density decrease?

Pressure:
Does an incompressible fluid mean that it cannot compress under pressure but can increase in volume if pressure decreases? If so, increasing the pressure (constant volume) should increase the temperature, and in turn, decrease viscosity. And decreasing the pressure will increase the volume (since it volume is not constant when pressure decreases). Therefore, should viscosity remain the same?

There's this passage I read about viscosity which is a little confusing:
http://hypertextbook.com/physics/matter/viscosity/

thanks

2. Jan 26, 2009

### pranaam_001

Re: viscosity

At ordinary pressure the viscosity of gases increases with the increase in temp..and the viscosity of most fluids decreases..the reason being at an increased temperature the frequency intermolecular collision in gases increases..which increases the viscosity as the flow reduces..for liquids at an increased temp the molecules get energy to break the intermolecular forces of attraction...hence the flow increases..

3. Jan 27, 2009

### Andy Resnick

Re: viscosity

First things first- an incompressible fluid means the change of volume (equivalently, density) with pressure is zero. That's unconnected with changes of density with temperature.

Second- that website is ok, but the fluids discussed (honey, glass, etc) are not Newtonian fluids, which can cause confusion. There is also kinematic and dynamic versions of viscosity, one is corrected for fluid density. If you like, viscosity is a fluid version of friction.

A Newtonian fluid is defined as one that the viscosity does not change with rate of shear. It is allowed to vary with any other property, AFAIK. There is no theory worth anything yet that can predict the viscosity of a real fluid (again, AFAIK). There are other fluids (Bingham, shear thickening, shear thinning, lots more) where the viscosity does depend on shear rate.

For example, toothpaste is approximately a Bingham fluid. If you do not squeeze the toothpaste tube, the toothpaste does not flow (very high viscosity). Squeezing the tube and pushing it out increases the shear rate, the viscosity goes down, and the toothpaste comes out of the tube. Then it again comes to rest on the toothbrush since the shear rate dropped.