[Atomic-S (OP)]

Does anyone know what is the least viscous liquid known at room temperature and pressure? And also at ANY temperature and pressure?

[another_someone]

The least viscous at any temperature must be superfluid liquid helium.

George

[Atomic-S (OP)]

Ah yes; of course!

[eric l]

Viscosity is rather complex.  In fact, very few liquids behave according to Newton's law.  And if you want the "dynamic" viscoisty of the old days (viscosity over density) mercury will flow faster than water if driven by gravity alone.  I don't have the exact data here, but at room temperature the viscosity of merrcury will be lower than 2 mPa.s (water would be about 1 mPa.s) while density would be about 13,5.  Dynamic viscoity would be about 7 times lower !  And you would not need to cool to - 200°C or lower as with liquified helium.

[Soul Surfer]

Ether and acetone are the liquids with the lowest viscosities at room temperature that I have seen and checking out my tables of physical constants they are the lowest viscosity common substances.
They have about the same viscosity as water near its boiling point.  At room temperature the viscosity of water is about four times this but it is still pretty runny.  The viscosity of mercury at room temperature is a bit higher than water but they are both about the same just above the freezing point of water

Learn, create, test and tell
evolution rules in all things
God says so!

[eric l]

Viscosity is rather complex.  In fact, very few liquids behave according to Newton's law.  And if you want the "dynamic" viscoisty of the old days (viscosity over density) mercury will flow faster than water if driven by gravity alone.  I don't have the exact data here, but at room temperature the viscosity of merrcury will be lower than 2 mPa.s (water would be about 1 mPa.s) while density would be about 13,5.  Dynamic viscoity would be about 7 times lower !  And you would not need to cool to - 200°C or lower as with liquified helium.

[Soul Surfer]

Ether and acetone are the liquids with the lowest viscosities at room temperature that I have seen and checking out my tables of physical constants they are the lowest viscosity common substances.
They have about the same viscosity as water near its boiling point.  At room temperature the viscosity of water is about four times this but it is still pretty runny.  The viscosity of mercury at room temperature is a bit higher than water but they are both about the same just above the freezing point of water

Learn, create, test and tell
evolution rules in all things
God says so!

[Mjhavok]

Where do you get these physical constants tables. On the net?

[Soul Surfer]

Google  (+viscocity  +water +ether +alcohol +mercury  ) produces some lists of viscocities of common substances

I just used a table of physical constans that I have had since I was a student.

Learn, create, test and tell
evolution rules in all things
God says so!

[Mjhavok]

Thanks Ian.

[Atomic-S (OP)]

Acetone, you say? Well, that makes sense, come to think of it. Acetone is quite "loose" in a container.

[Bored chemist]

Have you really been waiting since August 06 for an answer? In case you have, pentane is slightly less viscous than ether.

[lightarrow]

CS₂ and Br₂ seems very fluid too:

http://www.engineersedge.com/fluid_flow/fluid_data.htm

Anyway, tables in internet seems quite discordant:

http://www.engineeringtoolbox.com/absolute-viscosity-liquids-d_1259.html

[Bored chemist]

Those tables are allowed to be discordant. They are tables of different quantities.
Viscosity in poise is not the same as viscosity in stokes. There's a factor of the fluid density.

[lightarrow]

Those tables are allowed to be discordant. They are tables of different quantities.
Viscosity in poise is not the same as viscosity in stokes. There's a factor of the fluid density.

Right, thank you.

[tommya300]

A yard of grog has the least viscosity, it runs faster than all of these.  [:0]

[aricjoshua]

It is permissible for such tables to be discordant. It's a set of tables with various amounts.
The viscosity of poise differs from the viscosity of stokes. There's a fluid density issue to consider.

[evan_au]

Is viscosity even defined near the critical point of a phase diagram?

[Bored chemist]

Is viscosity even defined near the critical point of a phase diagram?

Yes.
It's the rate of momentum transfer per unit area divided by the velocity gradient.
What is less well defined is whether you are talking about a liquid, a gas or something else.

On a dull "answering the question" note, liquid butane and propane have lower viscosities at room temperature than ether. However, to stay liquid at that temperature, they must be kept under pressure.