Monday, November 15, 2010

A Polymer Contradiction

Over the weekend I remembered an unusual proposal for polymers that seems to have fallen by the wayside and I don't know why.

Polymers of course have a very high viscosity and their solvents have very low viscosities. Yet there are a few circumstances when mixing them results in an even lower viscosity (or least, an "apparent" viscosity). The specific example I am thinking of high molecular weight polyethylene oxide (PEO) in water. At dilute concentrations, the polymer can stifle the formation of turbulent flow, and thereby allow more liquid to be pumped at a given pressure.

The application that was most discussed was to use this system in fire fighting - more fluid for a given would be a good thing, or (semi) equivalently, being able to spray the water further also be helpful.

For reasons that I heard, this application never caught on. Anyone know why?

Polyacrylamide (PAm) of high molecular weight can also pull off this trick. When I was a TA at the University of Illinois for the undergrad ChemE unit ops lab, one of the experiments involved a large water tank that drained through a horizontal pipe at the bottom into a drain trench in the flow [1]. The tank was first filled with water and the vertical distance that the water shot was recorded as a function of the head. The tank was refilled and some PAm was added [2] and dispersed, and amazingly the water shot out quite a bit further.

[1] Yes, the polymer went right down the drain. This was not a problem, as PAm is used in water treatment plant to help flocculate and settle particulates - we were simply helping out the water treatment plant.

[2] One of the responsibilities of the TA was to prepare a 5 wt% solution of the PAm in water. This could take a while to prepare - the guide from the profs was 4 hours - the PAm had a tendency to clump together as is common with many water soluble polymers so if you tried to rush it, you ended up with a snotty mess and not a really thick, viscous liquid. Since I was studying polymers, I knew of a shortcut - use ice water, not tap water. The cold water limits the rate at which the polymer dissolves and prevents clumping. After everything is dispersed, you heat it up and your done. The other TA's always wondered how I could mix the concentrate up in 30 minutes. (Shhh, a magician never tells his secrets!)


Frank Van Haste said...


Re: PEO and "wetter water", the other interesting app was a polymer ejection system for submarines. By artfully ejecting PEO into the boundary layer you could delay separation and the onset of turbulence, thus reducing drag and the boat would go faster.

Full scale tests were done and the effect was demonstrated. I do not know (and probably couldn't tell you, if I did) whether the system was ever used operationally.



John said...

I never heard about that application. It seems a little impractical carrying all the PEO around, but maybe for a certain subclass of submarines, it might be practical. I wonder if it cut down on the acoustic footprint too?

Materialist said...

Sounds like you're talking about drag reducing agents (DRA) which are a big business in transporting petroleum products in pipelines.

Eric F. Brown said...

I like your method of mixing the polymer with water. Counterintuitive, but it works.

Frank Van Haste said...


The public sources I've seen state that there was an acoustic benefit in polymer ejection...but I wonder if that wasn't because with the lower hydrodynamic drag you could get equal speed using less horsepower - and hence, less radiated noise.

In other words, you could have the knots or you could have the noise reduction, but not both.