Monday, December 08, 2008

Viscosity - It's only half the story

Most people's introduction to rheology is via fluid mechanics, which certainly is a fine place to start. In the texts, the material property of the fluid that exhibits itself throughout the equation is the viscosity. For simplicity's sake (again, this is quite fine for starters), the viscosity is taken as a constant - an idealization known as a Newtonian fluid. As one's studies advance into more advanced cases, non-Newtonian fluids are studied. In most cases, the viscosity is then allowed to depend on the shear rate. A simple example is the power-law fluid, a strictly empirical and therefore potentially DANGEROUS model (another topic for another day), where the viscosity is proportional to the shear rate raised to a exponential value, h = gn-1 (Yes, it should be g with a dot over it, but coding that is beyond my HTML ability - any pointers?)

Again, all this is fine and nice. But at some point, it needs to be understood that viscosity is a SECONDARY material property, meaning that there are underlying properties that are even more valuable to understand. These are the storage and loss moduli, commonly denoted as G' and G". These properties can be measured in a dynamic state, one where a time dependent strain (such as a harmonic oscillation)is applied to the sample. If your material is a perfect solid, then it should deform in phase with the deformation, while if it is a perfect fluid, it should be 90 degrees behind the deformation (viscosity is proportional to the shear rate, not the shear). Non-Newtonian materials will generally show both of these characteristics to some degree, meaning that by performing dynamic measurements, you can generate curves for G' and G". In other words, viscosity is only half the story. Interpreting these curves is not simple and that is a big part of the reason that rheology is not taught to undergraduates, but there is considerably more information in the curves than a simple viscosity curve can ever show (in most cases being limited to a zero-shear viscosity region and a power-law region.)

As a rheologist, I am often asked for viscosity curves which might be what the client needs, but in most cases, going the extra step and preparing the moduli provides far more information on what is important to the client. Since most of them are not aware of rheology or the power of it, they don't know to ask for it, but are always happy with the results.

You might be curious if you can keep this going and look for even more fundamental properties than the dynamic moduli noted above. In fact you can. Further work shows that the relaxation spectrum, but devising this from the moduli is an ill-posed inverse problem that is difficult to complete. You ultimately end up with a distribution and thereby even more information than you find from just 2 modulus curves.

Friday, December 05, 2008

Lance Armstrong

In an earlier part of my life, I used to race bicycles. Any attempts to state how good I was would undoubtedly cause laughing-induced-cardiac failure in those that I raced against, so let's just say this: being a racer, I was faster than any non-racer (the types that go on long century rides or down to the store for an ice cream). This may sound like a tautology, but there is more to it. I knew plenty of people that put in long hours but never got their liscence to race, and consequently never got very fast. There is something about getting that liscence that forces you to either get serious enough so that you don't embarrass yourself or to quit and never mention that you raced at all. Keep in mind that bicycle racing is totaly different than running, where finishing a marathon dead last is still considered a good thing. In bicycling, you need to stay with (or ahead of) the pack or you are pulled from the race. After spending $20 on an entry fee and money and time getting to the race to have your visions of fame and glory end in less than five minute will motivate you one way or the other.

I still follow the sport and am quite curious about how well Lance Armstrong will do in his comeback. Elizabeth Kreutz has quite a few behind-the-scenes pictures of this (search through the index for "Lance's Comeback"), and what struck me were the number of distractions that Lance has to face. Given what I have to face in my professional career, I can barely find an hour to focus on reading a journal article or two. It's always done in smippets here and there. How this guy can deal with these distractions and THEN put 100% concentration into his bike is beyond me - regardless of whatever results he will ultimately achieve. Yes, it is his job to train, but I still wonder how he can set everything of modern life aside for so long.

Tuesday, December 02, 2008

Adieu Hercules

Hercules is no more. It's recent acquisition by Ashland Chemical brought to an end a tale (and trail) of tears and death by a thousand cuts. I had a front row seat for a short period (more about this below), as my first job after school was with Hercules in the Packaging Film Group (which was really only a polypropylene film group).

Hercules started back in 1912 as part of the anti-monopolistic breakup of DuPont. Originally making only black powder, the company expanded so that by the 1980's, it was in pulp and paper chemical, polypropylene resin and film, and even had an aerospace division. And then things started to fall apart. They sold off their share of the PP joint venture (Himont, with Montedison of Italy as the partner), followed by the PP film business which was rapdily becomeing a commodity requiring ever bigger investments in larger machines, followed by the aerospace division, followed by the tackifying business... There just wasn't much left.

Regarding my first job: I started in late 1989 as a newly minted Ph.D. I was part of the R & D organization, but management decided that the researchers needed more practical experience so they had some of the people located at the manufacturing sites. In my case, this was Terre Haute. This wasn't a bad situation, as we were called in often to help with manufacturing problems, but we had no responsibility for any of the problems, which was a good thing, as there were plenty of problems. In one case, we came within 6 hours of shutting down a customer. To say this was a major customer was an understatement. This customer was (and still is) so large that pretty much everyone on the face of the planet has heard of them. There was yelling and screaming in meetings that I've not heard since. Once the crisis was over, things were never the same due to the internal shufflings that followed. But to my young eyes, the writing was clear: I left for greener pastures within a year.

And so it is adieu Hercules, not au revoir, but adieu.

Monday, December 01, 2008

Where are the acorns?

Not even remotely related to anything polymeric, we also have an extreme shortage of acorns this year at our house in Minnesota and a corresponding increase of squirrels below the bird feeder. No one is complaining, as in more normal years the deluge was hazardous to everyone. As for causes, I would chalk it up to the plentiful rain here during the pollination period (despite the contrary claim of the expert in the article.)

The oak pollen was excessively heavy on the ground this year. We do pay attention to the oak pollen as it is a yearly milestone in the late spring. After the snow has melted, we start bringing out the patio furniture, sweep the deck, blow the grit off the driveway... Only after oak pollination do we bring out the fabrics - umbrellas and chair cushions. We didn't know any better when we first moved into the area, and found out the our ignorance turned everything a bright yellow. As it is, we still have to reclean the porch screens and the tile floor in it, but it is a relief to be down with the pollination as then we can really enjoy the outdoors. And we really did notice that the pollen on screens and floor was unusually heavy. Since there was more pollen on the ground, it doesn't surprise me that there was less in the trees to pollinate.

This year we also have a black coating on everything under the oak trees. It's weird stuff. Water won't touch it, as is the case with a wide range of other solvents: turpentine, limonene, IPA. 409 works (quats). I'm wondering if there is any connection. This coating didn't appear until mid July or so. It wasn't a one-time event eiher as I had to clean some of the deck tables multiple times. Does anyone have any ideas?