Thursday, August 30, 2012

Viscoelasticity: It's Not Just for Polymers Anymore

Viscoelasticity is that intriguing phenomona that blurs the lines between a solid and a liquid. Classic liquids are viscous and not elastic, and classic solids are elastic and not viscous, but modern materials such as molten polymers show parts of both and are called viscoelastic. When I first was learning of viscoelasticity many years ago, it was accepted on faith that all liquids would show viscoelastic behavior, even water, if examined closely enough. There was no data, just logic, to support this. That has now changed.

Researchers have now measured elastic behavior in water (free access until ~ September 14, 2012 with registration). I found the report fascinating for a number of reasons. First, the tests were completed on macroscopic samples. The published data was for 0.125 mm thick layers of water, but the researchers had gone to samples as thick as 0.5 mm. Secondly, the results were just what was expected. For instance, here is a strain sweep:
and just like any other viscoelasic material, G' (the storage modulus - i.e., solid characteristic) is greater than G" (the loss modulus - i.e., liguid characteristic) at small strains, with both functions decreasing nonlinearly as the strain increases, and also crossing over somewhere in the process. If I removed the numerical values from the y-axis, this would look just like plots that I make in the lab for any old polymer or gel or what have you. There are also these frequency sweep plots made at 2 separate strains. The first is at small strains showing that the water is solid-like
and the second made at very high strains showing that the water is purely liquid.
G' isn't in the picture - literally. What is so frustrating is that the authors only hint at what occurs between these plots as the strain is increased:
"It is interesting to note that the evolution from the low strain amplitude sinusoidal wave to the large strain amplitude sinusoidal-like wave does not correspond to a simple shift of the phase but to a strong modification of the signal."
More research is needed! And quickly please![*]

But lastly, the coolest part of this research is that is was done not with an ultrasophisticated instrument build just for this purpose but using a rather mundane and ordinary rheometer - an ARES2! Yes, it was modified slightly - the parallel plates were made from nonporous alumina (40 mm diameter of course), and the input and output signals were monitored by a 7-digit voltmeter rather than with the standard hardware/software, but that's it. Nothing more was done. This testing is something that pretty much any rheology lab in the world could measure. It's that simple.

[*] There is one plot in the article that was made at an intermediate strain, but to discuss it would require getting too involved in the rheological details, and besides, that one plot is only one place along the transition from the low to high strains. It's not enough. I still want to see the whole evolution from small to large strains and there are a lot of gaps to fill in.


Andrew Sun said...

I'm still very suspicious although I trust the reviewers of this manuscript. Water should be definitely a viscous fluid under ambient temperature observed over 0.1 s of time (or shorter). So the elasticity dominated mainly because of the confinement effect? The geometry of the confinement is still at a macroscale.

Wouldn't the measured elasticity be caused by the interfacial intention when the volume of water sample was reduced to such extent?

On the other hand, however, I always guess that water has a significant viscoelasticity at high frequency. Hot water pouring into a cup sounds different (in sound frequency) than cold water may be due to the viscoelastic difference caused by the temperature. This difference due to temperature is quite significant so if this is due to the viscoelasticity the thermorheological property of water is in fact very obvious. However this occurs at a high freqeucy regime or even high Re number regime (pouring a lot of water suddenly into a cup in order to create some noise).

This report is interesting but

John said...


Great comments. I do hope you take the time to read the report as there is more there than just what I mentioned, and yet it is only 4 pages long.

I agree with you suspicions, but these reseachers have been working in this area a while. I suspect the elasticity arise from the hydorgen bonding network. It would be pretty easy to work with heavy water to prove/disprove this. And also using hydrogen-bond disrupting materials in the water. The effects on viscosity would be minimal, but they could greatly alter the elasticity.

Zephir said...

The water autothixotropy was observed many times before (1, 2). It may be linked to the cluster medicine and alleged homeopathic activity of water.