Monday, December 21, 2015

Wrapping Up: Results from the Stanford Plasma Physics Lab

Hi all,

This is post is meant to summarize the work I've done at the Stanford Plasma Physics Lab on my water explosion project. School and other pursuits have occupied the majority of my time since my last post, so I've only been working on the project on-and-off since then. However, I was able to finish the experiments I set out to do over the summer of 2014, and I intend to report the results here.

The water does seem to contribute internal energy to the explosions. More and different research is needed to shed light on exactly how this happens.

To pick up where I left off last post:
  • I won the $1500 research grant I applied for, and used the money to buy a dedicated capacitor discharge circuit (here, from Information Unlimited), independent of the capacitor bank for the lab's plasma gun. The capacitor bank is 100 uF and can withstand up to 5 kV, though I've only ever charged it to 3 kV
  • I decided to wait on the shock-wave-time-of-flight-based measurements for explosion energy since:
    • the calorimeter-based measurements yielded satisfactory results, and
    • I don't currently know much about solving partial differential equations numerically.
Using my custom-made, hand-milled acrylic calorimeter, my custom thermistor feedthrough, Information Unlimited's capacitor discharging setup, and several high-current and high-voltage probes, I was able to get several complete measurements of a water explosion's input and output energies. And I have videos to prove it:






Notice that there are visible sparks around the chamber in both cases, and in the second video there's debris in the water following the explosion (compare the frames at 0:18 and 0:20). My guess is that this is the steel electrodes (or something on them) degrading. I tried estimating the expected energy released from ablating steel in water, to see if this might be a significant contribution to the energy recorded by the calorimeter.

Based on measurements from this article and currents typically seen in the discharge loop, I gathered that about 0.1 milligrams of steel is eroded from the electrodes in each shot. (This might not seem to be enough to color the water as it does, but note that the steel that does get eroded gets eroded very thoroughly). The standard enthalpy of formation of iron(iii) oxide is -824 kJ/mol and the molar mass of most steel is around 56 g/mol. Thus, we should expect to release on the order of 824/56*0.1 = 1.5 J from oxidizing the steel in the electrodes, which is negligible given our input energies.

Typical current and voltage waveforms for a shot through water
Calorimeter temperature over time
I was able to get data from five different shots before my acrylic explosion chamber gave out on me (read: exploded). In each shot, I charged the 100 uF capacitor bank to approximately 3 kV, for a total stored energy of ~450 J.

Table 1: Input and output energies for the measured water explosions.
(You can read my previous post for more details about my methods on calculating these energies).

As you can see, the net explosion energy exceeded the input pulse energy in every shot observed. In shot 1, the measured explosion energy even exceeded the total energy stored by over 200%.

Some points about table 1:
  • Both input pulse energies and output explosion energies are highly variable, and do not seem to be directly correlated.
    • Shots 1 through 3 have similar pulse energies but widely different explosion energies.
    • Shot 4 has the lowest pulse energy but the second highest explosion energy.
  • Shots 1 through 3 and 4 through 5 seem to be in two different classes as far as input pulse energy is concerned. This may have to do with the amount of contact resistance or dissipation in the circuit, which may change from shot to shot.
  • In each shot I charged the 100 uF capacitor bank to 3 kV, for a total stored energy of 450 J. Thus the efficiency of energy transmission from capacitor bank to calorimeter seems to vary significantly from about 80% in shot 1 to about 6% in shot 4.
  • The uncertainties on each measurement were calculated by propagating errors. I calibrated the calorimeter by submerging it in an ice bath, heating it with a known wattage, and measuring the resulting temperature change. From this, I estimated the uncertainty in the calorimeter's temperature measurements. I used this and the tolerance ratings on my voltage and current probes to compute the uncertainties displayed in table 1. 
My conclusion from these results: there's definitely something going on here. Unfortunately, I don't have the time, resources, or chemistry background to figure out what. I can, however, leave you with an unsubstantiated theory. I've speculated before (at the end of the post) on what mechanism might underlie these water explosions, but I've since come across some new papers and have some new ideas.

Graneau thought the formation of droplets in the water explosion was crucial to the release of energy in a water explosion. He attributed a the energy release to a difference in the heat capacities of bulk water and steam. However, heat capacity is a macroscopic thermodynamic property resultant from the statistical behavior of a large number of molecules in a bulk phase. Such macroscopic properties lose meaning at a microscopic scale, and if the explosions' strange properties are indeed due to the formation of micron-scale water droplets, then a microscopic explanation is probably required.

In the past, I proposed that the rapid formation of water droplets allows "weak" hydrogen bonds to collapse to "strong" hydrogen bonds in the water. Sun et al. (2013) found evidence that the lengths of both inter- and intra-molecular bonds in water depend on the number of neighbors the water molecule has. Water molecules with fewer than 4 neighbors tend to have longer hydrogen bonds and shorter covalent bonds. This has the net effect of making the bond "stronger," i.e. the bond has a more negative binding energy. Thus, when you have a lot of water molecules with a lower-than-average number of neighbors, as in a droplet or near an interface, you will see some rather unusual bond reorganization.  Water explosions are not the first bizarre phenomenon this bond lengthening and contracting might help explain; it's also been implicated in the Mpebma effect.

Illustration of two possible hydrogen-covalent bond lengths: the "weak" red state and the "strong" blue state
Figure from Zhang et al. (2013)

There is further evidence that water behaves weirdly at interfaces. Gerard Pollack of the University of Washington (check him out here) has done extensive research on water chemistry and has proposed the existence of a "fourth phase" of water, something between the liquid and solid phase, that forms at interfaces between bulk water and some hydrophilic surface. He describes the phase as a gel-like layer with an unusually high pH. It forms in layers of a few hundred microns, which Pollack calls "exclusion zones" since they tend to exclude solutes. These exclusion-zone layers likely form on the droplets created by a water explosion and may play an important role in the explosions' properties.

Gel-like “exclusion zone” boundary layer, from Pollack,  (2013)

If a water explosion releases energy, the second law of thermodynamics requires that the water produced by the explosion be something other than normal water; burning gasoline doesn't produce more gasoline. Pollack's "fourth phase" seems like a good candidate for the byproduct of a water explosion.

I've proposed two theories for the mechanism behind water explosions, but both of these theories still need substantial experimental and theoretical expansion. For example, Zhang et al. (2013) predict that there is a temperature dependence on the bond contraction; the O-H covalent bond contracts more (i.e. binding energy becomes more negative) when the water is at a higher temperature. Furthermore, Pollack has shown that incident light tends to expand the exclusion zone. Both of these observations suggest further experiments (e.g. firing the explosions at different water temperatures and at different exposures to light) that may shed more light on the nature of these explosions. However, as I've said, I don't currently have the time to pursue these or other experiments further. 

If anyone out there is interested in exploring this project further, please comment below and we can chat. Otherwise, I imagine this will be the last update I make to this blog for a while. Thanks for reading! This project has been a lot of fun. 

References:
[1] Comment on “A Theory of Macromolecular Chemotaxis” and “Phenomena Associated with Gel–Water Interfaces. Analyses and Alternatives to the Long-Range Ordered Water Hypothesis” G. H. Pollack, The Journal of Physical Chemistry B 2013 117 (25), 7843-7846 DOI: 10.1021/jp312686x 

[2] C.Q. Sun, X. Zhang, X. Fu, W. Zheng, J.-l. Kuo, Y. Zhou, Z. Shen, and J. Zhou, Density and phonon-stiffness anomalies of water and ice in the full temperature range. J Phys Chem Lett, 2013. 4: 3238-3244. 

[3] Zhang, X., Huang, Y., Ma, Z., & Sun, C. (2013). O:H-O Bond Anomalous Relaxation Resolving Mpemba Paradox. arXiv:1310.6514.

7 comments:

  1. These exclusion-zone layers likely form on the droplets created by a water explosion and may play an important role in the explosions' properties.Fred R

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  5. I think you misunderstood Graneau, he didn't say the energy came from forming water droplets, he said: "The loss of intermolecular bond energy in the conversion from liquid to fog must be the source of the explosion energy"(Graneau et al. 2000) the breaking of the intermolecular hydrogen bond is the source of the energy. as all else goes, good job!

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