Rob's Blog Archive

December 11, 2011

The Solid State Physics of Dogshit










It is said that Eskimos have hundreds of words for snow.  Given that they run sled dogs too, I wonder just how many words they have for dogshit.  It is winter, and with temperatures ranging across the freezing point for both dogshit and snow and affecting the interface between the two, given all the resulting permutations and combinations, there should be more words for dogshit than for snow. 

This is my eighth winter during which I’ve been forced to practice the Montana chip shot.  And just like shots set up differently in golf, so do those for various pieces of dogshit.   But, with practice, most people can become adept at scooping the yard in winter.  Moreover, a little understanding of the underlying, and I mean really underlying, physics is useful.  This essay is about shit science.

The most important qualities of shit are its thermodynamic properties.  Dogshit is a colloid, not unlike granite or at least the magma from which granite solidifies.  It freezes at about 20F or so (dogshit, not granite) and as the temperature drops, becomes rock hard.  More importantly to we who clean dogyards, the best property of shit is frozen shit has a significantly larger coefficient of thermal expansion (COTE)  than ice (Note: The actual COTE for dogshit is not well documented.  What is clear is its magnitude is greater than that of ice for which the COTE is very close to zero.  The sign, however, is unknown.  I am in the process of submitting a request for an NSF grant to determine this.  This is clearly more important to our understanding of the universe we live in than determining if there’s life on other planets or looking for gravitational waves, so I expect a nice big check very soon).   As the temperature drops, the difference in contraction between the ice and dogshit results in thermal stress at the ice-dogshit interface, which in turn results in stress fractures.  The upshot is that after subzero nights (real zero, not Canadian zero), tap frozen dogshit with a shovel and it pops free, breaking along the fractures.  Nice big whole pieces too.  This is good shit.

Even we of southern climes have at least a second word for shit, diarrhea.  Whereas good shit pops free, diarrhea is much more problematic. 

Ever heard of a Kovar seal?  Usually, glasses and metals have very different coefficients of thermal expansion---heated metal expanding faster than heated glass.  Special glasses and metals are chosen to be used at a metal glass junction to prevent the fracturing that would otherwise occur as the junction is heated or cooled.  Kovar and borosilicate glass are a particularly favored pair.  One of the techniques used to create a metal to glass seal with a glass other than borosilicate is to vary the glass type between the borosilicate at the junction and the glass that is ultimately required.  By doing this and gradually changing the glass, thermal stress can be minimized. 

Diarrhea in snow accomplishes the same thing.  On the surface, we have pure shit.  At some depth, we have pure snow and ice (at least as pure as you get in a dogyard).   In between, we have a transition from one to the other.  This transition means no real thermal stress between the shit and the snow, nature’s Kovar seal.  Diarrhea is bad shit. 

For snow covered ground, the worst temperature is just below freezing.  Snow is hard, shit is soft, and both are sticky.  It’s kind of a shit tri-critical point.  One hasn’t truly lived until he calls an object “you piece of shit” only to find that he is, in fact, cursing a piece of shit. 

Finally, there is the temperature range above freezing where snow is wet and shit has returned to its original colloidal state, a viscous fluid.  At this temperature, the snow has a significant amount of moisture on its surface.  While not near as easy as the rock shit shortly after a subzero night, this condition is fundamentally workable.  Dogshit really doesn’t bond with wet snow particularly well but does stick to itself.  The end result is that it can sort of be rolled into a pile and then onto a shovel or into some other receptacle. 

As it ends up, Eskimos don’t have that many words for snow.  What they do have is a language structure that lets them add descriptors together to form  “new words” without much constraint, and early anthropologists and linguists reinterpreted this as many words for what was being described, in this case snow.  Which is great.  It means the same thing works for shit ---- So they do have as many words for dogshit as they do for snow after all.

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