As a kid, I was always a little anal when it came to toys. I never lost a game piece, and somewhere, there are probably still untouched sticker books, rolls of caps, packets of photosensitive paper and countless other long-forgotten expendables that I saved myself out of ever using. Childhood doesn’t last for ever but some of its habits do, and despite missing my calling as an environmental crusader, I still find myself deliberating much longer than warranted before replacing razor blades, changing guitar strings, or “premiering” new shoes.
Looking through the vast online materials repositories (smallparts, matweb, inventables), I found many with surprising properties—translucent concrete, foldable porcelain, memory wire—which made me realize that materials engineering must be pretty fun: in order to achieve X, I need a material with a bunch of properties—physical, chemical, structural, economic. Finding that material requires a combination of research into existing materials and applied chemistry, physics, and manufacturing. Materials engineers are like the Special Forces of the world of stuff (and in this analogy I’m the fat kid on the couch who just watched a be all you can be commercial). Which is all to say that when faced with the challenge of creating my own material, I started out with two goals: that it have surprising characteristics (like silly putty or magnets) and that it be reusable.
Despite years of science instruction from teachers desperate to convey science’s innate coolness, I managed to make it through school without ever encountering non-Newtonian fluids (or “oobleck” in middle school science speak). Quicksand is a good example of a dilatant or shear thickening fluid, ie, a fluid that behaves like a solid when subjected to shear force. Silly Putty, it turns out, also has dilatant properties, which is why it stretches if you pull it slowly but breaks if you yank it quickly. Other more complex dilatants are used in automotive power transmission applications. The DoD has been researching armor applications for years, and I suspect that D3O’s shock absorption technology relies on dilatants. The most readily available and accessible example of a dilatant is a 2:1 mixture of cornstarch and water. I made some to play with, thinking it might be interesting to fill a balloon with it. It wasn’t (it’s hard to subject the entire contents of an elastic sphere to any significant shear stress), but it is fascinating stuff to play with. I took slow-motion video of it turning solid as it pours and of its state changes as I dragged my fingers through it.
Moving on, I returned to online repositories of materials for other inspiration. I realized that not all materials are invented through brilliant accidentsa la vulcanized rubber; many are “discovered” by when existing materials are shaped or applied in new ways. Rather than trying to create a reusable material with unexpected physical properties from scratch, I set out to find one that wasn’t in any of the online repositories. It was, it turned out, readily available at K-Mart.
Moon Sand is a moldable substance that feels and looks like wet sand but is in fact dry (and never dries out). A little research turned up this patent, that along with a detailed recipe for Moon Sand, reveals that it is actually very fine sand coated in wax. This allows it to stick together when molded or sculpted but also gives it some other nifty properties. If you heat Moon Sand for about half an hour in an oven, the wax melts and the sand sticks together a little more permanently.
Armed with this knowledge, I set out to explore some more of its properties. I assumed that the wax’s adhesion to each sand particle must be greater than its adhesion to itself (otherwise heating the Moon Sand should have produced a puddle of wax and a pile of ordinary sand) so I reheated it and while it was hot broke it back apart. When it cooled, it had returned to its original, pre-baked state—reusable! What are some of wax’s other properties? One obvious one is that water beads off it. This seemed promising. What if Moon Sand is water proof, if it keeps its moldable properties even underwater? Turns out it does! Amazing! Unexpected! And as soon as it dries, it’s back to it’s original state with no visible or tactile deterioration. Reusable! The Moon Sand marketing dwells on how it allows kids to bring the fun of outdoors indoors without making a mess that mom can’t clean up with a single swipe of the vacuum cleaner.
This seems like a missed opportunity—this is sand you can play with in the bath tub! So, it turns out it’s not a missed opportunity at all, it’s variant differentiation: check out Aqua Sand, a branded version of hydrophobic sand that was developed to clean up oil spills.
For my final material exploration, I ordered a pound of sodium acetate (C2H3CNaO2), a non-toxic salt used to flavor salt and vinegar potato chips and for a variety of industrial chemical applications as well as in hand warmers, which is how I knew about it. When it’s hydrated and heated, a solution of sodium acetate can be supersaturated and then supercooled at room temperature. This means that thought it ordinarily freezes at 58ºC, it stays in liquid form well below that temperature provided there is no nucleation center for crystal formation. This same effect can happen with distilled water (normal water contains impurities for crystals to form around) as this meathead scientist discovered in Thailand:
Sodium acetate comes in two forms: trihydrate (the kind that melts near room temperature) and anhydrous (the fine powder that doesn’t). The stuff I bought for a dollar was labelled trihydrate but I discovered was in fact anhydrous when it caramelized in the bottom of a frying pan. I brought a pan of water to a boil and then turned it off, spooning and stirring in the powder until no more would dissolve. I then poured it into a clean jar and have since “set it off” and “reset” it about twenty times. Reusable and suprising!
There are a number of applications I can think of for this stuff. It’s used in hand warmers because its crystallization is exothermic (which makes sense: if the liquid is cooled way beneath its freezing point, when it freezes, the kinetic energy of the liquid has to go somwhere). A basic application would be to make a waterproof fabric filled with a thin layer of the stuff to make a hand-warming clutch for chilly winter soirees. A more involved application might be as a security measure: keep a diamond in a tank of the stuff and if someone tries to get into it, it solidifies. In that sort of a situation, it might make sense to use another chemical with similar properties but a much higher melting point so that the heat it gave off when crystallizing cooked the would-be thief alive! That’s got to be worth a multimillion-dollar defense research grant.