Subwoofer / large speaker
Stiff 1/4" plastic / Plexiglass sheet
Large, transparent plastic container with lid (I used an empty Utz cheeseballs tub)
Heavy-duty trash bag
Sinewave source with amplitude and frequency control (computer with soundcard, or a signal generator, etc.)
Optional: Black fabric to cover up all your gear
How it works:
The cornstarch and water mixture, sometimes called oobleck, is a semifluid mixture with unusual properties. A normal fluid has a constant viscosity regardless of whether or how it is moving. But when the long, stringy starch molecules mix with water, they tend to become entangled, and resist pulling apart if pulled too quickly. So, the resulting mixture tends to stiffen up and become more like a solid when pushed around (that is, when a shear force is applied to it), and relaxes into a more liquid state when less force is applied. A fluid with this behavior is sometimes called a shear thickening fluid or more generally, non-Newtonian fluid.
As the membrane applies force nonuniformly, natural "harder" and "softer" spots will form in the mix. These will tend to be self-reinforcing as the hard spots provide more resistance against the membrane, pushing them to become harder still, while the surrounding softer material can flow underneath them, sending them essentially crowd surfing. These discontinuities are constantly dissipating and re-forming throughout the mix. The complex interaction of these areas produces a writhing mess that seems to have a mind of its own!
This project uses a speaker and flexible membrane (trash bag) to produce the shear. If you search the internet / youtube for things to do with Oobleck, one of the most popular is to put it directly into a speaker and watch it dance. This does work somewhat, but it is not ideal because a) Speaker cones are (by design!) rigid, so the cone does not so much impart shear as simply hop the mixture up and down, and b) putting wet stuff in speakers is really rough on speakers. Using the speaker to pump a flexible membrane sealed over it helps protect the speaker from damage, and by flexing at one of its resonant frequencies, the membrane imparts localized shear forces to give more angry, writing monsteriness and less vibration. By changing the frequency to hit a different resonance, the 'attitude' of the monster can be changed too.
The highlighter (or rather, the juice from it) is extremely UV-reactive and will make the Oobleck light up a bright radioactive yellow-green. Finally, the plastic jar helps minimize the acoustic noise from the speaker, deters kids from sticking their fingers in it (if you care), and helps keep the Oobleck from drying out.
Step 1: Assembling the container - part 1
Speaker - the bigger the better (those intended for use as woofer/subwoofers are ideal). It doesn't have to be pretty, high-quality or remotely good-sounding, as long as it moves a lot of air. Slightly damaged or overdriven ('blown', scratchy-sounding) ones are OK, as the container will muffle most of the sound. Overdriving a speaker (melting/deforming the voice coil, causing it to rub against the magnet) is a common cause of failure, so you can probably find one curbside that will work for this.
Plastic sheet - This will act as a coupling between the speaker, container and the active Oobleck well, which will probably be of different sizes. I used a piece of 1/4" thick plexiglass. It needs to be at least as large in diameter as both the speaker and the jar. The type of material is not important, as long as it is stiff enough not to resonate with the speaker, and thick enough to form a nice well for the Oobleck to sit in.
Plastic Jar - Again, whatever's handy, but it should be thick (again, so the speaker does not cause it to move) clear enough to get a good look at what's inside, and have enough air volume that putting the lid on will not make it impossible for the speaker to move against the air inside. The supermarket in my area sells Utz brand snack foods in large plastic tubs that are just about perfect.
Trash bag - The thicker the better - you don't want it to stretch excessively or tear when the speaker is cranked. This will form the membrane the Oobleck sits on.
Caulk (RTV silicone, etc.)
Remove any grills/covers and measure the diameter of the speaker. Cut a hole in the plastic sheet about 1/2 to 2/3 the diameter of the speaker. Keeping this opening smaller than the speaker itself will concentrate more force onto the membrane. This cut may be easier said than done...fortunately, the hole does not need to be perfectly round. In fact, an unusual shape may even create a more interesting resonance pattern on the membrane, changing the Oobleck's behavior - this needs some further study :-) Just be sure to take a file and smooth off any sharp edges so they don't nucleate tears in the membrane later. Unless you have a CNC machine or hole borer laying around, your best bet might be a jigsaw (slow - you want to cut, not melt it), or taking it outside and "cutting" with a hot knife. If you do decide to melt-cut the material, be careful not to inhale any vapors that result.