This Instructable shows how to build a radioactive-looking, glowing goo monster that rises up, writhes and moves around of its own accord. It looks like the man-eating Blob from the horror movie by the same name, and makes a smashing Halloween decoration. The goo itself is an easy-to-make shear thickening fluid consisting of mainly cornstarch and water, with a speaker-driven membrane to provide the shear.

Supplies Needed:

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
Caulk
Yellow highlighter
Corn starch
Blacklight
Sinewave source with amplitude and frequency control (computer with soundcard, or a signal generator, etc.)
Stereo amplifier
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

First, assemble together the parts you need for this step. They are:

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.

Step 2: Assembling the Container - Part 2

Cut a disc of trash bag at least 1 inch, ideally 2"+ larger than the hole diameter. This will be the membrane. Lay it out on your work table and ensure it is lying flat. Place a thick bead of caulk around the outside of the hole in the plastic sheet, center this side over the trash bag membrane, then press it over the membrane so that the caulk bonds them together. It's OK (and preferred!) if some excess caulk squishes out around the hole. Use your finger to smooth this excess into the edge of the hole.

You want a nice, thick bead of caulk around the inside edge of the hole. This, combined with the caulk on the backside between the membrane and plastic, will keep the membrane from pulling loose at high amplitudes and keep the Oobleck from leaking out into the speaker. So, if none or very little caulk squished out, place a fresh bead where the membrane meets the edge of the hole, and smooth in. Allow the caulk to cure.

Now, cut the bottom out of the plastic jar. The hole should be larger than the hole cut in the plastic sheet. Again, the cut need not be a perfect circle, as long as the jar can sit flush against the sheet when finished.

Once the plastic-and-membrane has cured, place a thin bead of caulk around the outside ring of the speaker and place the plastic, centered, flush against it. It's important to make an airtight seal to maximize the pressure delivered to the membrane and minimize noise.

Finally, place a bead of caulk around the bottom of the plastic jar. Center and place the jar atop the plastic sheet. Set this whole business aside and allow to cure.

Step 3: Make Waves

To make the Oobleck do something interesting, the membrane has to be flexed at a sufficient amplitude and at a relatively low frequency (20-200Hz). Ideally, the drive frequency should be at or near the natural (resonant) frequency of the speaker-membrane-jar combo. The exact frequency depends on factors such as the speaker diameter and volume of air between the speaker cone and membrane (the air acts as a spring), membrane thickness and stiffness, amount of Oobleck sitting on it, jar volume, etc., so it will be best just to experiment.

You will need some way of generating a pure tone (sinewave) and control its frequency and amplitude. If you have an analog waveform generator, your work here is done. Most people don't. But you can easily do the same thing with an old laptop. Music editing / tracker / softsynth software often has this feature, and there are programs specifically to generate signals with the sound card. Windows users can use the freeware SignalGen (http://www.dr-jordan-design.de/signalgen.htm).

Once a suitable signal source is up and running, connect the line-out to the aux input of the stereo amplifier, and the amplifier output to the speaker. (If you are using a speaker with its own amplifier built in, e.g. a powered subwoofer, skip this step and connect directly to the speaker line-in.)

Step 4: Make Oobleck and Finish

Now that the Oobleck jar has cured, it's time to place some Oobleck inside. Placing the membrane flush with the bottom of the plastic has created a shallow well that will help keep the Oobleck in place. Since the weight of the Oobleck sitting on the membrane will dampen its motion, you want to use only a small amount, just enough to cover the membrane and allow for some to rise up and dance.

Using pliers, remove the back of the highlighter so you can extract the juice. (If no pliers are handy, just cut it open). This irridescent juice is highly reactive to UV light and extremely concentrated. The juice is contained in a cellophane-wrapped sponge inside the highlighter, vaguely remeniscent of one of those cellophane-wrapped fake crab sticks. Squeeze its juice into the Oobleck well, along with a couple tablespoons of water.

With the tone generator set around 100Hz, turn on the speaker/amplifier with the volume at minimum, and slowly raise the volume until the liquid is slightly agitated. Keep the volume low so that the water doesn't splash. Now, slowly sift or spoon in cornstarch, distributing it evenly across the membrane, until the fluid begins to thicken. Visible lines, ridges or other patterns may start forming on the surface as it thickens; try playing with the frequency to help the cornstarch mix in. Continue adding cornstarch until the water is absorbed (i.e. won't splash out when the volume is increased). Increase the amplitude, start sweeping the frequency and see what happens. If water splashes out or the mixture flows freely, add more cornstarch. If the mixture is too thick and won't move at all, add a tiny bit more water. Experiment! At some point, the mixture should begin writhing around like a living being. Continue adding cornstarch and adjusting the mixture until the membrane is entirely covered. To increase the height your Oobleck monster can reach up, you may need to add a bit more.

Place the lid on the jar to reduce noise and limit evaporation. If the speaker cabinet has a port, stuffing a rag in it will help reduce noise further. Both will alter the resonant frequency of the system, so the frequency may need to be readjusted for maximum effect.

Place the blacklight so that it illuminates the Oobleck jar. For better visual effect, cut a large 'X' in a piece of opaque fabric (or the remainder of the garbage bag) and slip it over the jar to cover the speaker box and equipment.

Now, the only thing left to do is tell anyone who asks that you found the stuff in an unmarked toxic waste canister while digging in the backyard, and it nearly ate your arm off!