Picture of Eerie glowing blob monster
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
Yellow highlighter
Corn starch
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.

Wonderful use of non-Newtonian fluids with respect to halloween!

How'd your crowd react? 
You could add some sweet 'radioactive bars' (ie. fluorescent strips) or a spark-gap 'jail cell' to the inside jar to convince the little kiddies that it's safe to come look ;)
Drmn4ea (author)  T3h_Muffinator5 years ago
We set it out on the front porch a bit off to one side of the front door. Strangely, it was the adults that were all crowding around to play with it. The kids were much more goal-oriented (candycandycandycandycandy!) and tended to just beeline past it on their way to the door!

MrGreggan5 years ago
That looks really cool! What would it look like with a strobe blacklight?
Drmn4ea (author)  MrGreggan5 years ago
Blacklight strobes exist? I bet that would look pretty awesome. Besides the haunted-hause appeal of strobe lights - the shear membrane takes out a lot of the 'bounce' vs. oobleck directly in a speaker, but if you adjust the strobe frequency to match the speaker frequency (or some exact division of it), you shouldn't see any bounce at all, and the 'monster' should be that much more natural-looking.