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Spooky Cornstarch

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Tesladownunder

Sun May 07 2006, 01:43PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Inspired by a recent video I decided to do make my own cornstarch fun.
Essentially cornstarch liquid is a fluid whose viscosity is flow rate dependent. You can hit it hard with a spoon and it will strike it and bounce off, but can be poured slowly and drip from a spoon like a thin liquid. A real anti splash fluid.
On a flat plate under vibration the vibration is probably more marked at discontinuities and edges (like a wave breaks at the seashore). This leads to strange effects with cornflour which gets forced up into stalagmites, loops and bizarre slowly moving spooky shapes like pools of writhing undead...

Anyway it is very easy to do. Get the wife/girlfriend/significant other to make a reasonably thick cornflour/cornstarch mixture. (This is a common cooking ingredient). Get a 4 inch loud speaker (I used 4 inch, 4 ohm, 3.5W) and feed it with 50Hz/60Hhz. Use a 6VAC winding with a dropping resistor or use a variac. I needed about 2.0 VAC ie 1W.
Place some plastic cling film (Gladwrap in Australia) loosely over the speaker so that a spoonfull or two of the cornstarch mix will sit closely applied to the speaker cone.

I did try a frequency generator and different frequencies but 50 Hz seems fine, particularly as it dries a little and the viscosity rises.

Here is a sequence of still pics as the power increases.

Peter

ragnar

Sun May 07 2006, 02:30PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

That's.. um.. filthy, TDU =-D

You never cease to amaze me! :)

Keep up the great work.

Nik

Sun May 07 2006, 09:22PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

So I saw this and it was too simple no to try. I blew out the first speaker (it wasn't really a robust looking unit). I managed to get good results (posted here

) useing a 10w speaker drivin at 50hz and 101hz (that causes the beating). The I seemed to get the best "fingers" between 48hz and 62hz, out side of that I needed to increase the amplitude.

I also experimented with diffrent wave forms. Sin gave the best results, square did not form fingers (the speaker could not have handled the power), triangle took a higher amplitude then the sin but did produce fingers, sawtooth caused the cornstarch to splash out before fingers formed.

I may pick up a speaker that can handle a higher power and continue these experiments. If all goes well I might have a post for the projects forum.

*edit* NSFW ads on the link

Tesladownunder

Mon May 08 2006, 12:46AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

You must have been using a bit of power to blow the speaker since you can get pretty good cone excursion with the 1W I was using (amended figure). Sine wave is best since the low frequencies are what works and the higher harmonics with any other shape dont seem to help and probably tend to break the coupling.
I didn't have any other loose speakers readily available as I had blown them up with my capacitor bank in the name of science

You may get better results as the cornflour thickens as it dries a bit and doesn't flow as well.

I will repeat this and try for a video later. I suspect that there will be a bit of blurring though due to the vibration and that a flash camera shot does capture things more sharply.

Peter

Nik

Mon May 08 2006, 12:52AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

The cone in my speaker(s) arepretty steep so it takes alot more power to vibrate the whole volume of the liquid. The speaker was free from the side of the road so I don't know if it was previously damaged. The coil is fried (crunches and reads 0 ohms).

Alex

Mon May 08 2006, 01:54AM

Geometrically Frustrated
Registered Member #6 Joined: Thu Feb 02 2006, 04:18AM
Location: Bowdoin, Maine
Posts: 373

Wow, that was a very neat video. It's like waves crashing on the beach, but they don't break.

The last picture posted is particularly interesting because you can see a definate break in one of the loops.

Heiders

Mon May 08 2006, 03:48AM

Registered Member #268 Joined: Tue Feb 28 2006, 02:44AM
Location: Ontario, Canada
Posts: 48

It's also really cool when a drop of food colouring is added at different frequencies. You would expect it to spread out evenly, but it actually tends to stay on one side. The cornstarch is drawn up from the middle and dropped on the outside. Then more is dropped on top with the next wave, pushing the original stuff to the bottom while carrying the food colour, and keeps cycling on each "side", as it were.

(I was at Nick's house this afternoon during the experiment

)

TDU wrote ...
pools of writhing undead

Undead indeed!

Nik

Fri May 12 2006, 02:25AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I picked up a 10inch 100w speaker for 20$ cad to further my experiments with cornstarch. I have a photo to post tonight and i will write up some more tomorro.

Tesladownunder

Fri May 12 2006, 05:12PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I did a physics demo today for the opening of the Cosmology gallery at the Gravity Discovery Centre near Perth, Western Australia and showed some 80kV sparks, aerogel, magneforming, ferrofluid and the cornstarch demo. I had less than a days notice so it was rather hastily put together.

And the audience response? The ladies have all used corn starch and were fascinated by it, Small kids preferred the ferrofluid. Real men preferred the HV.... Science teachers liked the simplicity of the cornstarch.

Had to remake the cornstarch after about 1 hour, but it got a lot of use. As it got thicker it tried to escape by forming a ball and nearly jumping out.

No pics of the cornstarch as too busy doing the demos and the lighting wasn't good. Must make a video sometime.

Peter

1147453924 10 FT9119 Cosmologydemoferrofluid

Tesladownunder

Sun Oct 08 2006, 02:18PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I tried this with a subwoofer and the results were a bit disappointing initially.

Photo 1 shows the 12 inch subwoofer driven with 40 W at 50 Hz (12 VAC). Removing the plastic and putting the cornflour directly on the speaker improved things only a little. I later tried higher with perhaps 200 W but not much better.
Photo 2 Shows the setup with a frequency generator and a 100 W audio amp. I picked out the resonant frequency of about 22 Hz where cone excursion was greatest and up to one inch. then things started to move.
Photo 3 shows a blob in flight
Photo 4 shows a close up. Spooky

I guess I got lucky with a 50Hz resonance with the little speaker which in some respects was still the more dramatic demo and the smaller excursion makes things more visible.

Peter

1160317092 10 FT9119 Cornfloursubairbourne1full

1160317092 10 FT9119 Cornfloursubairbourne1close

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