Sharing another high speed

Because you can't get enough of wobbly gelatin...

Thurs-Demo: The Photoelastic One

Our department has an iPad. It's a lonely first-generation device: it doesn't get as much use as we hoped due to it's limited flexibility with computer projectors and video. Perhaps the newer versions would be more useful. That's not the point of my comment - the polarized light emitted by the iPad is. Last week I was able to use the iPad to view deformation in gelatin. Here's a quick movie of me rotating the iPad in front of my laptop's built in camera. The polarized light from the iPad is extinguished by the polarized filter I have in front of the laptop camera. All you see is a shiny reflection of the computer screen as a result.

My buddy Todd is (hopefully) putting together a post on light and polarization - perhaps he'll include some discussion on the orientation of the polarization of smart phones and sunglasses and such.

So this week, I tried something a little different. I pulled out some circular polarizers and used them to shoot some video of a steel ball dropped onto a dish filled with gelatin. I think the effect is both easier to see and easier to film due to the increased light coming from the setup.


That blue square in the background is one of the circular polarizing filters. There's another one in front of the camera.

The colored bands are a result of interference produced when the gelatin deforms. The spacing of the bands is proportional to the magnitude (amount) of strain/deformation in the material. Here's the experiment with a much larger ball (hence more stress, producing more strain):


For comparison, here's the same experiment in plane-polarized light:


In the write-up above, I've been using stress and strain somewhat interchangeably. That's because the gelatin deforms easily when subjected to pressure. The magnitude of the applied force/area (pressure) is the stress and the amount of deformation is the strain. So when we apply a little stress, we see a small amount of strain. With a lot of stress, a lot more strain develops. But in gelatin, it doesn't take much stress to produce strain. In more rigid solids, stress can develop without any observable strain. But that's a topic for later.

If you like, you could place a square block on the gelatin and see how the stress is distributed beneath and away from the base of the block (much like how stress spreads below and outward from a load placed on soil). Or you could take plaster or food coloring and inject a liquid into the gelatin and see how the shape of the fractures depends on the stress field around the intrusion (Callan has done this and there's a set of neat labs from a prof in Hawaii that talks about this technique in great detail).

I've found that using a gelatin mixture that's about 150% thicker than recommended for "finger jello" works best - and holds up reasonably well to the increased heat of the lights used for filming.

Hmmm, I've got the "updated" version of the Blogger interface and things are looking a bit strange. I hope this works. I suppose it's a good time to try and see if embedding videos works the same as it did before.

Thurs-Demo: The Next One Posted on Friday Because I Was Busy Thursday

This isn't a trend, but I seem to be rather busy on Thursdays. So much so that yesterday's demo didn't happen. Perhaps I'll describe what I was up to at some later point.

Today, however, I took advantage of the light output from an iPad. Many computing devices that have an LCD type screen emit polarized light. It's why I rarely wear sunglasses when I'm out taking pictures - the polarized glasses cut out too much - sometimes all - of the light from the review screen on the back. Wearing polarized sunglasses and looking at a computer can also be tricky.

But stick the iPad behind a transparent, isotropic material and then slap a polarized filter in front of the camera and you have an instant stress-field viewer. I redid the shot of a small cap gun charge through gelatin, but this time the only light was coming from an iPad.


This turned out much better than I had hoped. First, I didn't have a lot of light to work with. Second, many computer screens will produce scan lines because the screen is "refreshing" at some frequency (it's why video of older phosphor screen monitors "flicker"). But the iPad sent out a nice steady light (I made a blank white slide in keynote and just had that as the display).

Now I think I need to get my hands on some of those old fashioned 4"x4" rock thin sections and try photographing them...

NURD ALURT!

George Tekai posted a picture on his facebook wall. I had to steal it and change the caption. If you know the original attribution for this photo, lemme know (it's not my picture after all).

Pressure and Shear Waves in Springs

In an experiment fitting the vernal equinox, I was mucking about with springs today. I've been having trouble getting waves to show up well on the high speed camera. The ubiquitous "slinky" behaves a little oddly. Perhaps I'll write about shear waves in slinkys at some point. So I set up a few general springs in hopes of getting a pressure wave (longitudinal wave motion) that travels faster than the shear wave (transverse wave motion).

First run, shot at 1000 fps.

• P-wave travel time: 30/1000ths second


• P-wave velocity: 953 cm/sec



• S-wave travel time: 34/1000ths second


• S-wave velocity: 841 cm/sec

A second run, this time shot at 3000 fps.

• P-wave travel time: 107/3000ths second


• P-wave velocity: 802 cm/sec



• S-wave travel time: 127/3000ths second


• S-wave velocity: 676 cm/sec

Both show the shear (transverse) wave moving at a slower velocity compared to the pressure (longitudinal) wave. One of the methods we have to estimate the location of earthquakes is due to the difference in travel times. Kind of like using the number of seconds between the thunder and the lighting flash to estimate how far away the storm is. The large quake that struck Mexico today was probably located in part by the difference in P- and S-wave travel times.

Now I believe this demo is ready for my intro students...