FiO/LS 2011 - Tuesday Sessions

    Wow! Today was absolutely packed and quite exciting. First off I just invented my "Most Amusing Talk" award so I could give it to John Dudley, whose talk (slides available here) was very entertaining. He started off with a case of how he was wrong, then moved into a recount of how he was observing super continuum generation (where you get white light from almost single color laser light using special optical fibers) as a solution to the nonlinear wave equation and couldn't get it published anywhere (Science, Nature, and on down the food chain), but eventually got it into Applied Optics. Now these types of results make the cover of Nature Photonics! He suggested they start the journal "Nature Reinterpretations" which got a good chuckle from the audience.
    This was mixed with a recount of how the first nuclear weapon design (gun type) was not actually tested, but the engineers who were building it had a good idea it would work, so while the physicists were busy testing their bomb (implosion type), the engineers just dropped theirs! I really have no idea how this fit in, but interesting trivia anyway. Another good quote; "Fluid dynamicists were divided into engineers who observed things that could not be explained and mathematicians who explained things that could not be observed …" (Sir James Lighthill). I can think of some other fields, where this is true.
    Finally, he ended by showing just why science is exciting - the solutions they were seeing in their nonlinear optics experiments should have been possible in water (where the equations the optics people used were first developed). Despite searching, no-one had been able to produce the effect in water until the optics folks showed the way. And now we have rogue wave type solitons in water;

    The best part of the day, however, was going by the '"Mission: Optical" Student Chapter Competition'. The goal was to build a project for under $25 (US) that demonstrated an optics principle. So many good and fun ideas - I'll definitely be stealing many of these! My favorites were a balloon stretched across the end of a tin can, the other end was cut off so you could speak into it. A laser pointer was then attached pointing at a mirror (or CD) that was glued to the balloon. When you spoke into it, the balloon drum would vibrate, and the reflected spot would trace out a pattern on the wall. A great way of 'visualizing sound', and much easier, more direct and more robust than having motors drive mirrors to deflect the laser beam. Another group used legos, LED's, CD's and some ingenuity to build a 3D stroboscopic viewer. It almost worked. But it was enough of an ingenious idea, that I'd really like to give them credit for it.
    Other groups used just a bucket of water, sugar and milk to demonstrate as many different optical principles as they could: Reflection and refraction, the later of which changes with the addition of sugar to the water, total internal reflection in the bucket and the addition of milk for scattering. And another had a great setup that really made it easy for students to get into the scientific principle (just don't tell the kids that). They had kids split into groups and try different ways to heat water. Yet another demonstration of just why I think these conferences are awesome - I never would have thought of some of these ideas on my own, but now I can use them to teach others about optics.

Oobleck!

Here's a hands-on experiment that even the kids can try*.

  Oobleck (or corn starch in water) is a "Non-Newtonian fluid". When something tries to move through the oobleck, it tends to stiffen, and make it even more difficult for that object to move through. In the video below, sound waves from the speaker apply little pulses of pressure to the ooblek. Let's see what happens.

  When each pressure pulse (sound wave) is applied, the oobleck stiffens, and then relaxes when the pressure stops. This gives rise to the "fingers" of oobleck forming, then slowly wiggling around and breaking off. Look again at the video and watch the oobleck at the edges of the speaker. When the pieces break off and land on the part of the speaker that is not vibrating they stop stiffening and look almost like a normal liquid.

  Don't forget about the oobleck that's dripping off the edge too! It almost looks like a very thick syrup, similar to molassas. The same physics is happening here as well. As the oobleck slides down itself there's a force that causes it to stiffen up.

  Compare this behavior with plain water in a speaker cone (not responsible for musical tastes):

  Unlike oobleck, water doesn't get stiffer in the same way that oobleck does. If you have a bathtub full of water and you pull your hand through it it's difficult. That's the drag of water resisting you. Now if you drag your hand twice as fast it's just about twice as difficult. That's a Newtonian fluid, for those of you more advanced, it means that the drag of the water is linear in the rate of shear (how fast it flows over your hand).

   If we were to take our bathtub and fill it with oobleck (hypothetically, I wouldn't recommend doing this), we could pull our hand through it as well. Now if we were to drag our hand through it twice as fast, it resists us much more than twice as much. That is to say it gets stiffer as we increase the rate of shear, people call this dilatant.

  Alternatively we could imagine a material where if we were to pull our hand through twice as fast, it would actually be easier! This type of liquid gets softer as we go, or you may hear it as "shear thinning" or pseudoplastic.

  Oobleck is really a fantastically fun recipe and one that's so incredibly simple I really encourage you to do on your own: Mix 1 part water to 1.5 or 2 parts corn starch. Add some food coloring if you'd like. That's it!

  Another do-at-home non-Newtonian fluid recipe is that of Flubber.

*Always exercise caution when working with exposed electrical wiring, such as that which can be found in speakers, subwoofers, and other audio electronics. Children should always be supervised around electricity.