Sands of the San Francisco Exploratorium

When I bought my ticket for the Exploratorium, I asked, of course, where
there were exhibits that used sand. After initially pointing to one close by,
the guy behind the counter looked at me and asked if I wanted to avoidsuch things. Two thoughts immediately sprang into my mind: this could be a
fine example of creative, lateral, thinking but could it be that he had recently
encountered a visitor who had specifically expressed this desire? Someone who
wished to be given an itinerary around the exhibits that kept them as far away
from sand as possible? Someone with an allergy to the stuff? I quickly made it
clear that, no, such things were the primary focus for my visit, and received
helpful directions. Not that they were really necessary – within a few steps of
the entrance I encountered ten exhibits whose attraction depended on the
behaviours of granular materials.

The closest, the one which had been first pointed out, contained a strong
pair of magnets and a good supply of iron sand – like all the Exploratorium
entertainments, play was encouraged. It turns out that the iron sand was
extracted from Ocean Beach, on the coast just south of San Francisco:

Then I encountered “Seismic sand,” a clear, rotatable, disc filled with fine
sand, next to which hung a large drumstick. I was encouraged to beat
rhythmically on the disc and observe the patterns formed in the sand – so I did,
with gusto. Reflecting the fluid-like behaviour of the sand, the sign encouraged
comparison with earthquake liquefaction,
something that I thought a bit far-fetched; but it went on to say that the
patterns formed are due to “the complex interactions of the flowing sand with
the vibrational waves of the air inside and the motion of the walls of the
container itself. The dynamics of this process are not well
understood” (my emphasis – I always like such statements, when faced
with something created so apparently simply).

My next encounter was with a dune and ripple machine, blowing sand around –
the observer could manipulate the fan. The structures of windblown sand were
introduced in the associated description.

Next to the dunes was a device called “Landfall,” enabling the creation of “a
miniature landscape in sand.”This is somewhat difficult to describe, but see the
photos below. A large transparent sphere is divided in half by a panel that
contains holes which can be opened or closed using the silver knobs around the
circumference. The whole thing can be rotated by cranking a handle – rapidly or
slowly depending on the energy and age of the cranker – and the sand poring from
one hemisphere into the other creates endlessly varying granular landscapes:
below, a good approximation of talus slopes or even alluvial fans; other
possible landforms were invoked in the accompanying description. This was really
quite compelling to watch – I had to wait my turn for a while as a young cranker
expressed his determination to get all of the sand into one hemisphere (a
challenging task that eventually proved too much for his attention span and I
was able to move in).

Next, “Rift Zone.” By pressing a button, air can be jetted from below into a pile of very fine sand
and the resulting craters and slumps are dramatic. However, this was billed as
simulating “some of the geological features associated with rift zones” –
including lava extrusion: fun though it was, I thought this a little
misleading:

And then: **avalanches!**A stunningly-designed
container (of which I was quite envious) demonstrated the same processes of
self-segregation and stratification that I have described from my own kitchen
physics experiments. Kitchen
experiments, avalanches, making
cross-bedding, complexities of sand flowing out of a funnel – endlessly
entertaining (well, I find it so – as did any number of visitors I observed at
play in the Exploratorium).

Of course, playing in the sand often involves making patterns, and three of
the exhibits were designed exactly for that. A wonderful shape-shifting
vibrating pile of sand that responded in compelling but unpredictable ways to
visitor interaction, and several spinning discs on which designs could be
generated using a variety of tools – or, most popularly, simply one’s fingers.

And, finally, acoustics: two separate illustrations of Chladni patterns. I
wrote a bit about these in my book:

In the late eighteenth century, a lawyer, musician, and scientist in Leipzig,
Ernst Chladni, was determined to make sound waves visible. He succeeded in doing
so by covering glass or metal plates with sand and drawing a violin bow across
the plates’ edges. The vibrations provoked the sand grains into a frenetic,
leaping dance, not just randomly: they arranged themselves like Scottish dancing
groups into formations. The patterns were highly variable—stars with different
numbers of points, crosses, complex intersecting arcs. The science of acoustics
and the physical manifestation of sound was born… The patterns produced in
Chladni’s experiments were as much about the material on which they were
produced as about the sand itself, but the patterns seem to relate to underlying
natural behaviours. All granular materials indulge in some extraordinary pattern
making.

But this was the first time I had been able to play around with these
patterns myself, so play I did. The exhibit provided several differently shaped
plates which could be interchanged, and a simple means of varying the vibration
frequency of the plate. The accompanying panel gave useful hints as to
appropriate frequencies to try with different shapes – the results were quite
dramatic:

And, next to the vibrated plates was (above, right) a truly excruciating
further demonstration of Chladni’s patterns, this time produced using the
original method, pulling a bow across the edge of the plate. It worked, but the
process reminded me too closely for comfort of the days when my son was learning
to play the viola.

Now of course there is far, far, more in the Exploratorium beyond the sand
devices. There’s a terrific new section on perception errors and cognitive
biases, a working cloud chamber, biological and microscopic phenomena… all too
much to play with in one afternoon. But whether or not you have kids, a visit is
highly recommended if you’re in the Bay area – it’s a great “museum” that makes
science accessible as well as mysterious, compulsively interactive, and, above
all, FUN.

And, when you’re all played out, mentally exhausted by the mysterious
behaviours of granular materials, you can simply stroll outside, cross the road,
and sit on the beach with a great view of the Golden Gate Bridge. It’s all
magic.

(A side note: I was surprised not to find my book in the store - but I decided that they must have sold out) SIGNATURE

Comments

Richard Bready (2010-08-09):

Speaking of fun, I don’t know why you keep reminding me of toys I used to own, but I’m grateful:
http://www.sciplus.com/search.cfm?utm_source=internal&utm_medium=search&utm_content=cf&utm_campaign=celsearchtest&formfield1234567891=0&formfield1234567892=6&formfield1234567894=&term=93744&btnHand.x=0&btnHand.y=0
is a desktop 2-D version of Landfall.

Sandglass (2010-08-09):

And thanks, but I wish you hadn’t made me aware of this site - it might prove expensive! But it’s certainly fun, and fun is important…

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2010/08/sands-of-the-san-francisco-exploratorium.html