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 avoid such 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.