Guerrilla Science by Ernesto Altshuler

Author:Ernesto Altshuler
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

However, as I walked triumphantly back to the ESPCI, I realized that it would be a bit odd to see me entering through the main gate in the middle of the morning with a big, and potentially dirty plastic plate. So, as many dumpster divers probably do, I concealed the thing behind an electrical (or telephonic?) control box right at the corner of rue Monge and rue de Mirbel. 4 I’d come back late at night to complete my SDD activity.5 Once home—I was living at the time in an ESPCI apartment inside the institute—I washed and cut the plate into pieces that would fit into my luggage. The acrylic plates would land safely in my lab a few days later, before the excited faces of my students. Finally, we needed a nicely-shaped and relatively light granular material to fill up the Hele-Shaw cell. The problem was solved by a quick SDD operation in the garbage dump of the Chemistry department (University of Havana) where a few kilos of discarded ion-exchange resin, consisting of spherical grains, were collected.

The setup was completed (almost) with a Go-Pro video camera provided by Renaud—not taken from the garbage, as far as I know. Given its lightness, we fixed it to the shaker in such a way that it would oscillate synchronously with the Hele-Shaw cell: this guaranteed that we could take videos of the sinking process in a reference frame at rest relative to the granular medium. As suggested above, including accelerometers inside and outside the intruder would actually be very useful to study the sink process in a three-dimensional system, where the intruder would eventually escape any possibility of getting video-taped as it sank completely into the granular material. Finally, to help the fluidization process, a device to inject air in the upward direction was also installed at the bottom of the cell. Gustavo put together the setup with the (now) diligent help of undergraduate student Laciel Alonso.6

One might believe that a single accelerometer would be enough to follow the sink dynamics of the intruder—just as presented in the previous chapter. However, the vertical penetration of the intruder into the laterally shaken cell was relatively slow as compared to our previous experiments with ultra-light granular matter (see Chap. 4). So, a single accelerometer inside the intruder just measuring the vertical acceleration would yield a very noisy signal. I therefore proposed a “way around” the problem: comparing the outputs of two accelerometers measuring the horizontal (and not the vertical) acceleration of both the oscillating cell and the intruder: since the shaking acceleration was quite large, the horizontal signals would be easier to measure. But, how could they help us to figure out the sinking process of the intruder along the vertical? The idea was to compare the output of the two accelerometers—more exactly, to measure their temporal correlation.

Figure 5.4 illustrates qualitatively how the correlation helps us to know when the intruder has reached the “jammed phase” at the bottom of the granular medium. As

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