I had the same thought. The box in the photograph looks like acrylic, which has a strong triboelectric potential. https://en.wikipedia.org/wiki/Triboelectric_e

I had the same thought. The box in the photograph looks like acrylic, which has a strong triboelectric potential. https://en.wikipedia.org/wiki/Triboelectric_effect https://en.wikipedia.org/wiki/Triboelectric_effect Here's a paper by the authors: http://arxiv.org/abs/1307.2489 http://arxiv.org/abs/1307.2489 "The shear cell is nominally 125 cm × 26 cm in size, and the particles are weakly confined by either a 0.5 mm clear flexible PVC or 1.4 mm thick acrylic sheet in order to suppress out-of-plane buckling. The sheet is wiped with a damp cloth before the experiment to minimize triboelectrification."