To change the wall roughness we glued sandpaper of different grain sizes on the walls, leaving a small gap without sandpaper, to be able to detect the level of grains and measureh.Fig. 1showsthe appearance of the tube with and without sandpaper. We used
three different sandpaper average grain sizes: 28.5m, 46.25m
m.A good verticality of the oscillations is a crucial matter when studying the U-tube instability as was pointed out bySánchez et al.(2009). Our configuration based on coupled speakers, proved to be very effective in providing oscillations in one dimension. Great care was taken in order to level the system. The best sign of good
verticality is that the granular material tends to migrate to either
side of the tube. Furthermore, our results are averaged over ten
experimental runs, five of them with grains migrating to the left
side of the tube and five with grains migrating to the right side of
the tube.