- Why do the slopes of some piles of soil or sand slip very easily, while
other piles remain extremely stable? Answering this question holds the
key to the understanding of many fundamental physical questions, for
example the onset of landslides. For a long time scientists have assumed
that the surface roughness of the individual grains plays a keys role in
this process. However, this common-sense assumption has never been
verified experimentally. Here, for the first time, we provide the
necessary methodological concepts and succeed in proving this
hypothesis. We do so as follows.
We have invented a chemical recipe to increase the roughness of glass
and we use it to systematically tune the roughness of sub-mm glass spheres.
We measure the roughness of individual grains using light interference,
and the roughness of the whole granular sample using Coca-Cola.
We then measure the friction between the grains by looking at the slope
of the pile they form at the bottom of an hour glass.
We look at the correlation between roughness and friction. Our results
show that changing the roughness of grains allows us to tune the
friction between them, making friction a control parameter. And this
hands us the key to understanding real-life granular systems.
