Shake a can of mixed nuts long enough and the biggest nuts end up on top. Studied since the 1930s but still poorly understood, this phenomenon–called the Brazil nut effect–also occurs in batches of particles ranging from stones to powders.
The phenomenon is of more than academic interest. In drug manufacturing, for instance, such separations could lead to unevenly blended powders. That could throw off dosages in pills made from the mixtures.
In recent years, researchers found that the weights of particles in a granular mix–not just the grains' different sizes–are important factors in the Brazil nut effect. Now, a new experiment shows that the nature of the particles alone isn't enough to explain what's going on.