  Spontaneous stratification in granular mixtures has been recently reported by
H. A. Makse et al. [Nature 386, 379 (1997)]. Here we study experimentally the
dynamical processes leading to spontaneous stratification. Using a high-speed
video camera, we study a rapid flow regime where the rolling grains size
segregate during the avalanche. We characterize the dynamical process of
stratification by measuring all relevant quantities: the velocity of the
rolling grains, the velocity of the kink, the wavelength of the layers, the
rate of collision between rolling and static grains, and all the angles of
repose characterizing the mixture. The wavelength of the layers behaves
linearly with the thickness of the layer of rolling grains (i.e., with the flow
rate), in agreement with theoretical predictions. The velocity profile of the
grains in the rolling phase is a linear function of the position of the grains
along the moving layer. We also find that the speed of the upward-moving kink
has the same value as the mean speed of the downward-moving grains. We measure
the shape and size of the kink, as well as the profiles of the rolling and
static phases of grains, and find agreement with recent theoretical
predictions.
