  Grain segregation occurs under various conditions, such as vibration, shear
and mixing. In the gravity-driven shear flow, size segregation is triggered by
the percolation of small particles through the opened voids (kinetic sieving),
and large particles are accumulated in the upper layer. One hypothesis for the
upward migration of large particles is the squeeze expulsion mechanism, which
remains yet ambiguous due to limited evidences from either physical or
numerical experiments. Here we show statistically how the percolation of small
particles facilitates the upward movement of large particles. We found that in
large particles, the mechanical anisotropy (strong force network) coincides
with the geometric anisotropy (contact network), which indicates squeeze, and
the connectivity of large particles is much higher when they are squeezed
through small particles. The presence of small particles filling the voids
hinders the downward movements and provides dynamic steps for the climbing
large particles. Furthermore, increasing the coefficient of friction promotes
rotation and empowers more large particles to reach the top layer, implying
that the climbing particles tend to rotate relative to their neighbors. Our
findings of the microstructure and movement pattern of individual particles add
new evidences to the mechanism of squeeze expulsion and provide new perspective
for the study of segregation.
