Particle size segregation in a two-dimensional bed undergoing vertical vibration

We present experimental results on the size-ratio and acceleration dependences of particle size segregation behavior for a single intruder in a two-dimensional bed of monodisperse particles undergoing vertical vibration at low accelerations. Using trajectory maps based on digital high speed photography and computer image processing we find that, at all base accelerations, the intruder and surrounding particles move upwards at the same speed. The motor of such convective motion is slip planes and block motion of particles slipping past each other which push the intruder and surrounding particles upwards in a collective motion. Although some of these features have been previously observed, we use these observations to propose a mechanism for segregation at low accelerations. In this the intermittent steplike motion is found to be due to the finite frequency of slip planes and dislocations, causing the intruder and surrounding particles to move upwards in finite-size jumps. The stability of the intruder is not relevant in our experiments and no fundamental difference in mechanism between the so-called intermittent and continuous regimes exists; only the frequency of slip planes and block motions is important.