Rotating drum experiments on secondary avalanching in bidisperse particle-size segregation

• The surface angle of the granular bed and the rest time of the avalanche under different flow conditions are measured. • The relationship between the secondary avalanching and the trapped kink was established. • The evolution of the segregation pattern during the transition from discrete avalanche to continuous flow is analyzed. In binary-size granular segregation, the influence of secondary avalanching on segregation requires deeper investigation. The occurrence of this secondary avalanching is closely associated with the structural characteristics of trapped kinks. In this study, we introduce a dimensionless parameter—the ratio of avalanche rest time—to characterize the fluidity of the granular bed and employ a phenomenological model to describe the formation mechanism of trapped kinks. The experimental results reveal that the distribution of trapped kink follows an asymmetric probability distribution, with the theoretical predictions of the trapped kink model showing good agreement with the measured data. As the rotational speed increases, the frequency of secondary avalanching gradually decreases, while the avalanche rest time lengthens, and the average angle of repose rises, leading to a transition in the flow regime from avalanching to rolling. During this transition, the distribution of trapped kinks shifts from a lognormal distribution to an exponential distribution, the streak length progressively shortens, and streak merging becomes evident. These findings contribute to a deeper understanding of the mechanisms underlying avalanching segregation.