Wet granular rafts: aggregation in two dimensions under shear flow

Abstract

The aggregation and fragmentation of cohesive granular particles trapped at an air–liquid interface under shear flow are investigated by experiments and compared with numerical simulations. The cohesion owing to the formation of capillary bridges from a thin oil film covering the particles is found to dominate other particle–particle interactions. In the steady state the radius of gyration R_g of an N particle cluster follows a power law R_g ∝ N^1/δ. The fractal dimension δ of the clusters lies in the range between 1.5 and 1.6 and depends only weakly on the shear rate . The probability to find a cluster of N particles or larger P(N) displays an exponential tail P(N) ∝ exp(−N/N_c) with a characteristic cluster size N_c. A power law N_c ∝ ^−β with an exponent β = 0.65 ± 0.06 (β = 0.68 ± 0.02) is found in our experiments (simulations). The fractal dimension of the clusters and the value of β are consistent with an estimate of the largest stable cluster based on the balance between the capillary force and the viscous drag force.