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 Rg of an N particle cluster follows a power law R g ∝ N1/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/Nc) with a characteristic cluster size Nc. A power law Nc ∝ -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. © 2012 The Royal Society of Chemistry.