Fluid flows with cohesive particles are present in oil industry (e.g. natural gas/oil with hydrates, wax or asphaltenes), medicine (e.g. blood cells), nano- and ferro-fluidic applications (e.g. fluids with nanoparticles subject to the van der Waals and electrostatic interactions) and even in astrophysics (e.g. grains in planetary rings). Such flows may lead to formation of agglomerates that, for example in pipelines, may result in unwanted phenomena such as formation of deposits. The main process parameter governing this is the "collision efficiency", which is the ratio of the number of collisions resulting in agglomeration to the total number of collisions. This is commonly considered to depend on the relativemagnitudes of attractive and repulsive interactions during a collision. The effect of the particles' mechanical properties on the agglomeration efficiency has, however, not yet been studied. In this paper the agglomeration efficiency is studied as a function of inter-particle friction, stiffness, density and volume fraction by numerical simulation. By running direct numerical simulations (DNS) with Lagrangian particle tracking of a shear flow laden with solid particles, the parameters influencing the agglomeration efficiency are demonstrated and their effects quantified. Finally, an expression that relates the collision efficiency to the salient dimensionless physical parameters is proposed. (C) 2011 Elsevier Ltd. All rights reserved.