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In the final year of this project, we have continued the experiments studying the effect of cohesion on the structure of granular matter, and building on the study of flow properties diagnosed with the rotating drum apparatus in the early part of the project. In particular we observed there that the angle of repose increased and then decreased as the rotation rate was increased, and further depended on the viscosity of the wetting fluid. These earlier results were published in Physical Review E. In the studies since then we have concentrated on understanding the microscopic structure which results due to the addition of cohesive forces caused by capillarity, and the laws of addition of cohesive forces due to more than two bodies.  

We have completed a thorough investigation and analysis of a model two dimensional system of particles floating at the air-liquid interface. The meniscus formed around the floating spheres results in cohesive forces. While the force of attraction between spheres has been discussed theoretically and the exponential nature of the approximate force has been tested, the situation when more than two particles are present was not at all clear. In particular simple questions such as if superposition holds were unanswered. Our investigations over the past year and answered these questions. In particular we elucidated the forces between cohesive particles as a function of number particles starting with two spheres. We showed that superposition holds over a wide range of distances but fails where particles are closer than their size. We also estimated the effect of lubrication forces on the dynamics of their approach, and the non-trivial linear and lattice structures formed. We are currently preparing a manuscript for submission to Physics of Fluids.