A stochastic system of particles is considered in which the sizes of the particles increase by successive binary mergers with the constraint that each coagulation event involves a particle with minimal size. Convergence of a suitably renormalised version of this process to a deterministic hydrodynamical limit is shown and the time evolution of the minimal size is studied for both deterministic and stochastic models. (joint work with Anne-Laure Basdevant (Paris X), James R. Norris (Cambridge), Cl\'ement Rau (Toulouse))
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