About the role of physico chemical properties and hydrodynamics on the progress of a precipitation reaction : the case of cerium oxalate particles produced during coalescence of drops The size and morphology control of precipitated solid particles is a major economic issue for numerous industries. For instance it is interesting for the nuclear industry concerning the recovery of radioactive species from used nuclear fuel. The precipitates features which are a key parameter from the post precipitate processing depend on the process local mixing conditions. So far the relationship between precipitation features and hydrodynamic conditions have not been investigated. In this study a new experimental configuration consisting of coalescing drops is set to investigate the link between reactive crystallization and hydrodynamics. Two configurations of aqueous drops are examined. The first one corresponds to high contact angle drops (>90°) in oil as a model system for flowing drops the second one correspond to sessile drops in air with low contact angle (<25°). In both cases one reactive is dissolved in each drop namely oxalic acid and cerium nitrate. When both drops get into contact they may coalesce; the dissolved species mix and react to produce insoluble cerium oxalate. The precipitates features and effect on hydrodynamics are investigated depending on the solvent. In the case of sessile drops in air the surface tension difference between the drops generates a gradient which induces a Marangoni flow from the low surface tension drop over the high surface tension drop. By setting the surface tension difference between the two drops and thus the Marangoni flow the hydrodynamics conditions during the drop coalescence could be modified. Diols water mixtures are used as solvent in order to fix the surface tension difference between the liquids of both drops regardless from the reactant concentration. More precisely the used diols 1 2 propanediol and 1 3 propanediol are isomer with identical density and close viscosity. By keeping the water volume fraction constant and playing with the 1 2 propanediol and 1 3 propanediol volume fractions of the solvents the mixtures surface tensions differ up to 10 mN m for identical constant reactant concentration density and viscosity. 3 precipitation behaviors were identified for the coalescence of water diols recatants drops depending on the oxalic excess. The corresponding precipitates patterns are visualized by optical microscopy and the precipitates are characterized by confocal microscopy SEM XRD and SAXS measurements. In the intermediate oxalic excess regime formation of periodic patterns can be observed. These patterns consist in alternating cerium oxalate precipitates with distinct morphologies namely needles and “microflowers”. Such periodic fringes can be explained by a feedback mechanism between convection reaction and the diffusion.