Periodic precipitation patterns during coalescence of reacting sessile droplets

The coalescence behavior of two sessile drops that contain different chemical reactants (cerium nitrate and oxalic acid) and its impact on the formation of the solid precipitate (cerium oxalate) are investigated. With different liquids, the surface tension difference in the moment of drop-drop contact can induce a Marangoni flow. This flow can strongly influence the drop-drop coalescence behavior and thus, with reacting liquids, also the reaction and its products (through the liquid mixing). In our study we find three distinctly different coalescence behaviors ('barrier', 'intermediate', 'non coalescence'), in contrast to only two behaviors that were observed in the case of non reacting liquids. The amount of liquid mixing and thus the precipitation rate are very different for the three cases. The 'intermediate' case, which exhibits the strongest mixing, has been studied in more detail. For high oxalic acid concentrations, mainly needle-like aggregates, and for low concentrations, mainly flower-like precipitate morphologies are obtained. In a transition range of the oxalic acid concentration, both morphologies can be produced. With the applied coalescence conditions, the different aggregate particles are arranged and fixed in a precipitate raft in a regular, periodic line pattern. This confirms the drop-drop coalescence configuration as a convection-reaction-diffusion system, which can have stationary as well as oscillatory behavior depending on the system parameters. (authors)