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Abstract:
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”, “noncoalescence”), in contrast to only two behaviors that
were observed in the case of nonreacting 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.
