Drop production and tip-streaming phenomenon in a microfluidic flow-focusing device via an interfacial chemical reaction

Microfluidic flow-focusing technology is used to investigate the effect on drop formation due to the production of a surfactant via an interfacial chemical reaction. The reactants are an aqueous solution of sodium hydroxide (NaOH) and a mixture of oleic acid (C₁₇H₃₃-COOH) and mineral oil, for the dispersed and continuous phase fluids, respectively, at concentration ≤ 5 mM. In the absence of a chemical reaction, the drop shapes remain constant from just after breakup into droplets down at the flow-focusing nozzle until the drops exit the channel. In the presence of the chemical reaction, there is modification of the shape depending on the concentration of reactants. The drop speeds, O(10) mm/s, lengths, O(1-100) μm, and relative displacements, O(100-1000) μm, are measured for a variety of flow conditions with observable trends that correlate with the reaction rate, which we rationalize by using the Damköhler number to characterize drop production and transport in these types of flows.