Real-time probing of fast chemical reactions at the oil/water interface during microdroplet generation

Hypothesis Probing the interfacial reactions during microdroplet generation is experimentally challenging, limiting the understanding of the reaction dynamics at an early stage. Real-time monitoring of the temporal evolution of interfacial tension offers a quantitative approach for probing interfacial reactions and reveals the reaction rate-control mechanisms governing the interfacial reaction process. Experiments An automated microfluidic platform is developed to monitor the dynamic interfacial tension during microdroplet generation within a second, using oleic acid (HOA) and sodium hydroxide (NaOH) as a representative reaction system. The evolution of interfacial concentration of the reaction product is quantified by experiments using various reactant concentrations and flow rates, allowing for identifying the rate-control mechanisms. Findings The reactions are identified to be mass transfer-controlled or adsorption-controlled at different HOA concentrations with high NaOH/HOA concentration ratios, enabling the determinations of mass transfer boundary layer thickness and adsorption rate constant of HOA. A modified Damköhler number is introduced to characterize the transition between the rate-control mechanisms. Up to one-third of HOA in microdroplets is found to be consumed during the generation stage within one second, and smaller droplets exhibit higher consumption proportions.