Abstract
The coalescence of two different drops, one surfactant-laden and the other surfactant-free, was studied under the condition of confined flow in a microchannel. The coalescence was accompanied by penetration of the surfactant-free drop into the surfactant-laden drop because of the difference in the capillary pressure and Marangoni flows causing a film of surfactant-laden liquid to spread over the surfactant-free drop. The penetration rate was dependent on the drop order, with considerably better penetration observed for the case when the surfactant-laden drop goes first. The penetration rate was found to increase with an increase of interfacial tension difference between the two drops, an increase of flow rate and drop confinement in the channel (for the case of the surfactant-laden drop going first), an increase of viscosity of the continuous phase, and a decrease of viscosity of the dispersed phase. Analysis of flow patterns inside the coalescing drops has shown that, unlike coalescence of identical drops, only two vortices are formed by asymmetrical coalescence, centered inside the surfactant-free drop. The vortices were accelerated by the flow of the continuous phase if the surfactant-laden drop preceded the surfactant-free one, increasing the rate of penetration; the opposite was observed if the drop order was reversed. The mixing patterns on a longer time scale were also dependent on the drop order, with better mixing being observed for the case when the surfactant-laden drop goes first.
Highlights
Drop formation, transport, splitting, and coalescence are the main processes used in drop microfluidics.[1−4] The coalescence of drops has received growing attention over the last decade because of many potential applications, with a number of publications providing protocols for tailored drop coalescence using passive[5−7] and active[8−10] methods
There are several processes that contribute to the mixing accompanying the coalescence of drops with different compositions in a microfluidic device: recirculation inside the coalescing drop because of no-slip conditions on the channel walls, spreading of the content of the surfactant-laden drop over the surface of the surfactant-free drop because of the difference in the interfacial tension between drops, and penetration of the contents of surfactant-free drop into the surfactant-laden drop because of the difference in capillary pressure
The penetration rate in turn depends on the neck kinetics because under the same difference in the capillary pressure between the drops, the liquid velocity inside the neck, that is, the intrusion velocity, depends on the neck cross section
Summary
Drop formation, transport, splitting, and coalescence are the main processes used in drop microfluidics.[1−4] The coalescence of drops has received growing attention over the last decade because of many potential applications, with a number of publications providing protocols for tailored drop coalescence using passive[5−7] and active[8−10] methods. The two main advantages provided by coalescence of drops in microfluidics are the possibility to perform chemical reactions under highly controlled conditions and the use of very small amounts of reactants.[2,15] Such microreactors may be separated from each other by the continuous phase, and if reactants and reaction products are soluble only in the dispersed phase, there is no risk of cross-contamination. Using 1 mL of sample, it is possible to create thousands to hundreds of thousands of microreactors and obtain reliable statistics by varying reaction times and conditions. Microfluidic reactors have been successfully used, for example, to measure enzyme kinetic constants[16] or to synthesize nanoparticles[17,18] or hydrogel particles.[19]
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