Kalli, Maria; (2023) Surfactant effects on drop formation and flow characteristics in liquid-liquid microfluidic systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Microfluidic channels have been widely used to produce drops with controlled size, important in applications such as emulsification, inkjet printing and chemical analysis. Surfactants are often added to control drop size and provide emulsion stability by varying the interfacial properties. Due to the high flowrates required to form drops in microchannels, the formation process depends on the dynamic values of the interfacial tension, which are linked to the surfactant diffusion in the bulk solution and its absorption kinetics to the interface. To date, equilibrium values of interfacial tension are used in predictions, due to the limited range of available instruments that can provide measurements in the millisecond range, often relevant to microfluidic applications. Additionally, it is extremely difficult to visualise surfactants at the drop interface and measure their interfacial concentration. This thesis aims to understand the effect of surfactants during drop formation in another liquid phase in microchannels to improve future predictions. Three common surfactants, a cationic (DTAB, 20mM), an ionic (SDS, 11mM), and a non-ionic (TX100, 3.5mM), as well as two fluorescently tagged surfactants (R12, 1.5 mM and NBD-PC, 0.0125mM) were dissolved in an aqueous solution with 52% w/w glycerol, which was used as the dispersed phase (1132 kg/m3 and 7 mPa s). A low viscosity silicone oil (920 kg/m3 and 4.6 mPa s) was used as the continuous phase. Five patterns of drop formation were identified, namely squeezing, dripping, jetting, threading and tip streaming/jetting, whose boundaries were affected by the surfactant type and concentration. Specifically, the transition boundary between dripping and jetting patterns shifted to lower dispersed phase flowrates as surfactant concentration increased. The microfluidic channel was also used to measure the dynamic interfacial tension (DIT) down to 3 ms. In this approach a correlation was developed that linked the drop size to the capillary number and interfacial tension for systems with constant interfacial tension, i.e. systems without surfactants or with very high surfactant concentrations above the critical micelle concentration (CMC). For SDS and DTAB surfactants, the adsorption times were longer than the diffusion times for concentrations above CMC, but did not exceed the drop formation times. In the case of TX100 surfactant, adsorption times were comparable to drop formation kinetics at all studied concentrations and thus the adsorption step at the interface should not be neglected during drop formation. Using these dynamic values, it was shown for the first time that the boundaries between the dripping and the jetting patterns for all surfactants and concentrations studied, could collapse, resulting in a universal flow pattern map. A novel high-speed micro-Particle Image Velocimetry (µPIV) system was developed to follow the rapid formation of drops in the microchannels and obtain velocity fields during each stage. The experimental results demonstrated the strong influence of the surfactant on drop size, drop formation time, circulation patterns and velocity fields inside the drops and showed excellent agreement with available numerical predictions. Drop size and formation time decreased with surfactant concentration, as expected. The circulation patterns inside the drops decreased in intensity while higher velocities near the neck were seen as surfactant concentration increased. Finally, a micro-Laser Induced Fluorescence (µLIF) technique was developed to obtain the changes in the concentrations of the fluorescent surfactants at the interfaces. The times to reach equilibrium were compared between microfluidic approaches and tensiometry and possible causes for these differences were discussed.

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