Understanding flow-induced phase inversion of emulsions using microfluidics
Phase inversion emulsification (PIE) is a process of generating emulsions by inverting the continuous and dispersed phases of precursor emulsions. PIE is particularly useful when it is challenging to generate the target emulsions by conventional emulsification methods. One such case is the synthesis of polymeric nanoparticles, which requires production of very small droplets of viscous oils. Currently most, if not all, PIE processes in industry are performed as batch processes. Many studies have demonstrated considerable reductions in operation time/cost by changing a batch system to a continuous system. One way of inducing phase inversion in continuous processing is by flowing emulsions through precisely engineered channels and pore-arrays i.e. by flow-induced phase inversion emulsification (FIPIE). A clear advantage of this mechanism is that it can be simulated in microfluidic channels and thus direct observation and fundamental investigation of the PIE process is possible.^ It is shown that preferential wetting between the dispersed phase of the precursor emulsions and the channel surfaces is crucial for FIPIE. This means, O/W emulsions require hydrophobic channels for FIPIE and vice versa. A tapered design of the phase inversion channels (PICs) with homogeneous surface treatments is used to induce FIPIE. It is found that FIPIE is very sensitive to the amount of taper and is suppressed if taper angle increases above 5 degrees. The dynamic factors affecting FIPIE are investigated in terms of dimensionless parameters – Capillary number, which denotes the relative importance of surface tension and viscous effects and D/W, which is the ratio of size of droplets to the minimum width of the tapered PICs. Lower Ca and higher D/W are found to favor FIPIE. A mechanism of FIPIE is proposed based on the real-time visualization of FIPIE. As droplets passed through narrow channels, the continuous aqueous phase is sheared into a thin film surrounding the oil droplets. Rupture of this aqueous film is found to be the most critical mechanistic step of the process. The underlying physical phenomena driving film rupture are studied based on a balance of interfacial stresses. Finally, the effect of composition and molecular mass of surfactants on the stability of emulsions against FIPIE is studied. It is shown that surfactants, which provide thicker and more viscoelastic films at emulsion interfaces result in emulsions that are more resistant to FIPIE. The insights developed in this thesis can further the prospects of enabling continuous PIE on a larger scale.^
Kumar, Ankit, "Understanding flow-induced phase inversion of emulsions using microfluidics" (2016). Dissertations available from ProQuest. AAI10193673.