Perazzo, Antonio (2015) Microfluidics of Multiphase Flows. [Tesi di dottorato]

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Abstract

In this thesis, three kind of multiphase flow have been investigated with microfluidics methods, confocal microscopy and rheology: i) phase inversion emulsification, ii) emulsion interfacial tensiometry and iii) arylamine synthesis (based on a solid-liquid multiphase system). Multiphase systems are ubiquitous in industrial application, being emulsion one of the most relevant multiphase system. Emulsion microfluidics has been exploited as a mean to investigate emulsion morphology and flow behavior along with other well established technique such as optical microscopy and rheology. Phase inversion emulsification (the phenomenon by which the dispersed phase is switched into the continuous one) is one of the most popular route to obtain nano-sized droplet or capsules with tailored features and sometimes could also represent an inconvenient related with process operations, being the crude oil pipeline transportation one of the main example. Experimental techniques such as, rheology, confocal microscopy and microfluidics were used in order to obtain deep insight on such emulsification process. Microfluidic techniques have been applied also to characterize emulsion interfacial properties. Some limitations, e.g., droplet with surfactant covered interface, are related with the latter. Pendant drop tensiometry and capillary pressure tensiometry were used in order to elucidate and develop such droplet based microfluidic methodologies. A microreactor for the handling of a multiphase reactive flows for the synthesis of a pharmaceutical valuable arylamine via Buchwald-Hartwig reaction ( one of the most exploited reaction pathway in pharmaceutical industries) has been developed obtaining better performance with respect to the classical batch operations. The phase inversion emulsification, especially from the flow behavior point of view, has been investigated. The emulsion morphology has been characterized in detail by direct observation in confocal microscopy within microfluidic channels. Long term stable nanoemulsions (average drople size equals to 170 nm) with great energy saving have been obtained. Higher emulsions stability is associated with both small droplet size and low polydispersity of the droplet size distribution. Confocal microscopy can be exploited to follow the time evolution of the phase inversion process. Confocal imaging clearly shows bicontinuous structure formation in emulsification, that signs the two phases point of inversion. Rheological test showed an increased viscoelastic behavior in the proximity of the inversion. Cylindrical microchannel coupled with laser scanning confocal microscopy gave the opportunity to investigate tiny detail of multiphase system morphology. In the second part of the thesis, a method to measure interfacial tension in microfluidic divergent flow of emulsion was shown. Lowering of the interfacial tension due to droplet confinement has been noted and taken into account by scaling droplet deformation parameter. Results are comparable with literature data only in the case of pure droplet interface, while in the case of interface covered or partially covered with surfactants, microfluidic technique is not able as pendant drop to evaluate properly interfacial tension, maybe due to the effects of interfacial Marangoni flow and wetting effect in confined droplet flow as well as the effect of droplet interface elasticity that are not taken into account in the microfluidic model. In the final part we developed a microfluidic apparatus capable to deal with multiphasic reactive flow for the synthesis of a valuable pharmaceutical arylamine. Continuous flow microreactors exhibit a large number of advantages compared to traditional batch and macroscale flow reactors, such as the significant enhancement of transport phenomena, the safety of operation, the precise control of residence time, the possibility of automation and the ease of scale-up by operating several devices in parallel. Thus, the choice of a microfluidic approach for chemical synthesis meets sustainable and green chemistry requirements in terms of productivity, process handling, economic savings and operational safety. The microreactor, coupled with a highly active Palladium-N-heterocyclic carbene (NHC) catalyst, enabled the full conversion of the reagents within twenty minutes, even at very low catalyst concentrations. The influence of the microreactor operating parameters on the synthetic performance has been investigated showing that a slight increase in temperature allows faster conversion even at low catalyst loadings.

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