Fabrication of functional and structured particles by inkjet printing of emulsions

Abstract

Emulsion solvent evaporation is a well-established method for generating microparticles from solutions of polymers in volatile organic solvents dispersed in an aqueous medium. Previous work has shown that this approach can also be used to deposit particles by inkjet printing where the particles are formed during the drying of a liquid ink on a substrate. The particle size distribution, however, was very broad. Here I demonstrate that inkjet printing of oil-in-water emulsions produced by microfluidics can generate micron-sized particles with a narrow size distribution (coefficient of variation <6%) and that these particles can self-assemble into ordered arrays with hexagonal packing. The conditions under which drops can be printed with a minimum of break up and coalescence of the oil droplets in the emulsion are explored. Factors affecting the size of the particles and the morphology of the deposit are described. This study uses polystyrene in dichloromethane as a model system, but the approach can be generalized to the production of structured and functional particles.
Besides, I explore the mechanism of formation of core-shell microcapsules by emulsion solvent evaporation, where the emulsion contains a shell-forming polymer and a core-forming poor solvent dispersed in a good solvent as the oil phase. Evaporation of the good solvent induces an internal phase separation within the oil droplets of the emulsion, triggering the formation of microcapsules. First I study the wetting conditions necessary to induce the internal phase separation within emulsions. Then I construct a ternary phase diagram to predict the formation process, either polymer-rich phase or poor solvent-rich phase can phase separate from the bulk solution, depending on the ratio of polymer to poor solvent. Then the physical parameters affecting the phase separation and hence the final morphology are explored. Finally, the conditions are optimised to minimise the effect of pseudo partial wetting to obtain a uniform morphology of core-shell microcapsules.
Last, I demonstrate the strategy of emulsion solvent evaporation can be used in the generation of hierarchical Murray materials with multi-scale interconnected pores by inkjet printing nanoparticle-containing emulsions, where silica nanoparticles are pre-dispersed in the discrete phase. A continuous monolayer of well-defined, hexagonally packed silica microspheres with hierarchical macro-meso porous structure is obtained. The porosity of the Murray material can be readily tuned by varying the size of the primary silica nanoparticles. Factors affecting the morphology of the deposit pattern have been explored. This study uses solid silica nanoparticles in discrete phase as a model system to produce macro-meso porous material with two-scale interconnected pores, but the strategy can be generalized to the porous nanoparticles for the production of more structured Murray material with multi-scale pores, like macro-meso-micro hierarchical porous materials.