Phase Separation and Gelation of Biopolymers in Confined Geometries

Doctoral thesis, 2012

Many biopolymer mixtures exhibit segregative phase separation that generates regions enriched in one polymer and depleted in the other. Today, much is known of how phase separating biopolymer systems behaves in bulk phase, and the final morphology can be controlled and tailored with high precision. However, few experimental studies have examined such systems in restricted geometries, where the system might be affected and limited by surrounding surfaces. Restricted geometries may be of importance for the properties of multi-phase materials, such as foods and composite materials. In this thesis the effects of confinements on phase separating and gelling biopolymer mixtures has been investigated, with emphasis on the phase separation kinetics and the final morphology. Two biopolymer systems were used; gelatin–maltodextrin and whey protein isolate–gellan gum mixtures. Various types of restricted geometries such as microdroplets, parallel cover glasses, and networks of cellulose fibres were used to evaluate the effects of confinements. The structural evolution and the final microstructures were characterized by confocal laser scanning microscopy and image analysis. The findings showed that the size of a confinement had a marked impact on the resulting microstructure in both systems. The morphology observed within the studied confinements differed from those seen in the bulk phase and included core-shell, Janus-like microstructures in microdroplets and columnar structures in confinements with solid surfaces. In the whey protein isolate–gellan gum system, the structural evolution was similar inside large microdroplets and the bulk phase. It was also found that when the characteristic wavelength of the spinodal decomposition was comparable to the size of the confinement, a structure transition took place and core-shell structures were obtained. Furthermore, the confinement size influenced the initiation of the phase separation in the gelatin–maltodextrin system. Use of conventional emulsification and a microfluidic technique to produce microdroplets was also evaluated. The former method allowed examination of microstructures in emulsion droplets of different sizes within a single sample, ensuring the same composition and thermal history. Monodisperse microdroplets with highly reproducible internal morphology were generated using microfluidics. The internal microstructure was designed using different cooling protocols to control the phase separation and gelation kinetics, and the biopolymer concentration. Homogeneous (no phase separation), discontinuous, and bicontinuous microstructures were observed. The effects of confinement on the internal morphology were investigated by performing elastic Lennard-Jones simulations, which showed good correlation with the experimental structures. Initial studies of the relationship between the microstructure and the diffusion properties of phase separated gels provided promising results, which opens new possibilities to control such properties in microdroplets through careful design of the internal morphology. To summarize, the work underlying this thesis has demonstrated that exploitation of phase separated systems within confined geometries offers great potential to tailor materials with new functionalities.