Microscopic Characterization of Phase Separation in Mixed Carrageenan Gels Using Particle Tracking

The gelation mechanism and network structure of mixed κ-carrageenan (KC) and ι-carrageenan (IC) were investigated using particle tracking. The random motion of fluorescent particles (diameter = 100 nm) was utilized to probe local circumstances of the gelling carrageenan at the sub-micron scale. The mean square displacement (MSD) of particles was used to characterize the changes in the network structure on cooling and storage for 1 day. The individual MSD of particles in the mixture showed a large distribution after 1-day storage, indicating the emergence of microstructural heterogeneity in the gel. This heterogeneity was attributed to the frozen structure on the way to phase-separated network structure. The van Hove correlation plots suggested the presence of two groups of particles with fast and slow mobilities in the mixture. Plotting each MSD vs α with the MSD scaled as tα and t the lag time suggested a bimodal distribution with fast and slow particles. The presence of two groups of particles with different mobilities suggested that mixtures of KC and IC was frozen on the way to a phase-separated network structures made of KC-rich and IC-rich domains with a size of >100 nm due to the network formation of KC and IC chains.