DIRECT IMAGING OF NANOSTRUCTURE DEVELOPMENT IN BICONTINUOUS INTERFACIALLY JAMMED EMULSION
Bicontinuous interfacially jammed emulsion gels, or bijels, are nonequilibrium structures formed by jamming of colloidal solid particles at the interface between two partially miscible fluids undergoing spinodal decomposition. This natural processes can serve as a robust platform for the assembly of bicontinuous porous composite materials with tunable domain sizes across the nano- and micrometer scales. Theoretically predicted in 2005 and practically prepared for the first time in 2007, this new class of structured gels encouraged researchers in many fields to cope with fundamental and practical questions.
One example of a system, which could be jammed by silica nanoparticles is a well-studied water/lutidine (W/L) mixture. These two miscible liquids undergo spinodal decomposition at critical composition, above 34.1 oC. In the presence of silica nanoparticles (NP) this system forms bijel.
In order to understand bijel stabilization mechanism, we followed morphology changes in W/L solutions, stabilized by silica/alumina nanoparticles during bijel formation. Cryogenic scanning electron microscopy (cryo-SEM) served a main experimental tool to capture different states of mixture self-assembly in its native state. For our best knowledge, this is the first time, while bijels are studied at the nanometric scale.
According to a water/lutidine phase diagram, two chemicals are completely miscible at any composition below the lower critical solution temperature (LCST) and optically transparent. However, NP addition alters dramatically mixture viscosity and optical properties, while these differences are reflected in W/L/NP system morphology. We investigated structural changes by direct imaging of compositions with different W/L ratio below and above the LCST, while NP content is fixed to be constant. Our results are consistent with those of bulk measurement techniques, such as rheology and optical transmittance of the same mixtures.
In many application, removal of liquids is required. To study both, the final structure of dry material and the liquid removal process itself, studied bijels were subjected to freeze-drying inside the microscope while capturing liquid removal after different periods of time. Low temperature (around -100oC) and high vacuum (around 10-6mBar) in the microscope allow gentle liquid removal, preventing NP collapse and allowing in-situ capturing of final solid NP arrangement. After complete removal of liquids, the remained nanoparticles form micropores, separated by dense silica nanoporous aggregates. The combination between cryo-SEM of bijels and HR-SEM of corresponding solids, we studied how thickness and density of NP scaffold and dimensions of micropores can be tuned by varying of W/L ratio and of NP content only, without any additional treatment.