Insights into Formation of Bicontinuous Emulsion Gels via In Situ (Ultra-)Small Angle X-Ray Scattering

Nanostructured materials formed via kinetically controlled self-assembly processes gather more interest nowadays. In particular, bicontinuous emulsion gels stabilized by colloidal particles, called bijels, are attractive materials as they combine bulk properties of two immiscible liquids into an interwoven network structure. The limited understanding of the complex formation phenomena of bijels restricts the control over the synthesis, and so its applicability. In this work, in situ (ultra-) small-angle X-ray scattering is applied to gain insight into the phase separation and self-assembly kinetics of bijels formed via solvent transfer induced phase separation. An X-ray compatible microfluidic setup allows accessing the kinetics of the extrusion process with a millisecond resolution. The formation of such bijels out of a liquid precursor mixture is shown to occur via three consecutive steps. The first 7 ms of the extrusion are dominated by fluid dynamics. Then, the precursor mixture remains in an induction phase for 50 ms where nanoparticles start to self-assemble without structural development on the (sub)micron scale. From 50 ms on, an inward propagation of a liquid-liquid phase separation front occurs, besides the proceeding nanoparticle self-assembly obtaining (sub)micron-sized structures. This time-resolved monitoring technique offers valuable insights into the structural evolution of kinetically controlled materials and enhances our understanding of the formation of bicontinuous emulsion gels.