The bijel: a bicontinuous interfacially jammed emulsion gel E.M. Herzig, K.A. White, A.B. Schofield, W.C.K. Poon, M.E. Cates and P.S. Clegg* SUPA School of Physics, University of Edinburgh, Edinburgh, EH9 3JZ, United Kingdom * pclegg@ph.ed.ac.uk ABSTRACT The bijel is a new class of soft material comprised of two fluids arrested in a bicontinuous configuration on the mesoscale. We stabilize the fluid structure using an interfacial layer of silica particles. The particles be- come trapped at the liquid-liquid interface because they reduce the exposed area; this relies on the particles be- ing partially wetted by both liquids. We use water and 2,6-lutidine which have an accessible demixing transi- tion and the particles are silica with tuned wettability. Dispersing the particles in the mixed phase and then warming through the lower critical temperature into the demixed phase leads to our novel fluid-bicontinuous structures [1]. The two fluids of the bijel are in contact through the interstices in the particle monolayer. This, combined with the arrangement of domains, means that bijels are ideal for use as membrane contactors on the microfluidic scale. Keywords: particle-stabilized, Pickering emulsion, bi- continuous, gel, microfluidics 1 INTRODUCTION Emulsions can be stabilized by colloids and nanopar- ticles resulting in properties quite different to those of conventional emulsions [2]. Even for small nanoparticles the energy well is sufficiently deep that neutrally wetting particles are irreversibly trapped at the interface. Un- til recently all particle-stabilized emulsions created in- volved discrete droplets, although non-spherical droplets and bubbles can be made too [3]. A major new possibil- ity was suggested by large-scale computer simulations: a soft material (a bijel) might be formed comprised of a particle-stabilized emulsion with bicontinuous fluid do- mains [4]. The behavior of neutrally wetting particles dispersed in a binary fluid host were simulated following an instantaneous quench into the demixed region. As the two liquids phase separate the particles are swept up by the interface and, as the domains coarsened, the area fraction increases until the particles jam together. The final configuration of fluids and particles resembles the cartoon in Fig. 1. Experimentally we approach bijel creation by dis- persing neutrally wetting particles in a binary fluid [1]. Figure 1: A cartoon of the structure of a bijel. The two fluid domains are yellow and red respectively; the inter- face between them is coated by a monolayer of particles (which must exhibit partial wettability with the fluids). On quenching through the critical point the single-fluid phase will become unstable and the fluids undergo spin- odal decomposition. For a symmetric phase diagram this results in bicontinuous fluid domains; the size of these domains is characterized by a single length scale which grows with time. By contrast, shallow quenches through the binodal lead to phase separation via nucle- ation (droplets of the minority phase form). Discrete do- mains can also form via heterogeneous nucleation seeded by impurities and surfaces. To create the bicontinuous fluid domains we need a host fluid that undergoes spin- odal decomposition and we need this to take place in the presence of dispersed particles. To this end we use mix- tures of water and 2,6-lutidine with critical composition; the particles are fluorescent silica. 2 METHODS The water–2,6-lutidine system has a lower critical solution point (T c =34.1 ◦ C, x l =0.064); it demixes into phases with similar volumes and densities. The par- ticles are fluorescently tagged St¨ ober silica with a hy- NSTI-Nanotech 2008, www.nsti.org, ISBN 978-1-4200-8504-4 Vol. 2 657