(720c) Quantifying Diffusion across a Particle-Laden Interface

After the pioneering demonstration that catalytic nanoparticles can be localized at the interface of emulsion droplets, research in Pickering emulsions for catalysis, especially in continuous flow reactors, is now a rapidly expanding area. By positioning the catalyst either on the surface of the solid particles at the interface, or within the emulsion droplets, the reaction takes place at the interface and products are immediately removed from the reaction site, leading to high conversion and selectivity. The overall performance of such applications highly depends on the mass transfer of reactants/products across the interfacial layer of particles, which in turn depends on the surface properties of particles, surface coverage of the interface by the particles, and droplet sizes. Theoretically, the catalysis efficiency reaches its maximum when the reactant diffusion rate equals to the catalytic reaction rate, which corresponds to a critical droplet radius. However, so far this critical droplet radius has only been determined by trial-and-error, via experimental screening and empirical estimation. The fundamental bottleneck in predicting the optimal reaction conditions is due to our limited understanding of diffusion across a particle-laden interface. To fill this gap, we quantify the diffusive transport of a fluorescent dye across a particle-laden oil-water interface, in order to establish the relation between the permeability of the interfacial layer of particles, and its microstructure and properties. The mass transfer of Rhodamine B through the interface of a single, quasi-2D, particle-laden droplet is visualized in a Hele-Shaw geometry, where the concentration of transferred Rhodamine B is quantified according to its fluorescent intensity. The configuration of silica particles on the droplet surface is tuned by the wettability of the particles (modified via silanization), particle size (tens of nm to microns), and surface coverage. By comparing with bare droplets, we link the microscopic properties of the particles/droplets and their dynamics, to the performance of the mass transfer. Such fundamental insights will ultimately contribute to control the performance of Pickering-emulsion catalysis, paving the way towards optimized handling of complex, multiphase systems in continuous-flow Pickering emulsion.