Diffusion from particle-coated drops: the subtle role of particle size

Many natural and industrial systems involve particle-laden interfaces. Because interfacial particles prevent the coalescence and coarsening of drops, they hold promise for various applications requiring stable emulsions. Despite their remarkable ability to stabilize emulsions, it remains challenging to characterize how particles influence the interfacial transport of dissolved solutes. Here, we quantify the diffusion from a single particle-coated drop by confining it to a two-dimensional configuration. Using fluorescence microscopy, we extract the intensity profiles of the fluorescent dye as it diffuses from the drop, yielding spatio-temporal measurements of the concentration field. Over a range of particle sizes, the particles impose minimal resistance to diffusion. We rationalize this counterintuitive result with a mathematical model that couples interfacial mass transfer to a particle-coated interface. We show that the particle monolayer controls the temporal dynamics of the flux across the interface, hindering transport only at extreme coverage fractions beyond the close-packing limit. This framework reveals why particles often fail to hinder diffusion, offering new pathways to harness mass transfer in particle-stabilized emulsions.