Capillarity and convection-controlled assembly during the spreading of particulate suspensions on an air-liquid interface

Recent literature has seen an explosion of studies on the spreading of fluids on an air-liquid interface; however, none have examined the spreading characteristics of two-phase fluid materials. In this work, a drop of a concentrated suspension of PMMA particles in silicone oil was placed on an aqueous glycerol solution – air interface. Depending on the initial rate of spreading of the suspension, two outcomes were observed: the particles were either swept away by the spreading suspension, or organized into an array of two-dimensional networks. Increasing the particle size, particle volume fraction, substrate viscosity and salinity were found to promote network formation. The observations can be explained as a balance between the spreading-induced hydrodynamic forces that separate the particles from each other, and capillary attraction forces which lead to particle aggregation on the interface. These results have potential applications in printing and coating technologies, and in biology.