Role of capillarity and microstructure on interfacial viscoelasticity of particle laden interfaces

The correlation between interfacial interparticle forces, microstructure, and rheology of particle laden interfaces is characterized for a system of polystyrene particles at an air/water interface by using both aqueous salt concentration and particle surface concentration as control parameters. Characterizing linear, interfacial shear viscoelastic moduli and microstructure simultaneously using a custom imaging system attached to a double wall ring interfacial rheometer allows the relationship between these properties to be evaluated. Interfacial viscoelastic moduli magnitude is found to be determined locally by the degree of restricted particle motion, which is dictated to both by interparticle attraction due to capillarity and severity of caging caused by local microstructure. However, macroscale rheological behaviors, such as elasticity and yield, are tied to the mesostructural organization of local microstructure. Large domains of aligned hexagonal packed particles create elastic interfaces; when these domains begin to breakup, and smaller domains can flow on the interface, a transition to viscous like behavior is observed.