(243i) Micro and Macrorheological Methods At Fluid-Fluid Interfaces

Microrheological methods have been developed to probe the rheological properties of fluid-fluid interfaces, but orders-of-magnitude differences in interfacial properties measured with micro and macrorheological techniques have been reported in the literature. In bulk fluids, microrheological techniques have successfully been developed around the generalized Stokes-Einstein relation (GSER) and can be expected to agree with macroscopic measurements under conditions where the GSER is valid. For microrheological measurements at interfaces, the assumptions of GSER are not generally met due to the multi-component, multi-length scale environment of fluid-fluid interfaces. Instead, hydrodynamic models that relate the motion of a particle at an interface to the viscosity of the interface and the surrounding bulk fluids have been used to extract interfacial shear viscosities from particle-tracking experiments. Currently, models account for particle intrusion into the bulk phase, the effects of particle shape and the influence of Marangoni stresses. Notable omissions from these models are the effects of interface heterogeneity and electrocapillarity. We have investigated the orders-of-magnitude discrepancy between microrheological and macrorheological results at fluid-fluid interfaces and will discuss experimental results using both techniques on various interfaces.