Two-Particle Microrheology of Quasi-2D Viscous Systems

We study the spatially correlated motions of colloidal particles in a quasi-2D system (human serum albumin protein molecules at an air-water interface) for different surface viscosities η_s. We observe a transition in the behavior of the correlated motion, from 2D interface dominated at high η_s to bulk fluid dependent at low η_s. The correlated motions can be scaled onto a master curve which captures the features of this transition. This master curve also characterizes the spatial dependence of the flow field of a viscous interface in response to a force. The scale factors used for the master curve allow for the calculation of the surface viscosity η_s that can be compared to one-particle measurements.