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 eta(s). We observe a transition in the behavior of the correlated motion, from 2D interface dominated at high eta(s) to bulk fluid dependent at low eta(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 eta(s) that can be compared to one-particle measurements.