"Two-particle microrheology of quasi-2D viscous systems"
Vikram Prasad, Stephan A. Koehler, and Eric R. Weeks, Phys. Rev. Lett 97, 176001 (2006).

We study the correlated motion of colloidal particles in a quasi-2D system (Human Serum Albumin (HSA) 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 2-D 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. From the flow field and the correlated particle motions, we calculate a two-particle MSD (mean square displacement) for direct comparison with rheological measurements.