  We study the correlated motions 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.
