  We follow the diffusive motion of colloidal particles in soap films with
varying $h/d$, where $h$ is the thickness of the film and $d$ the diameter of
the particles. The hydrodynamics of these films are determined by looking at
the correlated motion of pairs of particles as a function of separation $R$.
The Trapeznikov approximation [A. A. Trapeznikov, \emph{PICSA} (1957)] is used
to model soap films as an effective interface in contact with bulk air phases,
that behaves as a 2D fluid. The flow fields determined from correlated particle
motions show excellent agreement with what is expected for the theory of 2D
fluids for all our films where $0.6 \leq h/d \leq 14.3$, with the surface
viscosity matching that predicted by Trapeznikov. However, for thicker films
with $h/d > 7 \pm 3$, single particle motion is faster than expected.
Additionally, while the flow fields still match those expected for 2D fluids,
the parameters of these flow fields change markedly for thick films. Our
results indicate a transition from 2D to 3D fluid-like behavior occurs at this
value of $h/d$.
