  We observe tracer particles diffusing in soap films to measure the
two-dimensional (2D) viscous properties of the films. We make soap films with a
variety of water-glycerol mixtures and of differing thicknesses. The
single-particle diffusivity relates closely to parameters of the film (such as
thickness $h$) for thin films, but the relation breaks down for thicker films.
Notably, the diffusivity is faster than expected for thicker films, with the
transition at $h/d = 5.2 \pm 0.9$ using the tracer particle diameter $d$. This
indicates a transition from purely 2D diffusion to diffusion that is more
three-dimensional. Additionally, we measure larger length scale flow fields
from correlated particle motions and find good agreement with what is expected
from theory of 2D fluids for all our films, thin and thick. We measure the
effective 2D viscosity of a soap film using single-particle diffusivity
measurements in thin films, and using the two-particle correlation measurements
in all films.
