  A thermal colloid suspended in a liquid will transition from a short time
ballistic motion to a long time diffusive motion. However, the transition
between ballistic and diffusive motion is highly dependent on the properties
and structure of the particular liquid. We directly observe a free floating
tracer particle's ballistic motion and its transition to the long time regime
in both a Newtonian fluid and a viscoelastic Maxwell fluid. We examine the
motion of the free particle in a Newtonian fluid and demonstrate a high degree
of agreement with the accepted Clercx-Schram model for motion in a dense fluid.
Measurements of the functional form of the ballistic-to-diffusive transition
provide direct measurements of the temperature, viscosity, and tracer radius.
We likewise measure the motion in a viscoelastic Maxwell fluid and find a
significant disagreement between the theoretical asymptotic behavior and our
measured values of the microscopic properties of the fluid. We observe a
greatly increased effective mass for a freely moving particle and a decreased
plateau modulus.
