  We study the motion of colloidal particles driven by a constant force over a
periodic optical potential energy landscape. Firstly, the average particle
velocity is found as a function of the driving velocity and the wavelength of
the optical potential energy landscape. The relationship between average
particle velocity and driving velocity is found to be well described by a
theoretical model treating the landscape as sinusoidal, but only at small trap
spacings. At larger trap spacings, a non-sinusoidal model for the landscape
must be used. Subsequently, the critical velocity required for a particle to
move across the landscape is determined as a function of the wavelength of the
landscape. Finally, the velocity of a particle driven at a velocity far
exceeding the critical driving velocity is examined. Both of these results are
again well described by the two theoretical routes, for small and large trap
spacings respectively. Brownian motion is found to have a significant effect on
the critical driving velocity, but a negligible effect when the driving
velocity is high.
