Measuring this dependence requires the ability either to track a sphere's motion in three dimensions, or else to position the sphere reproducibly within the slit. We adopted the second approach, using an optical tweezer to place a single sphere in the microscope's focal plane, releasing it for sec to measure its diffusivity, and then retrapping it. Optical gradient forces exerted by the tightly converging beam localized the sphere near the focal point despite radiation pressure and random thermal forces. The galvanometer drive was synchronized to the video camera's sync signal to ensure that data acquisition began at a reproducible interval after the optical trap was extinguished. We adjusted the focal plane's height in 1.0 increments to sample the diffusivity's dependence on the sphere's height in the slit pore.
Solid curves result from least squares fits to Eq.
Shaded regions are forbidden by sphere-wall contact, given the fit estimates for the wall locations. We determined the absolute positions of the glass walls by observing the influence of a steady Poiseuille flow on the particle's motion. Typical examples at the midplane of the slit pore appear in Fig. The peaks' drifts, plotted in Fig. Fitting to Eq.