Quasi-two-dimensional diffusion of single ellipsoids: Aspect ratio and confinement effects

We report on video-microscopy measurements of the translational and rotational Brownian motions of isolated ellipsoidal particles in quasi-two-dimensional sample cells of increasing thickness. The long-time diffusion coefficients were measured along the long (D-a) and short (D-b) ellipsoid axes, respectively, and the ratio, D-a/D-b, was determined as a function of wall confinement and particle aspect ratio. In three dimensions (3D), this ratio (D-a/D-b) cannot be larger than 2, but in quasi-two dimensions, wall confinement was found to substantially alter diffusion anisotropy and substantially slow particle diffusion along the short axis compared to 3D.