  We present here the measurement of the diffusivity of spherical particles
closely confined by narrow microchannels. Our experiments yield a
two-dimensional map of the position-dependent diffusion coefficients parallel
and perpendicular to the channel axis with a resolution down to 129 nm. The
diffusivity was measured simultaneously in the channel interior, the bulk
reservoirs, as well as the channel entrance region. In the channel interior we
found strongly anisotropic diffusion. While the perpendicular diffusion
coefficient close to the confining walls decreased down to approximately 25% of
the value on the channel axis, the parallel diffusion coefficient remained
constant throughout the entire channel width. In addition to the experiment, we
performed finite element simulations for the diffusivity in the channel
interior and found good agreement with the measurements. Our results reveal the
distinctive influence of strong confinement on Brownian motion, which is of
significance to microfluidics as well as quantitative models of facilitated
membrane transport.
