  Rotational diffusion of colloidal spheres has been studied rarely, in spite
of its importance in the kinetics of many processes involving friction or
alignment of surface functionalities. While for smooth hard spheres, it has
been found previously that rotational diffusion gets impeded only weakly with
increasing volume fraction, the picture changes drastically when surface
roughness is introduced. We show this for a system of undeformable all-silica
colloidal raspberries, studied with time-resolved 3D Confocal Scanning Laser
Microscopy (CSLM). We find that the strong surface roughness leads to a
significantly lower maximum volume fraction, preceded by a broad concentration
range in which the rotational Brownian motion changes signature from
high-amplitude diffusive to low-amplitude rattling. This strong rotational
slowdown occurs at significantly higher concentrations than for the
translations, thus presenting a second glass transition. In the regime where
the Mean Squared Angular Displacement (MSAD) is subdiffusive, significant
correlations are found between the translational and rotational motions. The
drastic rotational slowdown occurs when the particle intersurface distance
becomes comparable to the surface roughness amplitude. Concurrently, the number
of contacts exceeds a critical value. This picture is supported by observations
in a densely packed layer of raspberries at a smooth wall: here significant
rotational motions are found while the translations are almost completely
frozen.
