  We investigate the Brownian motion of boomerang colloidal particles confined
between two glass plates. Our experimental observations show that the mean
displacements are biased towards the center of hydrodynamic stress (CoH), and
that the mean-square displacements exhibit a crossover from short time faster
to long time slower diffusion with the short-time diffusion coefficients
dependent on the points used for tracking. A model based on Langevin theory
elucidates that these behaviors are ascribed to a superposition of two
diffusive modes: the ellipsoidal motion of the CoH and the rotational motion of
the tracking point with respect to the CoH.
