  Geometric confinement strongly influences the behavior of microparticles in
liquid environments. However, to date, nonspherical particle behaviors close to
confining boundaries, even as simple as planar walls, remain largely
unexplored. Here, we measure the height distribution and orientation of
colloidal dumbbells above walls by means of digital in-line holographic
microscopy. We find that while larger dumbbells are oriented almost parallel to
the wall, smaller dumbbells of the same material are surprisingly oriented at
preferred angles. We determine the total height-dependent force acting on the
dumbbells by considering gravitational effects and electrostatic particle-wall
interactions. Our modeling reveals that at specific heights both net forces and
torques on the dumbbells are simultaneously below the thermal force and energy,
respectively, which makes the observed orientations possible. Our results
highlight the rich near-wall dynamics of nonspherical particles, and can
further contribute to the development of quantitative frameworks for
arbitrarily-shaped microparticle dynamics in confinement.
