  Three-dimensional particle tracking experiments were conducted in a turbulent
boundary layer with friction Reynolds number $Re_\tau$ of 700 and 1300. Two
finite size spheres with specific gravities of 1.003 (P1) and 1.050 (P2) and
diameters of 60 and 120 wall units were released individually from rest on a
smooth wall. The spheres were marked with dots all over the surface to monitor
their translation and rotation via high-speed stereoscopic imaging. The spheres
accelerated strongly after release over streamwise distances of one boundary
layer thickness before approaching an approximate terminal velocity. Initially,
sphere P1, which had Reynolds numbers $Re_p$ of 800 and 1900, always lifts off
from the wall. Similar behavior was observed occasionally for sphere P2 with
initial $Re_p$ of 1900. The spheres that lifted off reached an initial peak in
height before descending towards the wall. The sphere trajectories exhibited
multiple behaviors including saltation, resuspension and sliding motion with
small random bouncing depending on both $Re_\tau$ and specific gravity. The
lighter sphere at $Re_\tau=1300$, which remained suspended above the wall
during most of its trajectory, propagated with the fastest streamwise velocity.
By contrast, the denser sphere at $Re_\tau=700$, which mostly slid along the
wall, propagated with the slowest streamwise velocity. After the spheres
approached an approximate terminal velocity, many experienced additional
lift-off events that were hypothesized to be driven by hairpins or coherent
flow structures. Spheres were observed to rotate about all three coordinate
axes. While the mean shear may induce a rotation about the spanwise axis,
near-wall coherent structures and the sphere's wake might drive the streamwise
and wall-normal rotations. In all cases where the sphere propagates along the
wall, sliding motion, rather than forward rolling motion, is dominant.
