  Turbulent resuspension, a process of lifting solid particles from the bottom
by turbulent flow, is ubiquitous in environmental and industrial applications.
The process is a sequence of events that start with an incipient motion of the
particle being dislodged from its place, continue as sliding or rolling on the
surface, ending with the particle being detached from the surface and lifted up
into the flow. In this study the focus is on the resuspension of solid
spherical particles with the density comparable to that of the fluid and the
diameter comparable with the Kolmogorov length scale. We track their motion
during the lift-off events in an oscillating grid turbulent flow. We measure
simultaneously the Lagrangian trajectories of both the particles freely moving
along the bottom smooth wall and the surrounding flow tracers. Different force
terms acting on particles were estimated based on particle motion and local
flow parameters. The results show that: \emph{i}) the lift force is dominant;
\emph{ii}) drag force on freely moving particles is less relevant in this type
of resuspension; \emph{iii}) the Basset (history or viscous-unsteady) force is
a non-negligible component and plays an important role before the lift-off
event. Although we cannot estimate very accurately the magnitude of the force
terms, we find that during the resuspension they are within the range of
$2\div10$ times the buoyancy force magnitude. The findings cannot be
extrapolated to particles, which are much smaller than the Kolmogorov length
scale, or much denser than the fluid. Nevertheless, the present findings can
assist in modeling of the sediment transport, particle filtration, pneumatic
conveying and mixing in bio-reactors.
