  The motion of small, spherical particles of finite size in fluid flows at low
Reynolds numbers is described by the strongly nonlinear Maxey-Riley equations.
Due to the Stokes drag the particle motion is dissipative, giving rise to the
possibility of attractors in phase space. We investigate the case of an
infinite, cellular flow field with time-periodic forcing. The dynamics of this
system are studied in a part of the parameter space. We focus particularly on
the size of the particles whose variations are most important in active,
physical processes, for example for aggregation and fragmentation of particles.
Depending on their size the particles will settle on different attractors in
phase space in the long term limit, corresponding to periodic, quasiperiodic or
chaotic motion. One of the invariant sets that can be observed in a large part
of this parameter region is a quasiperiodic motion in form of a torus. We
identify some of the bifurcations that these tori undergo, as particle size and
mass ratio relative to the fluid are varied. This way we provide a physical
example for sub- and supercritical pitchfork bifurcations of tori.
