  We present a deformation-dependent propulsion phenomenon for soft particles
such as cells in microchannels. It is based on a broken time reversal symmetry
generated by a fast forward and slow backward motion of a fluid which does not
progress on average. In both sections, soft particles deform differently and
thus progress relatively to the liquid. We demonstrate this by using
Lattice-Boltzmann simulations of ubiquitous red blood cells in microchannels,
as well as simulations for capsules and minimal soft tissue models in unbounded
Poiseuille flows. The propulsion of the soft particles depends besides the
oscillation asymmetry on their size, deformation type and elasticity. This is
also demonstrated by analytical calculations for a minimal model. Our findings
may stimulate a rethinking of particle sorting methods. For example, healthy
and malignant cells often differ in their elasticity. With the proposed method,
several cell types with different deformability can be separated simultaneously
without labeling or obstacles in a microfluidic device.
