  Sand is a highly dissipative system, where the local spatial arrangements and
densities depend strongly on the applied forces, resulting in fluid-like or
solid-like behaviour. This makes sand swimming challenging and intriguing,
raising questions about the nature of the motion and how to optimize the design
of artificial swimmers able to swim in sand. Recent experiments suggest that
lateral undulatory motion enables efficient locomotion, with a non-monotonic
dependence of the swimming speed on the undulatory frequency and the height of
the sediment bed. Here, we propose a quasi-2D granular model, where the effect
of the bed height is modeled by a coarse-grained frictional force with the
substrate. We show that the optimal frequency coincides with the second
vibrational mode of the swimmer and explain the underlying mechanism through a
characterization of the rheology of the medium. Potential implications in the
design of artificial swimmers are discussed.
