Abstract
A class of reptiles known as sand swimmers adapts to hot environments by
submerging beneath desert sands during the day and so provide a unique
probe into the dynamics of intruders in granular beds. To understand the
mechanism for swimming in an otherwise solid bed, we study a simple
model of periodic contraction and extension of large intruders in a
granular bed. Using an event-driven simulation, we find optimal
conditions that idealized swimmers must use to critically fluidize a
sand bed so that it is rigid enough to support a load when needed, but
fluid enough to permit motion with minimal resistance. Swimmers-or other
intruders-that agitate the bed too rapidly produce large voids that
prevent traction from being achieved, while swimmers that move too
slowly cannot travel before the bed resolidifies around them, i.e., the
swimmers locally probe the fundamental time scale in a granular packing.
