Undulatory intruder dynamics in water-saturated granular beds*

We will report an investigation of active intruder dynamics through complex media with {\texit Lumbriculus variegatus} moving through water saturated granular beds which are refractive index matched to enable observation of their motion inside the bed. These common marsh and pond dwelling worms display dual longitudinal peristaltic and transverse undulatory strokes. We will demonstrate that only the undulatory stroke plays a role in locomotion while swimming in water and in very shallow sediment layers which do not permit anchoring. The greater drag anisotropy of the rod like body in the sediments is shown to enable faster locomotion speeds compared to water because of drag-assisted propulsion. By constructing a magnetically driven soft filament robot, we will then describe the observed speeds as a function of undulatory strokes under prescribed conditions which include the undulation frequency and amplitude and the rheology of the medium.