Active intruder dynamics in granular suspensions

We discuss the dynamics of intruders in the form of solid rods, anguilliform robots, and limbless worms moving through sedimented granular medium based on experiments and rheology models coupled with resistive force theory. Index-matching techniques are used to directly visualize the motion of the intruder and the flow of the medium to diagonize the rheology of the medium. Friction, inertia and viscous forces are demonstrated to play important roles in determining the interaction of the medium depending on the intruder speed and depth, and the physical properties of the granular medium. We show that inertial and viscous numbers, first introduced in the context of steady linearly sheared medium, can be extended to unsteady flows as is the case in intruder dynamics by considering the shear rate given by the ratio of the intruder speed and size to describe the effective friction encountered by the intruder [1,2]. The effect of greater drag anisotropy observed in a granular medium is found to lead to greater propulsive force in active intruders and greater locomotion speeds compared to Newtonian mediums. The unique rheology of granular suspensions is then demonstrated to enable novel dual stroke biolocomotion [3].

[1] Micromechanics of intruder motion in wet granular medium, Rausan Jewel, Andreea Panaitescu, and Arshad Kudrolli, Phys. Rev. Fluids, 3, 084303 (2018).

[2] Effective drag of a rod in fluid-saturated granular beds, Benjamin Allen and Arshad Kudrolli, Phys. Rev. E 100, 022901 (2019).

[3] Burrowing dynamics of aquatic worms in soft sediments, Arshad Kudrolli and Bernny Ramirez, PNAS 116, 25569 (2019).