Meeting Abstract
Running up a sand dune is a challenging task due to several factors. First, sand fluidizes when an external force exceeding the material yield stress is applied. Second, at the angle of repose, the sand pile is unstable, such that small perturbations will cause fluidization. Ongoing studies in the lab show that sand specialist lizards exhibit lower running speed decrements than desert generalist lizards when running up inclined sand. Preliminary evidence indicates that impact angles of the foot relative to the sand differ between sand generalists and specialists, suggesting that differences in foot and leg movement can have dramatic effects on force generation. In this study, we experimentally examine the impact force normal to a flat plate against a bed of poppy seeds (~1 mm diameter). We test a range of impact speeds (0.01-1.2 m/s), substrate angles (0-40 degrees from the horizontal), and impact angles (0-40 degrees from gravity). When comparing the magnitude of force acting normal to the plate at varying substrate and impact angles, two regimes become apparent: 1) the gravity regime and 2) the inertial regime. In the gravity regime, the weight of particles dominates granular motion and causes fluidization of the substrate which correlates to divergences in the force depth relation when comparing force responses between substrate angles and intrusion angles. However, in the inertial regime, particle motion is dominated by the inertia of the intruder, which results in the force depth relation to converge between substrate and intrusion angle. Based on these results, we discuss the implications for the efficacy of lizards running up sand dunes.
