Liquefying the granular material: how limbless sand dollars achieve self-reorienting feat  

Locomotion on granular material poses a significant challenge to many organisms because this substrate can experience solid-fluid transition under the stress during locomotion through which energy may be dissipated. Many organisms like desert-dwelling lizard and snakes have developed strategies to effectively move on sandy substrates. As limbless echinoids with rigid skeletons with limited agility, sand dollars utilize the solid-liquid transition property of granular material to achieve self-righting and burrowing tasks. Our observations reveal that sand dollars address this challenge using their dense arrays of short, rigid spines to locally "liquefy" surrounding granular material. Through coordinated spine motion, sand dollars exploit the solid-liquid transition property of granular material to manipulate sand, eventually achieve self-reorientation and burrowing. To investigate how sand dollars effectively manipulate granular materials with the spine array, we combine biological observations with a robophysical model to quantify the kinetics of self-reorientation, spine–granular interactions, and the dependence of reorientation efficiency on structural parameters such as spine length, spacing, and actuation frequency. This study not only illuminates the granular manipulation strategies of a limbless organism but also offers design principles for limbless robots capable of locomotion and self-righting in yielding terrains and gives insights into how scale matters in different sizes of organisms when they interact with granular materials.