The effect of surface features on friction forces in granular media

Biological surfaces are covered with diverse features and textures whose functions are often unknown. In some cases, however, progress has been made; for instance, structures that serve as interface with underlying substrates, e.g., setae on gecko feet and microscopic spikes found on snake belly skins, are likely important for movement. Here, inspired by textures present on snake bellies, we investigate how textures affect frictional forces. Using 3D printing, we created and systematically varied texture feature sizes (from tens of microns to one centimeter), aspect ratios, and orientations printed on square plates that are then dragged (at constant speed) through granular material as a function of angle of attack. We find that textured surfaces do affect the magnitude and direction-dependence of frictional drag forces, with differences depend on the details and orientation of the textured pattern. Our results provide a starting point for understanding how differing animal surface textures may alter locomotive capabilities in order to adapt to a changing environment.