Surface instabilities generated by a slider pulled across a granular bed

We report an instability of a slider slowly dragged at the surface of a granular bed in a quasistatic regime. The boat-shaped slider sits on the granular medium under its own weight and is free to translate vertically and to rotate around the pitch axis while a constant horizontal speed is imposed. For a wide range of parameters (mass, length, shape, velocity) a regular pattern of peaks and troughs spontaneously emerges as the slider travels forward. This instability is studied through experiments using a conveyor belt and by means of two-dimensional discrete elements method simulations. We show that the wavelength and amplitude of the pattern scale as the length of the slider. We also observe that the ripples disappear for low and high masses, indicating an optimal confining pressure. The effect of the shape, more specifically the inclination of the front spatula, is studied and found to drastically influence both the wavelength and the amplitude. Finally, we show that the mechanical details (friction, cohesion) of the contact point between the slider and the pulling device is critical and remains to be fully understood.