Figure 7. Frequency dependence of the force necessary to move a round body cyclical through sand.

A round piece of aluminium was moved using of a home-built apparatus (see supplementary material S5) 100 mm underneath the sand surface in a sinusoidal path with an amplitude of 60 mm. Because the current for the linear motor was controlled to perform the desired movement, the current consumption was a direct measure for the force the motor has to overcome. The minimum of the required current can be seen at about 3 Hz.