Self-ratcheting Stokes drops driven by oblique vibrations

We develop and analyze a minimal hydrodynamic model in the overdamped limit to understand why a drop climbs a smooth homogeneous incline that is harmonically vibrated at an angle different from the substrate normal [Brunet, Eggers and Deegan, Phys. Rev. Lett. 99, 144501 (2007)]. We find that the vibration component orthogonal to the substrate induces a nonlinear (anharmonic) response in the drop shape. This results in an asymmetric response of the drop to the parallel vibration and, in consequence, in the observed net motion. Beside establishing the basic mechanism, we identify scaling laws valid in a broad frequency range and a flow reversal at high frequencies.