Skin friction on oscillating interfacial bodies

The hydrodynamic drag a body experiences when moving along a fluid interface is well studied in literature for bodies of large length scales, but is relatively unexplored for bodies at the capillary scale. It has been previously shown that the drag on centimeter-sized disks sliding on water, referred to as “sliders”, is dominated by skin friction due to the Blasius boundary layer that forms in the fluid beneath the body [1]. Here, we investigate the drag experienced by sliders forced to oscillate along an air-water interface.  Scaling arguments suggest that in the unsteady limit (i.e. high Strouhal number) the boundary layer beneath the slider can be modeled as a Stokes (oscillatory) boundary layer.  We validate our model via experimental measurement by embedding a slider with a permanent magnet and externally forcing it with an oscillating magnetic field.  We extract the response amplitude and phase shift of the slider and compare it to what is predicted by our boundary layer drag model.

[1] G. Pucci, I. Ho, and D. M. Harris.  Friction on water sliders. Scientific Reports. 2019; 9 : 4095.