SAW-driven motion

A small droplet of water on a SLIPS has a completely mobile contact line, and will readily roll off when the surface is tilted at even a small angle. We have also shown that a droplet on a SLIPS is easily transported on a horizontal surface by impacting it with a Rayleigh Surface Acoustic Wave (SAW). The experimental arrangement is shown schematically below, although the thicknesses of the substrate, ZnO and oil layers are drawn at an exaggerated scale compared to the droplet.

Schematic diagram of droplet on SLIPS surface impacted by a SAW
Schematic illustration of a droplet on a SLIPS impacted by a SAW

The image below shows three frames from the motion of a droplet on a SLIPS that has been impacted by a SAW from the left. We can clearly see that the drop has moved to the right.

Droplet on a SLIPS impacted by a SAW

The motion of a droplet impacted by a SAW can also be seen in the video below.

When we impacted a similar drop on a superhydrophobic surface with a SAW, the drop was not transported, as seen in the sequence below.

Droplet on a superhydrophobic surface impacted by a SAW

The shape of the drop changes because the SAW perturbs the surface such that it moves both horizontally and vertically. This results in the drop penetrating into the “nails” of the superhydrophobic surface, as seen in the final image of the sequence.

Publication

Slippery Liquid-Infused Porous Surfaces and droplet transportation by Surface Acoustic Waves J.T. Luo, N.R. Geraldi, J.H. Guan, G. McHale, G.G. Wells and Y.Q. Fu, Physical Review Applied 7 (2017) 014017.