Controlling vibrating droplets

Texture ratchets - microstructured surfaces - can be used to move and direct droplet motion using vibration. This could lead to low cost lab-on-a-chip applications. Instead of using pumps and valves, all you need is to vibrate the surface at the resonant frequency of the drops you want to move.

Karl Bohringer’s group from the University of Washington published an investigation of this phenomenon in Advanced Materials (DOI:10.1002/adma.201104446), showing that individual drops can be controlled and directed, even when several different drops are on the surface at the same time.

As illustrated in the picture, the texture ratchets can also be used for upside-down transport, and yet the surface is simply made from polydimethylsiloxane that has been textured with a track made of arced rungs, surrounded by pillars, much like hydrophobic surfaces. The droplets align their leading edge to an arced rung. As the surface moves upwards, the droplet expands its footprint, stretching along the rungs, and then as the surface moves downwards the drop contracts, keeping its leading edge where it is, and therefore slowly edging along the track.