Funded PhD studentship now available: “Superhydrophobic spacing, symmetry and stretch.”

Figure: A drop lands on a superhydrophobic surface, splashes, and rebounds. Three sequential images with time labelled in milliseconds. Filmed using a Photron SA1 high-speed camera in collaboration with Dr Michael Taylor, ESR Christchurch.

Figure: A similar experiment to the one above, with a more symmetric rebound and one smaller drop ‘jetting’ from the surface at a faster speed than the bulk.

Check out a movie showing some more drop impacts: Movie

The importance of drop impacts is instantly familiar to us from raindrops, sprinklers, sprays, ink-jets, painting, and so on. Scientific interest in drop impacts has never been stronger, due to their rich complexity, intrinsic beauty, and the ready availability of high-speed video. Drops landing on extremely water-repellent (‘superhydrophobic’) surfaces are particularly interesting, because they often bounce, or form interesting splashes and ‘crowns’. It has recently been shown that the symmetry of drop impacts can match the microstructure of posts. We have studied the impact of drops on superhydrophobic New Zealand leaves [1], and are working to survey drop impacts on polymer micro-pillars, arranged in various patterns.

Theory describing drop impact on structured surfaces is under-developed, because we cannot fully describe how the fluid and the microstructure interact. A key point is for superhydrophobics is that effective viscous losses are low, which must be due to surface effects. A boundary condition on the surface should be directional, accounting for pinning on ‘pillars’ (including contact line motion), and air flow in the microstructure. Our experimental work aspires to provide tailor-made data for testing and developing such theory.

To support this research, we have a high-speed camera (Photron Fastcam SA5) which is capable of HD defininition at 10,000 frames per second. We encourage collaborations using this camera, whether in dynamic microfluidics or otherwise.

[1] Fritsch, A., Willmott, G. R. and Taylor, M., “Superhydrophobic New Zealand Leaves: Contact Angle and Drop Impact Experiments,” J. Roy. Soc. New Zeal., DOI:10.1080/03036758.2013.782879 (2013).