Droplets on Shaped and Gradient liquid Surfaces
Solid surfaces can be geometrically shaped to control how they interact with liquids. For example, a droplet on a set of hydrophobic solid ridges will be in a suspended (solid-air) Cassie-Baxter superhydrophobic state. If the ridges become progressively wider along their length (a gradient in wettability) a driving force is created from the end with the larger fraction of air to the end with a larger fraction of solid surface. However, the droplet must overcome contact line pinning to start moving and the distance it can travel is limited by contact line friction.
To overcome contact line pinning friction, a liquid-layer can be impregnated into a thin coating on a solid so a droplet only ever interacts with a liquid surface. We can create a shaped liquid surface using a liquid coating on a shaped solid, such as narrowing ridges shaped into spiral patterns (video).
Video of droplets moving around curved paths.
Since liquid surfaces have low contact line friction, but are adhesive to droplets, we can hanging droplets can self-propel (video).
The same principles can be used with a flat surface with a comb of wedge-shaped regions comprised of liquid-like and liquid-infused surfaces to create low-friction self-propelled motion in either direction towards the end with the greater wettability (Video).
Publications
- Self-propelled droplet transport on shaped-liquid surfaces G. Launay, M. S. Sadullah, G. McHale, R. Ledesma‐Aguilar, H. Kusumaatmaja and G. G. Wells, Scientific Reports 10 (2020), 14987
- Bidirectional Motion of Droplets on Gradient Liquid Infused Surfaces M. S. Sadullah, G. Launay, J. Parle, R. Ledesma‐Aguilar, Y. Gizaw, G. McHale, G. G. Wells and H. Kusumaatmaja Communications Physics 3 (2020), 166