Hierarchical Length-scales

We have seen here that textured superhydrophobic surfaces easily shed water drops. This includes surfaces with multi-scale texturing. However, this doesn’t stop water condensing within the textures of these surfaces. The resulting droplets are strongly pinned to the surface and are hard to move. However, if the surface is impregnated with a lubricant, water can no longer condense within the textures. This is true whether the lubricant completely wets the underlying surface or not. As a result, the water drops are no longer pinned and can be easily moved. This has the potential to be used to harvest water from fog, as is done by the desert beetle (although the beetle doesn’t use a SLIP surface to harvest water).

In fact, the resulting SLIP surfaces are slippery to a wide range of materials and not just water, a property known as omniphobicity. We can see some examples in the videos in the supporting information of publication [2] below. They include a demonstration showing that machine oil with metal and plastic shavings will contaminate a polycarbonate lens, but when given a SLIPS coating, the lens sheds the contaminant.

Other videos (which can be found here) include demonstrations that:

  • A glass flower dipped in a container of liquid asphalt will remain covered in asphalt when removed from the container. However, when the glass flower has a SLIP surface, the asphalt falls of the flower when it is removed.
  • Water drops stay on the surface of an aluminium sheet, but when give a SLIPS coating, most of the water is shed off.
  • Wet cement slides off a SLIPS-coated glass surface but not an untreated surface. Furthermore, dry cement is much easier to remove from the SLIPS-coated glass.

Publications

  1. Enhanced Condensation on Lubricant-Impregnated Nanotextured Surfaces, S. Anand, A.T. Paxson, R. Dhiman, J.D. Smith and K.K Varanasi, ACS Nano 6 (2012), 10122-10129
  2. Hierarchical or Not? Effect of the Length Scale and Hierarchy of the Surface Roughness on Omniphobicity of Lubricant-Infused Substrates, P. Kim, M.J. Kreder, J. Alvarenga and J. Aizenberg, Nano Lett. 13 (2013), 1793-1799
  3. Self-propelled droplet transport on shaped-liquid surfaces, G. Launay, M.S. Sadullah, G. McHale R. Ledesma-Aguilar, H. Kusumaatmaja and G.G. Wells, Sci. Rep. 10 (2020), 14987