Bioinspired Self-Repairing Slippery Surfaces with Pressure-Stable Omniphobicity

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Entry by Emily Redston, AP 226, Spring 2012

Reference

Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity by T.S. Wong, S.H. Kang, S.K.Y. Tang, E.J. Smythe, B.D. Hatton, A. Grinthal, and J. Aizenberg. Nature 477, 443-447 (2011)

Keywords

omniphobicity, biomimetics, self-healing, surface texture, SLIPS, liquid-repellent surface

Introduction

Researchers have classically drawn inspiration from the lotus effect when designing synthetic liquid-repellent surfaces. Water droplets are able to easily roll off a lotus leaf because they are supported by surface textures on the composite solid-air interface. However, this approach has several inherent limitations, restricting its applicability. For instance, this design does not work well with organic liquids or complex mixtures with low surface tensions. Therefore, in this paper, the authors propose an alternative approach that derives from systems like the Nepenthes pitcher plant. Instead of using microstructures to repel impinging liquids directly, these systems use them to lock-in an intermediary liquid that then acts by itself as the repellent surface. With this idea, Wong et al. designed their own synthetic liquid-repellent surfaces, which they named 'slippery liquid-infused porous surface(s)' or SLIPS. These systems consist of a film of lubricating liquid locked in place by a micro/nanoporous substrate.

The heart of this design lies in the fact that a liquid surface

  • is intrinsically smooth and defect-free
  • provides immediate self-repair by wicking into damaged sites in the underlying substrate
  • is largely incompressible
  • can be chose to repel immiscible liquids of virtually any surface tension

The authors made the SLIPS based on three criteria:

  • 1. the lubricating must wick into, wet, and stably adhere to the substrate
  • 2. the solid must be preferentially wetted by the lubricating liquid rather than by the liquid that one wants to repel
  • 3. the lubricating and impinging test liquids must be immiscible

The SLIPS creates a smooth, stable interface that nearly eliminates pinning of the liquid contact line for both high- and low-surface tension liquids, minimizes pressure-induced impalement into the porous structures, self-heals and retains its function following mechanical damage, and can be made optically transparent.

Results

Figure 1. Omniphobicity and high-pressure stability of SLIPS. a, Time-sequence images comparing mobility of pentane droplets ((γA= 517.260.5mN m-1, volume ~ 30 μl) on a SLIPS and a superhydrophobic, air-containing Teflon porous surface. Pentane is repelled on the SLIPS, but it wets and stains the traditional superhydrophobic surface. b, Comparison of contact angle hysteresis as a function of surface tension of test liquids (indicated) on SLIPS and on an omniphobic surface reported in ref. 9. In the inset, the advancing and receding contact angles of a liquid droplet are denoted as θadv, and θrec, respectively. SLIPS 1, 2 and 3 refer to the surfaces made of Teflon porous membrane (SLIPS 1), an array of epoxy posts of Pressure (atm) geometry 1 (pitch ~ 2 μm, height ~ 5 μm, post diameter ~ 300 nm) (SLIPS 2) and an array of epoxy posts of geometry 2 (pitch~900 nm, height~500 nm–2 μm, post diameter ~300 nm) (SLIPS 3). Error bars indicate standard deviations from three independent measurements. c, A plot showing the high pressure stability of SLIPS, as evident from the low sliding angle of a decane droplet (γA = 23.6 ± 0.1 mN m-1, volume ~ 3 μl) subjected to pressurized nitrogen gas in a pressure chamber. Error bars indicate standard deviations from at least seven independent measurements.

Some Neat Videos

Movie 1 -- This movie demonstrates the fast recovery of the liquid-repellent function of a SLIPS after critical physical damage. As seen from the movie, the crude oil droplet is pinned on a nanostructured superhydrophobic surface (without lubricating fluid), while the droplet maintains its mobility on the SLIPS. Extra-light crude oil (from Appalachian Basin, USA) was used as the test liquid for demonstration.

Movie 2 -- This movie demonstrates the excellent ice-repellency of a SLIPS, as compared to a nanostructured surface. As seen from the movie, an ice block (formed from a frozen water droplet of ˜100 µL at –4 °C and ˜45% relative humidity) slides on the SLIPS under the influence of gravity. In comparison, an ice block of the same volume remains strongly pinned on a superhydrophobic nanostructured surface without a lubricating layer.

Movie 3 -- This movie demonstrates the self-cleaning ability of a SLIPS. As seen from the movie, carbon dust is seeded onto the SLIPS and can be easily removed by sliding an ethanol droplet across the surface.

The rest of the videos can be found here

Conclusions

These SLIPS have a wide range of applications: biomedical fluid handling, fuel transport, self-cleaning windows and optical devices, anti-fouling, anti-icing, and many others. Anti-icing is only briefly touched upon in the paper, but I personally find it very interesting. I know that one of the major uses of anti-icing devices is for wind turbine blades. While SLIPS are certainly an extremely clever design, I wonder if they will be able to hold up to the harsh weather faced by turbine blades. The authors mention that they are planning on exploring the limits of the performance of SLIPS for long-term operation and under extreme conditions, such as high flow, turbulence, and high- or low-temperature environments. It will certainly be interesting to see how that turns out, but I have no doubt that SLIPS will have a big impact regardless.