Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity

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Introduction

Most of current state-of-the-art superhydrophobic surfaces are based on the lotus leaf effect, where they owe their superhydrophobic nature to the composite air-solid-liquid interface (Cassie-Baxter model) which reduces solid-liquid contact area resulting in liquid repellency. This approach works well for liquid such as water, but not for liquid with small surface tensions such as most hydrocarbons. Wong et al has designed a new kind of liquid-repelling surface based on a completely different physical principle, inspired by the pitcher plant Nepenthes. The surface consists of a sponge-like material which is filled with a lubricating film that is smooth to the atomic level. Any liquid that is immiscible with the lubricating liquid can then easily be rolled off. Moreover, the surface, dubbed Slippery Liquid-Infused Porous Surfaces (SLIPS) can withstand high pressures and exhibit impressive self-repairing capabilities.

keywords

biomimetics, wetting, superhydrophicity, omniphobicity, self-repair

Results

The design of SLIPS is shown schematically in figure 1. There are three criteria for which SLIPS work: (1) the lubricating liquid must wick into, wet and stably adhere within the substrate, (2) the solid must be preferentially wetted by the lubricating liquid rather than by the liquid one wants to repel, and (3) the lubricating and impinging test liquids must be immiscible.

Surface1.png

Figure 1. Design of SLIPS. a, Schematics showing the fabrication of a SLIPS by infiltrating a functionalized porous/textured solid with a low-surface energy, chemically inert liquid to form a physically smooth and chemically homogeneous lubricating film on the surface of the substrate. b, Comparison of the stability and displacement of lubricating films on silanized and non-silanized textured epoxy substrates. Top panels show schematic side views; bottom panels show time-lapse optical images of top views. Dyed pentane was used to enhance visibility. c, Scanning electron micrographs showing the morphologies of porous/textured substrate materials: an epoxy-resin-based nanofabricated post array (left) and a Teflon-based porous nanofibre network (right). d, Optical micrographs demonstrating the mobility of a low-surface-tension liquid hydrocarbon—hexane (γA = 18.6 ±0.5 mN m-1, volume ~ 3.6 μl)—sliding on a SLIPS at a low angle (𝛼=3.00).

The SLIPS surface is shown to be able to repel a huge range of liquid from water to hydrocarbons. The water repelling abilities are tested by measuring the contact angle hysteresis and the sliding angle.

Surface2.png

Figure 2. 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.

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Figure 3. Repellency of complex fluids, ice and insects by SLIPS. a, Movement of light crude oil on a substrate composed of a SLIPS, a superhydrophobic Teflon porous membrane, and a flat hydrophobic surface. b, Comparison of the ability to repel blood by a SLIPS, a superhydrophobic Teflon porous membrane, and a flat hydrophilic glass surface. c, Ice mobility on a SLIPS (highlighted in green) compared to strong adhesion to an epoxy-resin-based nanostructured superhydrophobic surface (highlighted in yellow). The experiments were performed outdoors (note the snow in the background) when temperature and relative humidity were –4 0C and ~45%, respectively. Note also the reduced frosting and the resulting transparency of the SLIPS. d, Demonstration of the inability of a carpenter ant to hold on to SLIPS. The ant (and a drop of fruit jam it is attracted to) slide along the SLIPS when the surface is tilted. Note that the ant can stably attach to normal flat hydrophobic surfaces, such as Teflon. All scale bars represent 10 mm.

SLIPS are also able to repel complex liquids such as blood and crude oil. Moreover, they are optically transparent and are able to withstand high pressure up to 680 atm.

Personal Thoughts

The best ideas are often the simplest. The Authors have provided a very simple and cheap way to manufacture slippery surfaces that can repel a huge broad of fluids, including complex fluids like blood. There are many possible applications for this raised by the authors, such as anti-fouling and self-cleaning surfaces. Another possible application (not mentioned by the authors) for this is in microfluidics and lab-on-a-chip applications. One huge stumbling block in truly portable μTAS systems is the need for some form of pumps to facilitate the flow of fluids. Whitesides group has invented paper microfluidics (Martinez et al, 2010) where they made use of the capillarity action of paper to transport tiny volume of chemical reactants for various μTAS applications without the use of any pumps. SLIPS surfaces can be used in microfluidic systems to render the surfaces so slippery that allows easy transport of fluids such as blood, perhaps even without the use of pumps, driven perhaps by gravity.

References

1. T.S. Wong, S.H. Kang, S.K.Y. Tang, E.J. Smythe, B.D. Hatton, A. Grinthal & J. Aizenberg, "Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity", Nature 2011 2. A.W. Martinez, S.T. Phillips and G.M. Whitesides, "Diagnostics for the Developing World: Microfluidic Paper-Based Analytical Device", Anal. Chem. 2010