Lubricant-infused micro/nano-structured surfaces with tunable dynamic omniphobicity at high temperatures

The ability of surfaces to repel various liquids (i.e., omniphobicity) at elevated temperatures has broad practical applications in refinery processes, ethanol concentration, fuel transportation, district heating, and protective fabrics.^1,2 State-of-the-art approaches have been based on the “lotus” effect, where micro/nano-structures are carefully designed to maintain an air layer between the structures, forming a stable interface between the substrate and the applied liquid.^3,4 This superhydrophobic (SH) interface, however, becomes unstable for low-surface-tension liquids due to their enhanced ability to wet surfaces.⁵ Since surface tension decreases with increasing temperature, which further destabilizes the liquid-air interface, it is very challenging to design surfaces that can repel a wide range of liquids at high temperatures.

Early work on omniphobicity employed photolithography to create complex re-entry geometries on a surface that could then repel liquids with surface tensions down to ∼17 mN/m (e.g. heptane) at ambient temperature.^6,7 Other approaches to generating re-entry geometries that achieve omniphobicity were later demonstrated.^8–10 It was also shown that small liquid droplets can bounce off an extremely hot superhydrophobic surface reminiscent of the Leidenfrost effect, provided that the heat transfer between the solid and liquid is sufficient to generate a vapor layer, which can only occur when the temperature of the solid substrate T_solid is significantly above the liquid's boiling point (T_solid > 150 °C for water).¹¹ Recent progress in creating superhydrophobic/superoleophobic materials based on a fluorinated silica network and rare-earth oxide ceramics addressed the problem of developing mechanically robust surfaces that retain liquid-repellency (quantified by contact angle at ambient temperature), after annealing/sintering of the substrate at high temperatures up to 1000 °C.^12–14 While these surfaces have high-temperature stability during their respective manufacturing processes and remain liquid-repellent after cooling down to room temperature, they did not demonstrate liquid repellency at elevated temperatures. Oleophobic surfaces based on polymethylsilsesquioxane coating, on the other hand, have been shown to repel organic liquids at 250 °C.¹⁵ Nonetheless, omniphobic surfaces that can repel various fluids at elevated temperatures (when either T_liquid or T_solid or both are above 200 °C) remain rare due to the fact that surface tensions of some organic liquids, such as heptane and octane, can fall below 10 mN/m at T > 200 °C.^16,17

Recently, we developed a radically different concept in designing omniphobic surfaces, termed as Slippery Liquid-Infused Porous Surfaces (SLIPS), in which a suitably treated micro/nano-structured surface is infused with a lubricant, as shown in Fig. 1(a). Any foreign liquid droplet immiscible with the underlying lubricant can then easily slide off at a small tilting angle of <5°.¹⁸ We demonstrated how SLIPS coating can be applied to a variety of solid substrates, including polymers, metals, glass, and ceramics, such that arbitrary materials begin to show superior liquid-repellency, anti-biofouling, and anti-icing properties.^19–21 Lafuma and Quéré outlined the requirements for thermodynamic stability for such a system, which was further elaborated by Smith et al.^22,23 Here, we report that SLIPS retain their excellent liquid-repelling properties at high temperatures up to T_solid,liquid = 200 °C for a wide range of liquids, including those with low surface tension. This is in contrast with representative SH surfaces that were shown to lose their liquid-repellency even at moderate T_liquid ∼ 90 °C.¹ 

SLIPS can be created from a SH-surface by infusing it with a lubricant such as perfluorinated oil. A micro-structured surface consisting of a hexagonal array of posts with dimensions: diameter d = 1 μm, height h = 10 μm, and pitch l = 4 μm, as illustrated in the inset of Fig. 1(b), was fabricated using replication techniques described elsewhere.²⁴ The final substrate was made of UV-cured polyurethane (NOA 61, Norland) that was then fluorinated to render its surfaces SH. At room temperature, this SH-surface repels water with a contact angle of 165° ± 5°, but when 200 ml of hot water (T_liquid = 95 °C, stained with methylene blue) was splashed onto the surface, the surface was clearly wetted, as shown in Fig. 1(b). The same solid substrate infused with perfluorinated polyethers (Krytox 100, DuPont, lubricant thickness = 13 μm) retained its water-repellency even at 95 °C.

To confirm that the decrease in surface tension, γ, was the cause of the transition from a non-wetting to a wetting state, we measured the contact angle hysteresis (CAH) for water-ethanol mixtures of increasing ethanol (EtOH) concentration (0–70% wt.) on the SH-surface and SLIPS to simulate the effect of decreasing water tension with increasing temperature, T_liquid, as summarized in Fig. 1(c). Near boiling, water has a surface tension of 58 mN/m as opposed to its room temperature value of 72.8 mN/m.²⁵ For SLIPS, CAH remained small (∼2°) for all EtOH concentrations, whereas for the SH-surface, there was a transition from non-wetting to wetting when the liquid droplet reached a critical surface tension, 50 < γ_critical < 60 mN/m. The CAH for 6% EtOH was 18°, whereas for 7% EtOH, the receding contact angle was ∼0°, while the advancing contact angle was 130°, i.e., CAH = 130°. This sharp pinning transition is consistent with the wetting observed for water at T_liquid = 95 °C, where γ_water = 59.8 mN/m approaches γ_6%EtOH, shown as a dashed line in Fig. 1(c). A similar wetting temperature, T_wetting = 94° ± 1°, was observed for a SH-surface dipped in hot water for 1 min to ensure that the surface and water were at the same temperature, i.e., T_solid = T_liquid.

While these Norland-polymer infused surfaces show promise in applications that involve high-temperature water transport and district heating, they are unsuitable for ultra-high temperature applications involving liquids with high boiling point, since Norland 61 polymer begins to degrade at T_solid ∼ 150 °C. To investigate the effects of T > 100 °C, a Teflon membrane (pore size ∼0.2 μm, thickness ∼50 μm), which resists degradation up to 350 °C, was chosen as the solid substrate. Fig. 2 demonstrates that a Teflon membrane infused with perfluorinated Krytox 105 easily repels a 50 μl droplet of crude oil at T_SLIPS = 200 °C. Even at a small incline of 10°, the droplet moved at a speed of ∼1.5 cm/s, whereas the crude oil droplet on the bare Teflon membrane (SH state) was pinned to the surface. The crude oil used contained mostly light oil fractions with a boiling temperature of around 150 °C. SLIPS remained liquid-repellent even when the crude oil droplet started to boil. At T = 200 °C, crude oil is known to have a surface tension of <20 mN/m.^26,27

It is important to note that liquid-repellency of the material is associated with and can be described in terms of how quickly liquid droplets can move on and hence be removed from the surface. This in turn is significantly affected by the viscosity of the lubricant, η_lubricant, since the viscous force associated with the trapped lubricating film scales linearly with η_lubricant.^23,28,29 Unlike for the case of SH-surfaces, liquid droplets on SLIPS will, therefore, become more mobile with initial increase in temperature, as η_lubricant decreases with increasing temperature. We measured the velocity of 20 μl droplets of crude oil moving on a heated Teflon membrane infused with various lubricants (perfluorinated polyethers, DuPont Krytox 100, 103, and 105 with chain length n = 10–60 that have similar surface tensions of γ = 17 ± 1 mN/m but different viscosities η_101,103,105 = 20, 140, and 800 cP at room temperature respectively) and positioned at small 5° tilt angle at temperatures T_SLIPS = 20–200 °C. The results are summarized in Fig. 3. Each measurement was repeated 3–5 times and a thermocouple was used to monitor the temperature of the surface. For each case, the velocity of the droplet, V, first increases with T, as shown in Fig. 3(a), because the viscosity of Krytox oils decreases with increasing T_SLIPS, illustrated in Fig. 3(b), but only up to a critical temperature, T_c. With further heating beyond T_c, the Krytox oil evaporation rate becomes significant and the velocity of the droplets decreases due to pinning at exposed defects on SLIPS. T_c was observed to increase with increasing oil index number, because higher-index Krytox oils contain longer-chain polyethers. Fig. 3(c) shows the results of thermogravimetric analysis (TGA) for the different Krytox oils, which confirm that the higher-index Krytox oils evaporate at higher T_SLIPS, with maximum rate of evaporation occurring at T_max = 170, 270, and 395 °C for Krytox 100, 103, and 105, respectively. Inferring T_c directly from the thermogravimetric data is not trivial and it was found that T_c occurs well below T_max. Nevertheless, the values of T_c for the different Krytox oils obtained, T_c,100 ≈ 70 °C, T_c,103 ≈ 100 °C, T_c,105 > 150 °C are consistent with the maximum operating temperature T, shown by dashed lines in Fig. 3(a), provided by the manufacturer.

To further demonstrate the effects of η_lubricant on the mobility of liquid droplets, we measured the velocity of 20 μl water droplets down an incline of 50° for Teflon membranes infused with perfluorinated liquids (perfluoro-octane, FC-770, Krytox 100–107) of increasing viscosity, η_lubricant = 1–2500 cP. Fig. 4(a) shows a time-lapsed image of such an experiment, in which the positions of differently colored water droplets at equally spaced time intervals are juxtaposed. Each 30 cm lane was infused with different lubricants (increasing η_lubricant from left to right); the time-lag, Δt, from start to finish was 0.4 s. Fig. 4(b) shows a log-log plot of the velocity of water droplets and the viscosity of lubricant. For the most viscous lubricant, Krytox 107, which is ∼2500 times more viscous than water, the droplet moved at a speed of about 1 mm/s, whereas for perfluoro-octane, which is as viscous as water (η_lubricant ∼ 1cP), the maximum velocity at the end of the 30 cm track reached an extremely high value of ∼0.5 m/s. To put this into perspective, a similarly sized water droplet moves down a vertically inclined glass at a speed of ∼1 cm/s. In Fig. 4(c), the plot of the droplet displacement on perfluoro-octane-infused surface with time shows a quadratic relationship, i.e., the droplet was accelerating at a constant a = 1.15 m/s², c.f. g sin(50°) = 7.5 m/s². The discrepancy between the gravitational acceleration g sin(50°), and the observed acceleration, a, is probably due to friction between the underlying solid substrate and the liquid droplet. For Teflon infused with other lubricants, the droplets achieved their terminal velocities.

The results of the experiments depicted in Figs. 3 and 4 demonstrate that it is possible to tune the velocity of droplets on SLIPS continuously by a factor of 1000 on the same solid substrate by either (1) changing the temperature of the surface or (2) using lubricants of different viscosities. This capability has broader implications in terms of droplet manipulation on surfaces. On a textured, SH-surface, droplet mobility is dictated by the static contact angle θ_s and CAH, but the two parameters do not sufficiently characterize droplet behavior on SLIPS. The water droplets on different lubricant-infused SLIPS have similar θ_s = 115° ± 2° and CAH ∼ 3°, but nevertheless travel with a wide range of velocities, showing the importance of the lubricant as an additional source of friction/resistance for the droplet movement. Our results also suggest that within different temperature regimes, there is an optimum lubricant that maximizes the velocity of droplet: Krytox 100 for T_SLIPS < 70 °C, Krytox 103 for T_SLIPS = 70–120 °C, and Krytox 105 for T_SLIPS = 120–200 °C. We did not measure the mobility of crude oil droplets for T_SLIPS > 200 °C for reasons of safety, but TGA indicates that when infused with Krytox 105, SLIPS can retain its liquid-repellency up to T_SLIPS ∼ 300 °C, as depicted in Fig. 3(c). The lifetime of SLIPS at high temperatures is closely related to the evaporation rate of the lubricant and therefore increases with increasing Krytox index. (See supplementary material.³²)

Finally, SLIPS exhibit a much better heat transfer rate than SH-surface, which may be relevant in some industrial processes where heat transfer is important.^30,31 20 μl water droplets were placed on a SH-surface and its SLIPS counterpart (same surfaces as Fig. 1) that were kept at a temperature T_solid = 85° ± 5° using a thermal plate. The temperatures of the water droplet and the surfaces were measured using an infra-red camera, as shown in Fig. 5. Initially, both water droplets were at room temperature, but at equilibrium, the cores of the water droplets were at different temperatures: T_core,SLIPS = 45° ± 2° > T_core,SH = 32° ± 2°. The equilibrium temperatures, T_core,SLIPS and T_core,SH remained constant until all of the liquid droplets have completely evaporated. Potentially, this heat transfer rate can be further controlled by adjusting the thermal conductivity of the lubricants.

To summarize, we have demonstrated a system that is capable of repelling a wide range of liquids, from simple newtonian fluids such as water to a low-surface-tension complex fluids such as crude oil, across a wide range of temperatures up to 200 °C. In comparison, most state-of-the-art SH-surfaces fail at moderate T < 90 °C. We also outlined how to manipulate droplet's mobility on SLIPS continuously on the same solid surface by a factor of 1000 by either changing the temperature or using different lubricants. We believe that such tunable, robust omniphobic surfaces will have many uses in industrial processes that involve liquid manipulation and transport in challenging, high temperature conditions.

We thank P. Kim for helpful discussions regarding TGA. M.N.M. gratefully acknowledges a Fannie and John Hertz Foundation Graduate Fellowship and a NSF Graduate Research Fellowship. The work was supported partially by the ONR MURI Award No. N00014-12-1-0875 and by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000326. We acknowledge the use of the facilities at the Harvard Center for Nanoscale Systems supported by the NSF under Award No. ECS-0335765.