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

Omniphobic surfaces that can repel fluids at temperatures higher than 100 °C are rare. Most state-of-the-art liquid-repellent materials are based on the lotus effect, where a thin air layer is maintained throughout micro/nanotextures leading to high mobility of liquids. However, such behavior eventually fails at elevated temperatures when the surface tension of test liquids decreases significantly. Here, we demonstrate a class of lubricant-infused structured surfaces that can maintain a robust omniphobic state even for low-surface-tension liquids at temperatures up to at least 200 °C. We also demonstrate how liquid mobility on such surfaces can be tuned by a factor of 1000.

Omniphobic surfaces that can repel fluids at temperatures higher than 100°C are rare. Most state-of-the-art liquid-repellent materials are based on the lotus effect, where a thin air layer is maintained throughout micro/nanotextures leading to high mobility of liquids. However, such behavior eventually fails at elevated temperatures when the surface tension of test liquids decreases significantly. Here, we demonstrate a class of lubricant-infused structured surfaces that can maintain a robust omniphobic state even for low-surface-tension liquids at temperatures up to at least 200°C. We also demonstrate how liquid mobility on such surfaces can be tuned by a factor of 1000.
The ability of surfaces to repel various liquids (i.e. omniphobicity) at elevated temperatures have broad practical applications in refinery processes, ethanol concentration, fuel transportation, district heating, and protective fabrics. 1,2 State-of-the-art approaches have a D. Daniel and M.N. Mankin contributed equally to this work b Present Address: Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, PA 16802 c Authors to whom correspondence should be addressed. Electronic mail: tswong@psu.edu and jaiz@seas.harvard.edu 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 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, which could then repel liquids with surface tension 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][9][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 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][13][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 surface based on polymethylsilsesquioxane coating, on the other hand, has 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 lubricating fluid, as shown in Fig. 1a. Any foreign liquid droplet immiscible with the underlying lubricating fluid can then easily slide off at a small tilting angle of < 5°. 18 We demonstrated how such a principle 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][20][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 the best superhydrophobic (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 d = 1 µm, height h = 10 µm, and pitch l = 4 µm, illustrated in inset of Fig. 1b, was fabricated using fast replication techniques described elsewhere. 24 The final substrate was made of UV-cured polyurethane (NOA 61, Norland) which 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. 1b. 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 waterethanol mixtures of increasing ethanol (EtOH) concentration (0-70% wt) on the SH-surface and SLIPS to simulate the effect of water surface tension drop due to increasing temperature, T liquid , as summarized in Fig. 1c. Near boiling, water has 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. 1c. 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 .  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 lubricating layer, η lubricant , since the viscous force associated with the trapped lubricating film scales linearly with η lubricant . 23,28,29 Unlike SH-surface, liquid droplets on SLIPS will therefore become more mobile with initial increase in temperature, as η lubricant drops with increasing temperature. We  Fig. 3a, because the viscosity of Krytox oils decreases with increasing T SLIPS , illustrated in Fig. 3b, 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 Krytox oils index, because higher-index Krytox oils contain longer-chain polyethers. Fig. 3c shows the results of thermogravimetric analysis, which confirms that the higher-index Krytox oils evaporate at higher T SLIPS , with maximum rate of evaporation occurring at T max = 170, 269, 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 .   Finally, SLIPS exhibit a much better heat transfer rate than SH-surface and this may be relevant in some industrial processes where heat transfer is important. 30,31 20 µL water droplets were placed on a lotus-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 infrared 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 fluid such as water to a low-surface-tension complex fluid 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 continuouly on the same solid surface by a factor of 1000 by either changing the temperature or using different lubricants. We believe that such a 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