Slipperiness and stability of hydrophilic surfaces coated with a lubricating fluid

Abstract

In recent years, many research groups have studied the slipperiness of rough or porous hydrophobic surfaces infused with a lubricating fluid. Here, we alternatively present a simple method to fabricate stable slippery surfaces on hydrophilic samples, which are more commonly and widely used. At room temperature, hydrophilic samples lubricated with silicone oil did not present a slippery surface for water drops, which were observed to sink into the oil layer due to the inherently hydrophilic surface. Annealing these samples at higher temperatures, however, caused the silicone molecules to covalently bind the silicon surface, hence making the surface hydrophobic. At an optimized annealing temperature and annealing time, the surfaces showed excellent slipperiness, with negligible contact angle hysteresis and low sliding angles. Water drops on the slippery surfaces were found to be enveloped, or cloaked, in a thin layer of the lubricating oil, which minimized the oil–water interfacial energy. Upon sliding, these oil-cloaked water drops slowly removed lubricating oil, which resulted in the degradation of the slipperiness. This degradation was prevented when using large water drops or a continuous flow of water.

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Introduction

Inspired by Nepenthes pitcher plants, the fabrication and study of lubricated fluid-infused slippery surfaces are emerging as a hot topic for fundamental surface science as well as for their enormous practical applications.^1–4 Various research groups have attempted to fabricate slippery surfaces using different lubricating fluids infused on rough or porous substrates.^5–18 Upon coating a suitable lubricating fluid on such substrates, test drops of liquid can slide very easily on them with very low friction. Due to the lubricating behavior, these slippery surfaces have been found very useful in various ways including reducing drag when transporting a liquid, inhibiting the formation of ice and frost while preserving optical transparency, serving as an anti-biofouling agent, enhancing condensation and fog harvesting, and siliconizing syringes, to name a few.^13,19–27 Slippery surfaces have been found to be more stable if the underlying substrates are rough or porous, as such substrates provide larger surface areas and greater capillary adhesion for the lubricating fluid.^8,15 Therefore, rough or porous substrates are preferred while fabricating the slippery surfaces. Also the lubricant has to cover the entire surface, and the substrates therefore have to be completely wetted with the lubricating fluid. Test drops of liquid, which are meant to slide on the lubricated infused slippery surfaces, should be immiscible with the lubricating fluid. Also the test liquid should be non-wetting to the substrate surface.^12,15 For this reason, hydrophobic substrates are always used for slippery surfaces when testing aqueous drops. Aizenberg et al. observed the disruption of lubricated films on non-silanized (hydrophilic) epoxy-based substrates whereas silanized (hydrophobic) epoxy yielded a stable and quite slippery lubricated film.¹⁵ Quéré et al. also showed two possible outcomes for test liquid drops, that they either float or sink, and which outcome results being determined by the surface tension of the liquids and the surface energy of solid substrates.²⁸ Different fabrication techniques are used by various research groups to fabricate the lubricated fluid-infused slippery surfaces. Aizenberg et al. used porous nanofibrous Teflon membranes for the infusion of perfluorinated lubricating fluid, which yielded surfaces that were highly slippery for various test liquids, e.g., water, glycerol, ethylene glycol, alkanes, biofluids, crude oil, etc.¹⁵ These surfaces were found to be slippery under up to 700 atmospheres of pressure. These investigators also demonstrated their slippery surfaces to lower the nucleation temperature of super-cooled water and thus provide alternative anti-ice and anti-frost surfaces.²⁹ They also fabricated slippery surfaces through a layer-by-layer deposition of silica nanoparticles followed by silane functionalization and fluorinated lubricant coating.³⁰ These surfaces were also observed to be highly slippery for a variety of test liquids and were found to be very resistant to getting damaged. Varanasi et al. used lithographically patterned hydrophobic silicon substrates infused with either silicone oil or ionic liquid as the lubricating fluid to produce surfaces slippery for aqueous drops.¹² They observed that the aqueous drops may, depending on the interfacial energy, become enveloped, or “cloaked”, with a thin layer of lubricating fluid. Based on analyzing the trajectories of coffee particles immersed in aqueous drops, they concluded that these drops actually roll rather than slip. Their lubricated surfaces also showed enhanced condensation of water droplets compared to dry substrates, and this feature can be very useful in heat transfer applications. They also demonstrated that these slippery surfaces reduced drag by about 16% in laminar flow.³¹

In all these studies, either the substrates used were inherently hydrophobic or an additional step of hydrophobic coating was performed before infusing a lubricating fluid. Using hydrophobic substrates limits the choice of surfaces whereas the hydrophobization process has its own limitations and concerns such as chemical compatibility of the substrates with large-area fabrication, multi-step fabrication process, and cost. Alternatively, if the hydrophobization of hydrophilic substrates can be performed while or after coating a lubricating fluid using a convenient process, the overall fabrication procedure would remain relatively simple. Here we demonstrate that annealing hydrophilic substrates after coating them with silicone oil (lubricating fluid) results in the hydrophobization of the substrate surface, which subsequently improves the adhesion of the lubricant as well as making the surface more slippery. We tested various annealing parameters to produce the most effective slippery surface with the smallest contact angle hysteresis and largest slip velocity. The substrates remained hydrophobic even when the lubricating fluid was washed away after annealing, confirming the hydrophobization of the substrate surface. This method can be generalized to all the metal oxide and ceramic surfaces where –OH bonds can be introduced. As the sliding water drops in our experiments became cloaked with a thin layer of oil, the slipperiness of the surface began to degrade. Such a degradation was found to be dependent on the size of the sliding drops, and no decrease was observed at all for a continuous flow of water. Once degraded completely, the slipperiness of the surface could be recovered simply by recoating it with the lubricating fluid.

Experimental section

Polished silicon (Si) wafers cut into 2 cm × 2 cm pieces were used as substrates for slippery surfaces. These substrates were cleaned by ultrasonicating them in ethanol, acetone and toluene for 5 min each followed by oxygen plasma cleaning for 30 s, which resulted in hydrophilic surfaces on which water completely spread. Silicone oil (Sigma-Aldrich, kinematic viscosity ∼370 cSt) was used as the lubricating fluid, which was spin casted onto the cleaned Si substrates. Subsequently, the samples coated with the lubricating fluid were annealed at different temperatures and for different durations to optimize the slipperiness of the surface as well as the stability of the lubricating fluid. The wettability of the surfaces coated with lubricating fluid was measured with an optical contact angle goniometer (OCA35, DataPhysics Germany) using the sessile drop method. Drops of de-ionized (DI) water with a volume of 10 μL were used as the test liquid to investigate the slipperiness of the fabricated surfaces. The slipperiness of the surface was quantified by measuring the velocity of a drop of water on the surface when this surface was titled by 10°. To investigate the stability of the lubricating fluid, a volume of 50 mL of water was dispensed drop-wise (with the volume of each drop being 10 μL) on the slippery surfaces, followed by measuring the slip velocity using the 10 μL water drops. We also investigated the effect of the volume of the dispensed drop on the stability of the lubricating fluid.

Results and discussion

Cleaned silicon substrates, having a thin native SiO₂ top layer, were found to be inherently hydrophilic as well as oleophilic with water and oil contact angles of 10° and 0°, respectively. Spin coating silicone oil with varying rpm values provided lubricating films with different initial thicknesses. Water drops on the as-coated silicone oil films were found to be unstable as they sank in the oil due to the underlying hydrophilic silicon substrate. ESI Movie S1† shows the sinking behavior of a water drop on a substrate freshly coated with silicone oil. Strong polar interactions between the hydrophilic Si surface and water molecules resulted in the sinking of the water drops inside the silicone oil, making the surface non-slippery. So the most important requirement in using hydrophilic substrates as slippery surfaces is to first make them hydrophobic. This was achieved by annealing the substrates coated with lubricating fluid at different temperatures and for different periods of time. Aizenberg et al. found that to a achieve a stable slipperiness, the lubricating oil should be wetting (oleophilic) and the test liquid should be non-wetting (hydrophobic) on the substrates. In the present case (with water and silicone oil as the test liquid and lubricating fluid, respectively), total energy was calculated for water on the Si substrate (“configuration 1”), oil layer on the Si substrate (configuration 2) and water on the silicone oil that was on the Si substrate (configuration 3) as shown in Fig. 1. Conditions for a stable slippery surface can be written in terms of the energy difference between configuration 2 and configuration 1, and that between configuration 3 and configuration 1 using the equations

ΔE₁₂ = E₁ − E₂ = (γ_o cos θ_o − γ_w cos θ_w) + (γ_w − γ_o) (1)

ΔE₁₃ = E₁ − E₃ = (γ_o cos θ_o − γ_w cos θ_w) + γ_wo (2)

where γ_w, γ_o are the surface tensions of water and silicone oil, respectively, γ_sw, γ_so and γ_wo are the interfacial tensions between the solid and the water, between the solid and the oil, and between the water and the oil, respectively, and θ_o and θ_w are, respectively, the silicone oil and water contact angles on the Si substrate.

Fig. 1 Schematic diagram of configurations 1–3 to calculate the total energy of the system. Configurations 1–3 correspond to water on substrate, oil on substrate, and water on oil on substrate respectively.

For stable slippery surfaces, the energy differences ΔE₁₂ and ΔE₁₃ should be positive. For the freshly coated samples that we made, the values of these energy differences were determined to be ΔE₁₂ = 1.1 mJ m⁻² and ΔE₁₃ = −92.5 mJ m⁻². As a result, the test water drops sank into the lubricating oil layer, and these samples were relatively non-slippery.

When annealing Si substrates coated with silicone oil, silicone molecules have been shown to become covalently bonded to the Si surface and hence modify γ_s, γ_sw and θ_w.^32,33 And indeed, in our experiments, the energy differences for the annealed samples became positive, with ΔE₁₂ = 94.3 mJ m⁻² and ΔE₁₃ = 0.65 mJ m⁻², thus preventing the test water drops from sinking, and hence stabilizing the slipperiness of the surfaces. Subsequently, the effect of annealing temperature and time was investigated to optimize the annealing parameters. Fig. 2 shows the effect of annealing temperature and time on the slip velocity of drops of water on the fabricated surfaces. First, Si substrates coated with silicone oil at different rpm values were annealed at different temperatures for 90 min to determine the optimum annealing temperature. As the annealing temperature was increased above 50 °C, the slipperiness improved, as indicated by the increasing water drop velocity (Fig. 2(a)). Initially, no improvement in the slipperiness was observed from room temperature to 50 °C, but further increasing the annealing temperature up to 200 °C improved the slipperiness as indicated by the increasing drop velocity. Further increasing the annealing temperature above 200 °C, however, destroyed the slipperiness of the samples since these temperatures, specifically 200 °C and 250 °C, are the boiling and flash points of silicone oil, and the lubricated film began de-wetting (breaking into drops).

Fig. 2 (a) Slip velocity of a test liquid (water) drop as a function of annealing temperature for lubricated films prepared at different rpm values and with the annealing time kept constant at 90 min. (b) Optimization of annealing time when the annealing temperature was kept constant at 150 °C. (c) and (d) show images of the optical contact angles of water drops on non-annealed and annealed samples.

The results of our slip velocity test (Fig. 2(a)) suggest that annealing at 150 °C provides the optimum slipperiness for the surface we fabricated as the drop velocity was observed to be the highest for this temperature. Fig. 2(b) shows the optimization of annealing time for samples annealed at 150 °C. As the annealing time was increased, the slip velocity increased, indicative of improved slipperiness. The highest slip velocity was obtained for samples annealed for 90 min, which apparently provided enough time for the silicone molecules to covalently graft with the Si surface. Therefore, the most slippery surfaces were fabricated by annealing freshly coated samples at 150 °C for 90 min (see ESI Movie S2†). Fig. 2(c) and (d) show images of optical contact angles of water drops on silicone-oil-coated non-annealed and annealed samples. For the non-annealed samples, water drops sank inside the lubricated layer, and hence showed a much lower apparent average contact angle (∼53°). For annealed samples, the water drops did not sink and showed an average equilibrium contact angle of around 108°, which is similar to the contact angle of water drops on a monolayer of silicone molecules. We may predict that varying the thickness of the silicone oil (prepared by changing the spin coating rpm from 2000 up to 8000) would not affect the slipperiness of the sample surface, based on the results of Fig. 2 and due to the slip velocity for the annealed samples being independent of the lubricant thickness in the given thickness range. Therefore, optimized slippery surfaces were fabricated by spin coating silicone oil at 8000 rpm and then annealing at 150 °C for 90 min, and these surfaces were used for all subsequent experiments.

To verify that the Si surfaces were chemically modified upon annealing, which is essential for attaining stable slipperiness as suggested by the energy calculations (cf. eqn (1) & (2)), lubricant was slowly removed from the non-annealed and annealed samples, and the water contact angles and drop velocities were measured. Lubricant-coated samples were washed in an aqueous 2 wt% surfactant (sodium dodecyl sulphate) solution produced by ultrasonication, and the water contact angle and drop velocity were measured as a function of washing time. Fig. 3 shows the slip velocity and static contact angle of water drops on silicone oil-coated non-annealed and annealed samples as a function of washing time. For the non-annealed samples, the slip velocity remained zero throughout the lubricant removal time as the water drops were subjected to strong polar interactions with the hydrophilic Si surface. The static water contact angle on non-annealed samples decreased as the lubricant was removed due to thinning of the silicone oil layer and increasing exposure to the hydrophilic Si surface. Upon complete removal of the lubricant, the water contact angle became ∼24°, which is very close to the contact angle on the bare Si surface. This confirms that for non-annealed samples, the Si surface remained hydrophilic and all silicone oil molecules can be removed via ultrasonication of the surfactant solution in a few minutes. On the other hand, for the samples annealed at 150 °C for 90 min, the slip velocity decreased as a function of lubricant washing time. This happened because the surfactant solution slowly removed silicone oil molecules from the lubricant layer, reducing its thickness and thus decreasing the slip velocity. Upon complete washing of the lubricating layer, the slip velocity again became zero, indicative of the surface having become non-slippery. The average initial water contact angle for the annealed samples was observed to be 108°(±3), and did not vary as a function of the amount of lubricant washing. Even after complete removal of the lubricant, the water contact angle remained 108°. This confirms that even after complete removal of lubricant from the annealed samples, there was at least a monolayer of silicone molecules on the Si surface that could not be removed. A monolayer of silicone molecules can resist being removed from the Si surface by surfactant washing only if the silicone molecules are covalently bonded with Si surface. Such a difference in covalent bonding explains why the water contact angle decreased on the non-annealed samples but not on the annealed ones as a function of lubricant removal. Fig. 4 shows a detailed XPS spectrum of Si 2p, C 1s and O 1s core lines of bare and annealed Si samples. A survey scan confirmed the presence of Si, O and C in the bare and annealed samples. Bare Si substrate showed standard Si bonds (O–Si–O (103.6 eV) and Si–OH (102.5 eV)), C bonds (C–H (284.6 eV), C–N (286.3 eV) and C–O_x (287.1 eV)) and O bonds (O–Si–O (532.2 eV) and C–O_x (533.6 eV)). The annealed samples showed Si bonds (O–Si–O (103.6 eV), Si–C (100.3 eV), and Si–O–Si (101.9 eV)), C bonds (C–Si (284.5 eV), C–H (284.8 eV), and C–O_x (286.7 eV)) and O bonds (Si–O–Si (531.7 eV), O–Si–O (532.2 eV), and C–O_x (533.6 eV)). The binding energy values of these peaks matched standard references.^34,35 The appearance of Si–O–Si and Si–C peaks in the annealed samples confirmed the covalent bonding of silicone molecules with the bare Si surface upon annealing.

Fig. 3 Drop velocities and contact angles of 10 μL water drops upon removing (by washing) the lubricant from non-annealed and annealed slippery surfaces.

Fig. 4 XPS core spectrum of Si 2p, C 1s and O 1s of bare and annealed samples.

These covalently bonded silicone molecules make the Si surface hydrophobic as well as oleophilic, which provides the necessary condition for a stable slippery surface. Contact angle hysteresis measurements were also taken on the annealed slippery surfaces. The hysteresis was determined by measuring the advancing and receding contact angles while increasing and decreasing the water drop volume. Due to the slipperiness of the surface, the three-phase contact line of a water drop was able to move easily upon increasing and decreasing its volume, thus resulting in very low hysteresis (Δθ ∼ 2°). This observation confirmed the lubricated surface to be homogeneous, defect free and very smooth.

The slipperiness of annealed surfaces degrades if many water drops are dispensed through them. Therefore, we tested the stability of the slippery surfaces subjected to dispensed water drops, and the results are summarized in Fig. 5. Here, the slipperiness of the surface was investigated after 50 mL volume of water (for 10 μL drops) was dispensed dropwise onto the surface. Contact angle hysteresis was also measured along with slip velocity to predict any change in the surface energy and morphology. Slip velocity decreased monotonously after each step, which is an indication of the removal of the lubricant layer responsible for the slipperiness of the surface. After dispensing about 800 mL of water, the slip velocity decreased to zero, indicating an almost complete removal of the lubricant layer.

Fig. 5 Durability test of the slippery surfaces involving measuring the slip velocity (black squares, left Y-axis) and contact angle hysteresis (red squares, right Y-axis) as a function of the amount of water dispensed.

Varanasi et al. showed that water drops deposited on a solid surface coated with silicone oil are cloaked with a thin layer of oil.¹² The condition of the cloaking (when a thin layer of oil covers water drops) is governed by positive spreading coefficient of oil on water, i.e., S_ow = γ_w − γ_wo − γ_o, which was observed to be 8.5 mN m⁻¹ for the water and silicone oil system. This result indicates that the drops of water on these silicone oil-coated surfaces were always cloaked with a thin layer of the oil. As drops of water slide on the surface, silicone oil is also removed, resulting in the thinning of the lubricating film and reduction of the slip velocity. We found that this degradation in slipperiness depended on the volume of the dispensed drops of water. The smaller the volume of the dispensed drops of water, the greater will be the degradation in the slipperiness due to the larger surface-to-volume ratio of the smaller drops. Fig. 6 shows the effect of the volume of the dispensed drop on the durability of the slippery surfaces. Using 10 μL drops, a complete degradation of the slipperiness was observed after dispensing a total volume of 800 mL of water, whereas using 200 μL drops yielded an only 10% reduction in drop velocity for the same total amount of dispensed water.

Fig. 6 A durability test of slippery surfaces as a function of total dispensed water volume for three different volumes of drops.

Therefore, a total degradation of the slipperiness would probably require tens of litres of water. For a continuous stream of water, since the water could not become covered with oil, the slipperiness of the surfaces were found to be completely unaffected. When the slipperiness of a surface is completely or partially deteriorated, either due to water flow or some other reason, it can be quickly recovered upon re-coating the surface with the lubricant. For samples that had already been annealed, another annealing is not necessary in this case since the hydrophobicity is not affected. We also checked that these surfaces were slippery not only for water but also for other silicone oil–immiscible liquids such as glycerol, ethanol and ethylene glycol, and they showed different static contact angles and slip velocities.

Conclusions

Hydrophilic Si surfaces coated with silicone oil as a lubricating fluid showed very poor slipperiness since drops of water dispensed on these surfaces sank inside the lubricating layer immediately. To prevent the sinking of the drops of water, the Si surface has to be hydrophobic as well as oleophilic, which could be obtained by annealing the Si surfaces coated with silicone oil. Annealing at 150 °C for 90 min provided the optimized slippery surface as assessed by slip velocity, contact angle and contact angle hysteresis. Chemical modification of the lubricated Si surface upon annealing was confirmed by the observation of a systematic decrease of the slip velocity but not of the contact angle when this surface was washed in a surfactant solution. Also, the slippery surfaces were found to be degraded when many drops of water were made to slide on them. This degradation was due to the drops of water being cloaked with silicone oil, which slowly removed the lubricating oil during sliding. Using large drops or a continuous stream of water prevented the degradation due to a diminished or lack of removal of the lubricating fluid. When the slippery behavior is degraded, however, it can quickly be restored by recoating the lubricant onto the surface.

Acknowledgements

This research work was supported by Hindustan Unilever Limited, India and DST, New Delhi through its Unit of Excellence on Soft Nanofabrication at IIT Kanpur.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23140j

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