Versatile Pickering emulsion gel lubricants stabilized by cooperative interfacial graphene oxide-polymer assemblies

Abstract

Although a large number of water- and oil-based gel lubricants have found extensive potential applications in industrial and biomedical fields, developing new-type emulsion-based gel lubricants that may effectively integrate their characteristics and preponderances remains a significant challenge. Here a water-in-oil Pickering emulsion gel lubricant that is able to combine the high colloidal stability of traditional Pickering emulsions, the good swelling and corrosion resistance of oil-based gel lubricants, and the high cooling capacity of water-based gel lubricants prepared from a binary mixture of aqueous graphene oxide (GO) dispersion and diamino-functionalized polydimethylsiloxane oil solution in a broad concentration, pH, and water volume fraction range is reported. It can provide favourable lubrication for the Si₃N₄/steel and Si₃N₄/silicone tribopairs either in air or under water owing to the formation of a sturdy adsorbed oil film and ball-bearing actions of the GO particles. It can also be printed into various colourful 2D and 3D geometries upon direct extrusion into water, thanks to its water-in-oil nature and inherent shear-thinning and thixotropic properties, which further shows good prospects in underwater operations and artificial biomimetic organs. Our study may provide new insights into the design and preparation of novel semi-solid materials for diverse industrial, engineering, and biomedical applications.

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New concepts

We illustrate a new type of semi-solid lubricant called “Pickering emulsion gel lubricant”. Different from traditional oil- and water-based gel lubricants, the Pickering emulsion-based gel lubricants prepared from a binary mixture of aqueous graphene oxide (GO) dispersions and diamino-functionalized polydimethylsiloxane oil solutions in a broad concentration, pH, and water volume fraction range were capable of integrating the characteristics and preponderances of both semi-solid lubricants including the non-flowability, good thixotropic properties, and shear-thinning behaviour and water-in-oil Pickering emulsions such as high colloidal stability, good water absorption resistance, and effective anti-corrosion. As a consequence, they can provide favourable lubrication for metallic, ceramic, and silicon-based materials either in air or under water due to the formation of a high-strength physically adsorbed oil film and the rolling and repairing actions of GO nanosheets, which allows them to find extensive potential applications in not only mechanical engineering but also marine or navy industry. They can also be processed into a variety of colourful patterns and configurations using 3D printing techniques, which further show good prospects in underwater operations and repairing as well as fabricating artificial organs for in vitro biomedical applications. Our study may provide new insights into materials science and engineering.

1. Introduction

One of the major challenges in industrial and engineering fields is to validly overcome the friction and wear between two sliding moving parts of both large-scale equipment and a small-sized device used for aerospace, marine engineering, medical apparatus, precision machine tools, and so on. It is estimated that friction and wear account for nearly 30% of the total primary energy consumption and approximately 80% of the total machinery component failure worldwide every year.^1,2 To address such crucial issues, different types of lubricants including various gaseous, liquid, solid, and semi-solid lubricants have been developed to minimize the adverse effects of friction and wear.^3–10 Of the lubricants mentioned above, semi-solid lubricants (e.g., gel lubricants) are advantageous due to their non-flowability, anti-creeping and anti-irradiation characteristics, good thixotropic properties, high surface adhesion, and on-demand reversible transitions between liquid and solid states upon exposure to external stimuli.^9–13 As a consequence, gel lubricants, which are usually prepared by encapsulating lubricating base oils or water and different lubricant additives into a crosslinked polymeric or supramolecular network, have drawn particular attention in the past few decades.^{10–12,14–18} Moreover, it is worth noting that a gel lubricant for marine engineering and navy industry purposes should also possess effective underwater lubrication, corrosion resistance, and anti-swelling behavior.^19–22 A gel lubricant suitable for under-water 3D printing may also exhibit good prospects of being manufactured into not only devices and machinery components with fruitful shapes and geometries for either maintaining the normal operation of submerged instruments or helping repair the damaged machines and equipment^23–25 but also various biomimetic artificial organs such as cartilages, eyeballs and digestive systems for potential in vitro biomedical applications,^26,27 even though 3D printing is not an obligatory capability for a gel lubricant.

Nevertheless, although different kinds of oil- and water-based gel lubricants have found a broad range of tribological applications, it is equally important to develop novel emulsion-based gel lubricants that might be able to combine the advantages of oil-based gel lubricants in their excellent corrosion and swelling resistance and the superiority of water-based gel lubricants in their high cooling efficiency by effectively dispersing and crosslinking lubricating base oil (or water) droplets in water (or base oil) using amphiphilic surfactants, polymers, and particles.^6,28–30 To date, emulsion gels with the characteristics of both conventional gels and emulsions have been extensively applied for preparing functional materials for food chemistry, chemical engineering, and pharmaceutics owing to their tailorable and controllable microstructures and rheological properties.^19,31–33 By contrast, their prospects as functional lubricants have hardly been illustrated. In comparison with traditional liquid emulsions, semi-solid emulsion gels usually gain splendid colloidal stability against droplet coalescence, phase separation, and Ostwald ripening during continuous long-term storage.³¹ This makes them potentially ideal candidates for tribological applications after a rational design of their chemical compositions and internal microstructures, since a lubricant is normally repeatedly used until a complete deficiency and dysfunction is reached in practical applications.⁶

Preparation of emulsion gels with desired performances generally requires precise engineering of the interdroplet interactions, and the interactive forces can be either repulsive or attractive.^31,34,35 When the repulsive forces are dominant (i.e., the so-called “repulsive emulsion gel”), a high volume fraction of the dispersed phase (usually no less than 70%) is necessary to form stable gels,^36,37 and this may restrict the design and tuning of the lubricity of resulting gels. A few examples of emulsion gels constructed by attractive interactions between adjacent droplets (i.e., the so-called “attractive emulsion gel”) have been reported in recent years, but their types are mainly limited to oil-in-water.^38–40 The choice of water as the continuous phase may suffer the risk of leading to deteriorated lubricity, relatively low corrosion resistance, and unsatisfactory anti-swelling properties, which may confine their applications in underwater conditions such as marine environments.^6,18,23 In a word, the preparation of attractive water-in-oil emulsion gels capable of providing both excellent in-air and under-water lubrication and 3D printing into various configurations remains an appealing challenge.

In this study, we demonstrate that attractive emulsion gels with high lubricity can be fabricated by crosslinking water droplets in dimethyl silicone oil (DSO) on the basis of the interfacial assembly between hydrophilic graphene oxide (GO) nanosheets and oil-soluble diamino-functionalized polydimethylsiloxane (NH₂-PDMS-NH₂). The cooperatively assembled nanoparticle–polymer complex at the oil/water interface (usually termed as the “nanoparticle surfactant”)^41–43 can not only act as a binder for neighbouring water droplets but also inhibit droplet coalescence by the formation of a jammed interfacial film, thus allowing the production of Pickering emulsion gels in a relatively broad pH, concentration, and water volume fraction range. Owing to the generation of a sturdy physically adsorbed oil film and the rolling and repairing actions of GO particles, the gels can provide potent lubrication for both the Si₃N₄/steel and Si₃N₄/silicone tribopairs. Taking advantage of their strong adhesion and water-in-oil nature, the gels can effectively suppress their swelling and electrochemical corrosion of metal materials, making them able to maintain good lubricity when submerged in water. Their inherent shear-thinning and thixotropic behaviour also enables them to be printed into a variety of 2D patterns and 3D structures under water. Our work may provide new insights into the design and development of new-generation general-purpose semi-solid lubricants that can be applied to a diverse range of systems in materials science, mechanical engineering, and marine industry.

2. Results and discussion

The attractive water-in-oil Pickering emulsion gel lubricant was prepared by directly mixing an appropriate amount of aqueous GO dispersions with NH₂-PDMS-NH₂ oil solutions at ambient temperature under vortexing. The GO particles used for assembling Pickering emulsion gels were all ultra-thin nanosheets with an average lateral size of ∼1 μm as shown in Fig. S1a (ESI†). Fig. S1b (ESI†) shows that a pure water droplet deposited onto a solid substrate coated with a thin film of GO particles yielded a low contact angle of less than 29°, indicating a relatively high hydrophilicity of the nanosheets. In an optimal pH range (i.e., 4 to 10), the GO nanosheets could cooperatively assemble with NH₂-PDMS-NH₂via electrostatic interactions at the water/DSO interface and bridge the neighbouring water droplets together to form Pickering emulsion gels as schematically illustrated in Fig. 1a. Please refer to Fig. 1b and Fig. S2 (ESI†) for representative photographs and internal self-assembled structures of the gels, respectively. Fig. 1c and Fig. S3 (ESI†) show that a gel stained with a hydrophilic dye sodium fluorescein only yielded fluorescent signals in the dispersed aqueous phase, which was indicative of the formation of water-in-oil emulsion gels. Fig. S4 (ESI†) demonstrates that the as-formed Pickering emulsion gels showed no sign of phase separation after being stored at room temperature for 30 days, corresponding to a relatively high colloidal stability. The darkening of the colour of the gels over time was likely due to an increased packing density of the water droplets as displayed in Fig. S2 (ESI†).

Fig. 1 (a) Schematic illustration of the formation mechanism of the attractive water-in-oil Pickering emulsion gel lubricant. (b) Photographs of the GO/water/NH₂-PDMS-NH₂/DSO mixture at different pH values. (c) Fluorescent micrograph of a Pickering emulsion gel stained with sodium fluorescein at pH 7. (d) Equilibrium interfacial tensions of the water/DSO interface covered with different materials at pH 7. (e) Snapshots of aqueous GO droplets (pH = 7) immersed in NH₂-PDMS-NH₂ oil solutions upon slowly withdrawing the internal dispersed phase under both pendant drop (top) and constrained sessile drop (bottom) configurations. (f) Measurements of the squeezing and detaching behaviour of two freshly prepared aqueous GO droplets (pH = 7) in an NH₂-PDMS-NH₂ oil solution. Insets show snapshots of the experimental droplets. c_GO = 0.3 mg mL⁻¹. c_NH2-PDMS-NH2 = 10 mg mL⁻¹. Water volume fraction (V_W%) = 50%. T = 25 °C.

Fig. 1d shows that both pure GO and NH₂-PDMS-NH₂ did not exhibit high interfacial activities independently as they can only reduce the water/DSO interfacial tension from ∼37 mN m⁻¹ to ∼28 and ∼20 mN m⁻¹, respectively, after spontaneous adsorption at pH 7. Fig. S5 (ESI†) illustrates that the DSO/water mixture stabilized by GO independently showed prominent phase separations after only 12 h, demonstrative of an unsatisfactory interfacial activity of the particle emulsifier. Mixing the water-soluble nanosheets and oil-soluble polymer ligands at the interface resulted in a relatively highly interfacial active cooperative assembly (usually termed as the “nanoparticle surfactant”)^41–43 with an equilibrium interfacial tension of ∼10 mN m⁻¹ that contributes to the formation of emulsion gels.^31,40 Fig. S6a (ESI†) further shows that the interfacial tension of the GO/NH₂-PDMS-NH₂ nanoparticle surfactant declined as a function of the pH value and a relatively high interfacial tension above 20 mN m⁻¹ was obtained at pH 12 owing to a reduced interfacial activity from the deprotonation of both GO nanosheets and NH₂-PDMS-NH₂ ligands,^44,45 which consequently hinders an effective complexation between the nanoparticles and polymers at the interface and the formation of emulsion gels. This can be confirmed by a gradual increment in the interfacial tension of the GO solution/DSO interface and the water/NH₂-PDMS-NH₂ solution interface upon increasing the pH values (Fig. S6b and c, ESI†). As one can see in Fig. 1e and Fig. S7 (ESI†), the withdrawal of the dispersed aqueous phase from either a pendant drop or a “constrained” sessile drop led to significant wrinkling or buckling of the water/oil interface in a broad pH range from 2 to 10, which can be attributed to jamming of the nanoparticle surfactant at the interface and the formation of a “solid-like” interfacial film that can effectively prevent droplet coalescence and result in highly stable emulsion gels.^31,41–43 In comparison, a pH 12 GO droplet in polymer solution, a pure water droplet in NH₂-PDMS-NH₂ solution, and a pH 7 GO droplet in pure oil all cannot yield apparent wrinkles during an identical compression process (Fig. S7 and S8, ESI†), which verifies a crucial role of interfacial jamming of the nanoparticle surfactant in inducing the formation of a Pickering emulsion gel. To explore the reason why a pH 2 nanoparticle surfactant, which can both provide a low interfacial tension (∼2 mN m⁻¹) and a jammed interfacial film, can hardly result in Pickering emulsion gels, zeta potential measurements of pure GO particles in aqueous dispersions were carried out as shown in Fig. S9 (ESI†). It can be seen that the nanosheets at pH > 4 all possessed a relatively high thermodynamic stability against aggregation and coagulation as indicated by their zeta potential values of no more than −42 mV.^15,23 A relatively high potential value of ∼−21 mV was obtained at pH 2, suggesting relatively low thermodynamic stability of the nanoparticles and their tendency to undergo coalescence in the aqueous phase, thereby confining an effective crosslinking between water droplets.^15,23,31,40

Fig. 1f and Movie S1 (ESI†) illustrate the morphological changes of two aqueous GO droplets immersed in NH₂-PDMS-NH₂ oil solutions during a continuous squeezing and detaching process. One can see that the weight of the top droplet gradually reduced as the bottom droplet slowly approached and squeezed the top droplet. The weight of the top droplet started to re-increase and the bottom droplet became retracted when the repulsive force reached ∼−14.7 μN. The results further confirm the effectiveness of a jammed interfacial film of the GO/NH₂-PDMS-NH₂ complex in simultaneously suppressing the droplet coalescence and facilitating the droplet crosslinking during the formation of Pickering emulsion gels as schematically illustrated in Fig. 1a. The attractive force between the two contacting water droplets was found to be ∼1 μN from the dramatic decrease of the weight of the droplet at the detaching point and the detaching work of ∼0.3 nJ was determined from the integration of the region covered by the fitting curves.³¹

Fig. 2a–c and Fig. S10–S12 (ESI†) show that such stable water-in-oil Pickering emulsion gels can be produced in a wide particle (≥0.2 mg mL⁻¹) and polymer (≥2 mg mL⁻¹) concentration and water volume fraction (V_W%, between 30% and 70%) range. Fig. 2d and e demonstrate that the non-flowable Pickering emulsion gels could yield an elastic modulus (G′) higher than their viscous modulus (G′′) in a relatively broad shear frequency range from 0.1 to 100 Hz at pH values from 4 to 10 and water volume fractions from 30% to 70%, which can be correlated to their semi-solid nature and a relatively good shear stability.^15,17,31,46 Fig. S13a and b (ESI†) illustrate that the emulsion-based materials could yield a relatively broad linear viscoelastic region between 0.01% and 1%, which further verifies their gel-like behaviour. Their yield point and flow point were found to increase moderately from 0.75 and 5 Pa to 1.26 and 11.7 Pa, respectively, as the internal pH value increased from 4 to 10 and increased remarkably from 0.23 and 0.54 Pa to 1.64 and 15.6 Pa, respectively, as the water volume fraction increased from 30% to 70% as shown in Fig. S13c and d (ESI†). The results thus indicate the feasibility of achieving tuneable viscoelasticity and mechanical performance of the Pickering emulsion gels by simply changing their internal water-to-oil volume ratios. Fig. 2f displays that the gels owned a relatively high shear viscosity of >100 Pa s under a relatively low shear rate of 0.1 s⁻¹ and the viscosity decreased by nearly two orders of magnitude upon exposure to a relatively high rate of 10 s⁻¹. Such prominent changes in shear viscosity upon alternately varying external shear rates could be repeated for several cycles, suggesting the excellent thixotropic properties of the Pickering emulsion gels. This therefore ensures not only a good deformation-recovery and creeping-recoil capability essential for working as semi-solid lubricants but also a satisfactory rheological performance for acting as inks for 3D printing into various patterns and shapes.^23,47,48

Fig. 2 (a) Phase diagram showing the concentration of GO and NH₂-PDMS-NH₂ required for producing a Pickering emulsion gel. Insets show representative photographs of liquid emulsions and emulsion gels prepared with different particle and polymer concentrations at pH 7 and a V_W% of 50%. (b) Photographs of the liquid emulsions and semi-solid emulsion gels prepared with different water volume fractions. c_GO = 0.3 mg mL⁻¹. c_NH2-PDMS-NH2 = 10 mg mL⁻¹. pH = 7. (c) Fluorescent micrographs of Pickering emulsion gels having a V_W% of 30% (left) and 70% (right), respectively. c_GO = 0.3 mg mL⁻¹. c_NH2-PDMS-NH2 = 10 mg mL⁻¹. pH = 7. Scale bar = 100 μm. Rheological measurements of the Pickering emulsion gels prepared with (d) a constant V_W% of 50% and different pH values and (e) a fixed pH value of 7 and different V_W%. The testing shear strain was 0.5%. c_GO = 0.3 mg mL⁻¹. c_NH2-PDMS-NH2 = 10 mg mL⁻¹. (f) Viscosity of the Pickering emulsion gels upon alternately changing shear rates between 0.1 and 10 s⁻¹. c_GO = 0.3 mg mL⁻¹. c_NH2-PDMS-NH2 = 10 mg mL⁻¹. pH = 7. V_W% = 50%. T = 25 °C.

Fig. 3a and Fig. S14 (ESI†) show that the Pickering emulsion gel could provide a coefficient of friction (CoF) of 0.09 ± 0.04 for the Si₃N₄/steel tribopair under an applied normal load of 30 N and a sliding velocity of 80 mm s⁻¹ in air, indicative of a favourable lubricity. By contrast, pure DSO, the NH₂-PDMS-NH₂ oil solution, and the aqueous GO dispersion yielded CoF values of 0.13 ± 0.05, 0.12 ± 0.05 and 0.50 ± 0.04, respectively, suggesting an improved lubricity of the resultant Pickering emulsion gel compared to the base oil and that the potent lubrication of the gel was mainly derived from the dispersed oil phase. Fig. S15a (ESI†) illustrates that the gel lubricants could maintain a CoF of ∼0.13 even when the external normal load is as high as 200 N, indicating a good load-bearing capacity that is crucial for metalworking and heavy-equipment lubrication,^6,29,47 although the CoF of the gel lubricants generally increased as a function of the applied normal load. Their lubricity was relatively independent of the applied sliding velocity and their pH values and water volume fractions as evidenced in Fig. S15b and S16 (ESI†). Fig. S17 (ESI†) shows the variations in the CoF of the Pickering emulsion gels after being continuously used for 12 h under 10 N and 80 mm s⁻¹. It can be seen that the gels could maintain a very low and stable CoF of ∼0.02 without prominent increments and fluctuations throughout the whole test, which suggests an excellent long-term lubrication of the semi-solid lubricants that is necessary for putting into practical applications. OM observations in Fig. S18 (ESI†) depict that a large area of crosslinked aqueous droplets can still be observed inside the gel lubricants, although their size exhibited a certain reduction. The results thus verify the satisfactory stability of our prepared Pickering emulsion gels against external friction forces, which is necessary for their use as functional semi-solid lubricants.

Fig. 3 (a) CoF of the Si₃N₄/steel tribopair after being lubricated with the Pickering emulsion gel and its different compositions at 30 N and 80 mm s⁻¹. (b) Scanning electron microscopy (SEM) observations, (c) 3D surface profiles, and (d) wear volumes of the steel substrates lubricated with the different materials. Scale bars = 200 μm. (e) ECR measurements of the Pickering emulsion gel lubricant and its different compositions. (f) Schematic illustration of the lubrication mechanism of the Pickering emulsion gel lubricant. c_GO = 0.3 mg mL⁻¹. c_NH2-PDMS-NH2 = 10 mg mL⁻¹. pH = 7. V_W% = 50%. T = 25 °C.

Fig. 3b–d and Fig. S19 (ESI†) uncover that the steel disc after being lubricated with the Pickering emulsion gels yielded a small and shallow wear scar with a total volume of no more than 1 × 10⁶ μm³, which further certifies a good tribological performance of the gel lubricant. Whereas the metal substrates lubricated the pure oil, the aqueous GO dispersion, and the NH₂-PDMS-NH₂ oil solution, displaying much larger wear volumes of ∼2.2 × 10⁶ μm³, ∼3.1 × 10⁶ μm³, and ∼1.2 × 10⁷ μm³, respectively, and much more prominent abrasive scars, grains, and tracks on their surfaces. The results thus verify the significance of assembling different compositions into a Pickering emulsion gel to achieve maximum lubricity and the DSO lubricating base oil played a dominant role in providing an effective semi-solid lubrication. We also have studied the cooling performance of pure water, DSO and the Pickering emulsion gels by measuring their thermal conductivities. As shown in Fig. S20 (ESI†), deionized water possessed an excellent cooling capacity as indicated by its relatively high thermal conductivity at ∼0.58 W m⁻¹ k⁻¹, whereas the base oil was found to have an unsatisfactory cooling efficiency due to its relatively low conductivity at around 0.16 W m⁻¹ k⁻¹. After assembling the water and oil into Pickering emulsion gels using the polymer-nanosheet complex, a favourable cooling property with thermal conductivity values of no less than 0.39 W m⁻¹ k⁻¹ could be obtained. The cooling efficiency of gels can be slightly improved as a function of the water content. The results therefore certify our “proof-of-concept” of effectively combining the high friction and wear resistance of traditional oil-based lubricants and the high cooling capability of conventional water-based lubricants into one new-type emulsion-based gel lubricant.

The good lubricity of the gel lubricant was likely owing to, on the one hand, the oil component forming a sturdy physically adsorbed film on the surface of the tribopair as indicated by its relatively large electrical contact resistance (ECR) value at ∼0.15^16,29,30,49 (Fig. 3e) that can validly avoid direct contact between the ceramic ball and metal disc, and on the other hand, the GO nanosheets acting as a ball-bearing between the two contacting surfaces that can simultaneously turn partial of the sliding friction into rolling friction (i.e., the rolling action) and fill in the scratches and depressions on a worn surface to help repair the mechanical damage from the continuous friction (i.e., the repairing action).^15,17,23 The rolling and repairing actions of the GO nanosheets may also account for the enhanced lubrication of the Pickering emulsion gels in comparison with the lubricating base oil, although its adsorption capability on metal surfaces was relatively lower as indicated by its ECR value at ∼0.08 Ohm. The corresponding lubrication mechanism of the Pickering emulsion gels is schematically illustrated in Fig. 3f.

Thanks to the beneficial design of such an attractive water-in-oil Pickering emulsion gel lubricant that leads to not only a strong under-water adhesion on solid surfaces (e.g., steel disc, silicone wafer and glass bottle bottom) as shown in Fig. S21 (ESI†) but also a good anti-swelling performance making it capable of sustaining its pristine shape when submerged in water for more than 3 months as displayed in Fig. S22 (ESI†); effective under-water lubrication for both the Si₃N₄/steel and Si₃N₄/silicone tribopairs could be attained as demonstrated in Fig. 4a–f. The resultant CoF of both at ∼0.08 was comparable to those obtained under in-air conditions. An optimum friction resistance of the gel lubricant could be achieved at 10–30 N, 160 mm s⁻¹ and 5 N, 80–160 mm s⁻¹ for the two tribopairs with a minimum CoF of ∼0.022 and ∼0.031, respectively. The Pickering emulsion gel was still able to provide a beautiful load-carrying capability for the Si₃N₄/steel tribopair as a relatively low CoF of ∼0.14 could be achieved at an external normal load of 200 N. The high under-water lubricity of the gel lubricant may expand its potential applications in the fields of water transportation, marine engineering, navy industry, and biomedical lubrication.^19–22

Fig. 4 Comparison between the in-air and under-water lubrication of the Pickering emulsion gel for the (a) Si₃N₄/steel and (d) Si₃N₄/silicone tribopairs under 30 N and 80 mm s⁻¹ at 25 °C. Effect of the applied normal load (b) and (e) and sliding velocity (c) and (f) on the CoF of the Si₃N₄/steel and Si₃N₄/silicone tribopairs lubricated with the Pickering emulsion gel, respectively, under water at 25 °C. (g) Surface structure and topography of the pristine steel block, with the steel block directly immersed in deionized water for 10 h at 50 °C, pre-coated with a small amount of Pickering emulsion gels and then submerged in the corrosive media. Scale bars = 10 μm. c_GO = 0.3 mg mL⁻¹. c_NH2-PDMS-NH2 = 10 mg mL⁻¹. V_W% = 50%. pH = 7 (unless specially noted in g).

Fig. 4g further certifies a splendid anti-corrosion property of the Pickering emulsion gel lubricant that is essential for underwater operations.^19–22 One can see that a pure steel block after being immersed in deionized water for about 10 h at 50 °C exhibited profound colour changes with a large area of surface erosions and damages detected in the SEM images. In contrast, steel blocks pre-coated with a small amount of the Pickering emulsion gels at different pH values still preserved a surface topography nearly identical to that of the pristine one after being submerged in water for the same period. The anti-corrosion efficiency of the gel coatings could be retained for a prolonged period of 72 h as shown in Fig. S23 (ESI†). Electrochemical measurements in Fig. S24 (ESI†) illustrate that a bared steel working electrode directly submerged in 3.5 wt% NaCl solution yielded a corrosion current and potential of ∼1.38 × 10⁻⁵ A cm⁻² and ∼0.55 V, respectively. A prominent semicircular curve with a relatively small radius can be observed in the Nyquist plot. A |Z| value of ∼10⁵ Ohm cm² and a body phase angle of <18° at 0.01 Hz can also be obtained. In comparison, an electrode covered with a small amount of the Pickering emulsion gel on the top (Fig. S25, ESI†) showed no evident polarization curves and the radius of the resultant Nyquist plots was too large to be included in the measuring −Z′′ range of our instrument. A remarkable increment in both |Z| and the body phase angle at 0.01 Hz, which usually serves as an indicator of the corrosion-resistant efficacy of a material,^50,51 to ∼10⁶ Ohm cm² and ∼60° can also be detected. Similar findings can be discovered upon directly putting the electrode in the non-conductive DSO. The results thus demonstrate that our prepared water-in-oil Pickering emulsion gel lubricant can function as an efficient physical barrier for preventing any potential contacts between the steel working electrode and saline media and consequently result in a potent anti-corrosion performance.

Fig. 5a and Fig. S26 (ESI†) validate an inherent shear-thinning behaviour of the Pickering emulsion gel lubricant as its shear viscosity overall linearly decreased as a function of the external shear rate. This property in combination with the favourable thixotropic performance illustrated in Fig. 2f could enable the gel lubricant to be an ideal ink for high-precision 3D printing into diverse high-resolution and complicated configurations and geometries under water.^23,52,53Fig. 5b and c, Fig. S27 and Movie S2 (ESI†) exhibit the printed 2D spiral and snake-shaped patterns and more sophisticated and colourful structures such as word-arts spelling the abbreviation of our institute (i.e., “LICP”), a jumping dolphin, a bear paw, a notched apple, the Eiffel Tower, and a bubble-blowing fish using the Pickering emulsion gel ink stained with dyes. The gels can also be moulded into fruitful 3D shapes including an apple, a tower, a cat with different poses, a bone, a bulb, and a rocket upon direct extrusion into water as displayed in Fig. 5d, Fig. S28 and Movie S3 (ESI†). The 3D printed pattern was able to maintain its entity without noticeable collapse or swelling after being submerged in water for 30 days (Fig. S29, ESI†), corresponding to a high structural stability. The results therefore suggest that the attractive water-in-oil Pickering emulsion gels may also show good prospects of not only being processed into various self-lubricating machinery components and parts for achieving efficient operation of an under-water device and equipment or helping repair a damaged machine but also being manufactured into different biomimetic artificial organs such as cartilages, eyeballs, and digestive systems that are usually viscoelastic and lubricative for in vitro biomedical applications.^6,7 Together with their effective in-air lubrication, our study may provide a new paradigm for the design and development of novel semi-solid materials for industrial, engineering, and biomedical fields.

Fig. 5 (a) Variations in the shear viscosity of the Pickering emulsion gels with different internal pH values as a function of the applied shear rate. (b) Schematic illustration of under-water 3D printing of the Pickering emulsion gels and the resultant 2D spiral pattern. (c) Various printed colourful 2D structures using the Pickering emulsion gel inks. (d) Printed different 3D configurations from direct extrusion of the Pickering emulsion gels into water. c_GO = 0.3 mg mL⁻¹. c_NH2-PDMS-NH2 = 10 mg mL⁻¹. V_W% = 50%. pH = 7 (unless specially noted in a). T = 25 °C.

3. Conclusions

In conclusion, we have developed a new type of semi-solid lubricant that is able to combine the characteristics of both gel lubricants and Pickering emulsions based on a nanoparticle surfactant cooperatively assembled from water-soluble GO nanosheets and oil-soluble NH₂-PDMS-NH₂ ligands at the water/DSO interface. By generating a jammed interfacial film with high interfacial activity, the nanoparticle surfactant can produce stable water-in-oil Pickering emulsion gels in a broad pH, concentration, and water volume fraction range by facilitating an effective crosslinking between neighbouring water droplets and suppressing droplet coalescence simultaneously. Owing to the formation of a high-strength oil film on solid substrates and the rolling and repairing actions of the GO particles, the Pickering emulsion gels could provide potent lubrication for both the Si₃N₄/steel and Si₃N₄/silicone tribopairs either in air or under water, which may allow it to find a wide range of potential applications in systems of mechanical engineering and marine or navy industry. Taking advantage of their water-in-oil nature and inherent shear-thinning and thixotropic properties, the gels can be imparted with excellent swelling and corrosion resistance and the capability of 3D printing into various patterns and shapes upon direct extrusion into water, which may further show good prospects of being manufactured into soft machinery components for under-water operations and biomimetic artificial organs for in vitro biomedical applications. Our study may provide new insights into preparing functional gels for materials science and engineering.

Author contributions

J. H. and L. X. supervised the research. W. Y. prepared the samples and characterized them. W. Y. and X. S. performed the friction and wear tests. W. Y., Z. J. and L. Y. performed the 3D printing tests. L. X. drafted the manuscript. J. H. and L. X commented on and revised the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (22202220 and 22302215), the Talents Program of Chinese Academy of Sciences (E30247YB), the Talents Program of Lanzhou Institute of Chemical Physics (E0SX0282), the National Natural Science Foundation of Shandong Province (ZR2022QB190) and the Innovative Research Funds of Shandong Laboratory of Yantai Advanced Materials and Green Manufacturing (E1R06SXM07, E1R06SXM09 and E2R06SXM14).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh00364k

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