Weiyan
Yu
ab,
Zhongying
Ji
ab,
Yang
Lyu
b,
Xudong
Sui
ac,
Jingcheng
Hao
*bd and
Lu
Xu
*ab
aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail: xulu@licp.cas.cn
bShandong Laboratory of Advanced Materials and Green Manufacturing at Yantai, Yantai 264006, China. E-mail: jhao@sdu.edu.cn
cInstitute for Engineering Design and Product Development, Tribology Research Division, TU Wien, Vienna 1060, Austria
dKey Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials (Ministry of Education), Shandong University, Jinan 250100, China
First published on 7th June 2024
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.
New conceptsWe 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. |
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.
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 (VW%, 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. 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. 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 ∼0.1516,29,30,49 (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. 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.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh00364k |
This journal is © The Royal Society of Chemistry 2024 |