A dual-responsive microemulsion with macroscale superlubricity and largely switchable friction†

Siwei Chen ^ab, Hong Sun ^c, Jian Liu ^a, Jinyu Wang ^b, Hongsheng Lu ^b, Jingcheng Hao *^de, Lu Xu *^ae and Weimin Liu ^ae
aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail: xulu@licp.cas.cn
^bCollege of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China
^cSchool of Chemistry and Pharmaceutical Engineering, Shandong First Medical University & Shandong Academy of Medical Sciences, Tai’an 271016, China
^dShandong Laboratory of Advanced Materials and Green Manufacturing at Yantai, Yantai 264006, China
^eKey Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials (Ministry of Education), Shandong University, Jinan 250100, China. E-mail: jhao@sdu.edu.cn

First published on 5th March 2024

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New concepts We demonstrate a proof-of-concept of creating a smart liquid superlubricant with inherent dual responsiveness and anti-freezing performance building on a microemulsion system comprising n-hexane, water, surfactant DDACe ((C12H25)2N+(CH3)2[CeCl4]−) and ethylene glycol. Although responsive microemulsions have found extensive applications in industrial fields, there have been no reports on responsive microemulsions exhibiting either macroscale superlubrication or two convertible friction states where their coefficients of friction (CoF) can differ by more than one order of magnitude. Moreover, although traditional liquid superlubricants can provide ultralow friction and wear, effective control over the friction between two contacting surfaces is crucial for achieving accurate control of the operation and efficiency of instruments and also fabricating smart devices. Here we show that our prepared microemulsion can not only provide near-zero friction and abrasion for metallic materials in a broad temperature range but also abruptly and repeatedly switch its CoF by ∼25 fold based on thermally reversible microemulsion-to-emulsion transitions. Its potent freezing resistance shows potentials in working under harsh conditions (e.g., aerospace and polar environment). Its magneto-responsiveness shows good prospects in achieving continuous lubrication. Together with its ultrahigh stability allowing long-term storage, our study is envisioned to provide new insights into materials science and engineering.

1. Introduction

Friction, which hinders the relative motion between two contacting surfaces, leads to a large number of undesirable energy dissipation and machinery component damage worldwide every year,^1,2 especially for metallic materials that have been substantially applied in industrial fields. To eliminate or strongly diminish any adverse impact caused by friction, materials capable of creating a quasi-frictionless environment (usually termed “superlubricants”) have received extensive attention from scientists in the fields of materials science, colloidal chemistry, mechanical engineering and tribology since the 1990s.^3–7 However, in contrast, friction is sometimes necessary for maintaining the normal operation of a mechanical or biological system.^8,9 For instance, people cannot walk, speak, catch and get dressed in the absence of friction.^8,10,11 Moreover, precise control over the friction between two mutually interacted moving parts is crucial for achieving effective control of the efficiency, accuracy and reliability of both large equipment and small devices, which is of great importance for developing advanced technologies such as microfluidic devices, smart sensors or surfaces and lab-on-chip systems.^8,12–15 Hence, creation of lubricants that are able to switch between ultralow and relatively high friction states has become one of the major challenges in materials science.

Microemulsions (MEMs) are thermodynamically stable colloidal dispersions generally consisting of water, oil, surfactants and co-surfactants. MEMs in response to external stimuli including the temperature, pH, CO₂, light irradiation and magnetic field have found widespread applications in food, cosmetics, pharmaceutics, petrochemistry, nanotechnology and other industrial fields,^16–21 owing to their ultrahigh colloidal stability, large interfacial area, high transparency, strong amphiphilicity and controllable interfacial properties. The advantages of employing stimuli-responsive MEMs as lubricants include their powerful cleaning performance, good affinity to both hydrophilic and hydrophobic surfaces, unique properties in providing lubrication and cooling simultaneously due to the coexistence of lubricating oil/additive and water, and tuneable tribological performances based on variations in their physicochemical properties.^{16,18,20,22–24} As one of the most frequently used external stimuli to trigger the variations in both properties of materials and biological signals in the human body,^16,25–28 temperature is advantageous in its remote controllability, good reversibility and no accumulation of impurities such as inorganic salts in the system, which may therefore allow in-time and in-space control of the tribological behaviour of a MEM simply, rapidly and non-invasively. In addition to effective regulation of its tribological properties by switching an applied magnetic field “on” and “off”, a magneto-responsive MEM could become “active” upon exposure to external magnets and be manipulated by them.^16,29,30 This may further enable the MEM to overcome the effect of both gravity and “flow” of the solvents that are easy to cause creeping or even leakage of the liquid lubricants during the lubrication process.^16,31,32 As a result, an effective combination of the thermal and magneto-responsiveness may lead to smart MEMs that can not only significantly change their friction and wear upon alternate heating and cooling but also provide continuous lubrication after placing in an applied magnetic field. Nevertheless, to date, there have been no reports on responsive MEMs exhibiting either macroscale superlubricity or two convertible friction states where their coefficients of friction (CoFs) can differ by more than one order of magnitude.

Here we create a thermo- and magneto-responsive MEM (Fig. 1) capable of providing not only potent superlubrication for metallic materials but also switched the CoF between ∼0.006 and ∼0.15 on the basis of a temperature-induced reversible MEM-to-EM transition using a mixture of water, n-hexane, the cationic surfactant DDACe ((C₁₂H₂₅)₂N⁺(CH₃)₂[CeCl₄]⁻) and ethylene glycol (EG). We demonstrate that the MEM can form a high-strength interfacial physical adsorption film that is able to undergo tribochemical reactions to facilitate dissipation of the mechanical energy generated during the frictional process, thus allowing the lubricant to attain a near-zero CoF in a broad temperature range between −30 and 20 °C. At relatively high temperatures (e.g., 50 °C), the tribochemical reactions were highly suppressed and the superlubricity was lost as a result of dissociation and competitive adsorption between DDACe and EG. Together with its inherent magneto-responsiveness and anti-freezing performance, such novel smart and superlubricative MEMs are anticipated to find potential applications in a diverse range of systems in materials science and chemistry.

2. Results

2.1. Preparation and characterization of magnetic DDACe MEM

Surfactant DDACe with a critical micelle concentration (CMC) and dissociation constant (β) of 0.543 mmol L⁻¹ and 0.874 (Fig. S1, ESI†), respectively, and inherently paramagnetic nature (Fig. S2, ESI†) was synthesized by a one-step coordination reaction using the surfactant DDACl ((C₁₂H₂₅)₂N⁺(CH₃)₂Cl⁻) and magnetic halide CeCl₃ (see details in the ESI†).^33,34 Mixing aqueous solutions of DDACe with EG and n-hexane at optimal concentrations and stoichiometric ratios resulted in MEMs with magneto-responsiveness, high transparency and good colloidal stability. The pseudo-ternary phase diagram in Fig. 2a summarizes the amount of DDACe solution, EG and n-hexane required for acquiring a MEM. Fig. 2b illustrates the low-strength magnetic field-triggered migration of a MEM stained with dye methylene blue. As is shown, the viscous MEM could overcome its gravitational effect and be “levitated” through air within a few seconds after introducing a static 0.25 T cylindrical magnet suspended ∼1 mm above. Such magneto-responsive properties may enable the MEM to provide unique continuous lubrication under external magnetic forces by quickly filling up the scratches and grooves on a worn surface and also overcome the influence of gravitational or centrifugal forces that may cause creeping and even leakage of liquid lubricants during the lubrication process.^31,35Fig. 2c displays the representative photographs of the MEM prepared with optimal amounts of surfactant solution, glycol and oil, whereas Fig. S3 (ESI†) shows the photographs of the EM formed by the ternary mixture at non-ideal stoichiometric ratios. Fig. 2d shows that the clear and transparent MEM did not exhibit any changes in macroscopic appearance after 6-month storage, indicating high colloidal stability. Drop tests in Fig. S4 (ESI†) suggest that the MEM type was oil-in-water, as ∼5 μL of MEM droplets collapsed rapidly upon addition into water but could maintain integrity in n-hexane.³⁶ Dynamic light scattering (DLS) data in Fig. 2e demonstrate that the n-hexane droplets dispersed in the continuous aqueous phase had a hydrodynamic diameter (2R_H) of ∼60 nm, corresponding to the formation of a MEM of the DDACe/EG/water/n-hexane mixture.¹⁸ The droplet size only exhibited slight variations after storage at room temperature for 6 months (Fig. 2f), thereby confirming high thermodynamic stability.

Owing to the presence of n-hexane having a very low freezing point and an anti-icing additive EG, the DDACe MEM was demonstrated to exhibit potent anti-freezing performance as indicated by the differential scanning calorimetry (DSC) profiles in Fig. S5 (ESI†), where the MEM gave out a transition temperature as low as ∼−66 °C. Such a property may further allow the MEM to be suitable for extreme-environment lubrication such as aerospace, polar exploration, liquid-propellant rockets and cryogenic wind tunnel,^37,38 in which conventional water-based lubricants are easily frozen due to the low temperatures. It is worthwhile to note that, as illustrated in Fig. S6 (ESI†), although a similar MEM can be produced using magnetic surfactants DDAFe ((C₁₂H₂₅)₂N⁺(CH₃)₂[FeCl₄]⁻) and DDAGd ((C₁₂H₂₅)₂N⁺(CH₃)₂ [GdCl₄]⁻) with approximate CMC and β values (Fig. S1 and S2, ESI†) or even the traditional cationic emulsifier DDACl, the resultant MEM all exhibited relatively low stability or can only maintain long-term stability within a narrow surfactant concentration range as indicated by Table S1 (ESI†). This may confine their applications as industrial lubricants.

2.2. Thermoresponsive behaviour of the magnetic DDACe MEM

Fig. 3a–f show the thermoresponsive behaviour of a DDACe MEM. It can be seen that the MEM became turbid and the 2R_H of n-hexane droplets increased from ∼60 to ∼3000 nm with the temperature gradually elevating from 20 to 60 °C, which signifies heat-induced transition from a MEM to an EM. As the temperature subsequently fell down to 10 °C, the droplet size significantly reduced to ∼40 nm and the DDACe/EG/water/n-hexane mixture became clear and transparent again, which correlates with a reversible conversion back into an MEM. The oil droplets fully regained their original 2R_H with the temperature returning to 20 °C, thus demonstrative of the good reversibility and reproducibility of such MEM-to-EM transitions.

Fig. 3g–i indicate the possible mechanism that caused phase transition. As shown, the zeta potential of n-hexane droplets lowered from ∼62 to ∼17 mV along with a stepwise temperature increment from 20 to 50 °C, corresponding to a strong attenuation of the thermodynamic stability that favours aggregation and coalescence of the oil droplets.^14,39 The temperature elevation also induced the interfacial tension of the DDACe/EG mixture to increase by nearly 25 fold, which therefore strongly improved the interfacial energy of an n-hexane/water interface that hampers the formation of a stable MEM.¹⁸ By contrast, the interfacial tension of pure DDACe did not change prominently as a function of temperature, and the value was close to that of the surfactant/glycol complex at 50 °C. The results suggest that DDACe and EG likely dissociated with each other and a large amount of EG was squeezed out from the swelling surfactant micelles at relatively high temperatures. This essentially led to the destabilization of the oil droplets and heat-driven transition from a MEM to an EM.

2.3. Superlubrication and switchable friction of a dual-responsive DDACe MEM

Fig. 4a and Fig. S7a (ESI†) show a comparison of the tribological performance and resultant average CoF values obtained from multiple reproducible measurements between steel/steel tribopairs after lubrication with n-hexane, water, EG and DDACe MEM, respectively. One can see that both water and oil exhibited relatively poor lubricity with a CoF exceeding 0.5 at a relatively high normal load pressure of 2.2 GPa (i.e., 10 N normal load) and a sliding velocity of 0.1 m s⁻¹. A satisfactory lubrication could be provided when using EG, given that a relatively low CoF at ∼0.14 could be achieved under the same frictional conditions. Mixing the three together with the surfactant DDACe led to a MEM that was able to provide superlubrication as the CoF could be reduced to as low as ∼0.006.^5–7Fig. 4b and Fig. S7b (ESI†) further show that such a near-frictionless MEM cannot be produced using surfactant DDAFe, DDAGd or DDACl instead, since the resulting CoF was elevated by nearly one order of magnitude to all around 0.06. The results thus indicate the dominant role of a [CeCl₄]⁻ counterion for achieving superlubrication. Fig. S8 and S9 (ESI†) illustrate that the DDACe MEM was able to provide ultralow friction in an applied normal pressure range between 1.8 and 2.2 GPa (i.e., a normal load range between 6 and 10 N) with a negligible running-in period for achieving superlubrication, indicative of a good load-carrying capacity of this kind of superlubricant and its ability in effectively eliminating abrasions caused by a relatively long running-in period.^40,41 Its superlubricity was found to attain the highest at 2.2 GPa and 10 N and be relatively independent of surfactant concentrations. We did not employ a normal load and pressure higher than 10 N and 2.2 GPa due to the upper measuring limit of the friction and wear tester used in this study.

Fig. 4c and Fig. S7c (ESI†) show that the superlubrication could be maintained in a broad temperature range from −30 to 20 °C with a CoF kept at all below 0.01, and the DDACe MEM was certified to exhibit a very low CoF (∼0.012) even when the surrounding temperature decreased to a value (e.g., −60 °C) close to its freezing point (∼−66 °C), thereby verifying its capacity as a cryogenic liquid lubricant.³⁷ In contrast, at a temperature that is high enough to trigger the MEM-to-EM transition (e.g., 50 and 60 °C), the lubricity of the DDACe/EG/n-hexane/water mixture attenuated significantly with the CoF attaining as high as ∼0.15. Owing to the reversibility of temperature-induced phase transition, the mixture could abruptly and dramatically convert its CoF by approximately 25 fold between ∼0.006 at 20 °C and ∼0.15 at 50 °C as shown in Fig. 4d. Such switchable behaviour could be repeated several times without any noticeable changes in the CoF of both MEM and EM states throughout the whole tests, demonstrative of the longevity and reversibility of the thermoresponsive superlubrication. The good reversibility also ensured that the enhanced friction at relatively high temperatures was not an experimental artifact caused by water or oil evaporation. Fig. S10 (ESI†) further illustrates that n-hexane, water and EG all yielded slightly different CoFs at temperatures of 20 and 50 °C. It consequently demonstrates that the thermally induced switchable friction should not be an inherent character of the MEM components but a property endowed by surfactant DDACe. Moreover, as depicted in Fig. S11 (ESI†), the MEM could attain a stable CoF of <0.01 and ∼0.15 for continuous 20 h at temperatures of 20 and 50 °C, respectively. By contrast, a commercially available general-purpose oil-based lubricants (a mixture consisting of non-volatile liquid paraffin based oil and additives) can only maintain its CoF at ∼0.13 and ∼0.17 for about 14 and 11 h at the two studied temperatures, respectively. Unusual increments and fluctuations in the CoF correlated with the lubrication deficiency were discovered thereafter. The results suggest that the DDACe MEM can function as a potent liquid lubricant to provide relatively long-lasting super and normal lubrication at ambient and elevated temperatures, respectively.

The remarkable lubricity of the DDACe MEM can be verified by the wear track profiles displayed in Fig. 4e–m and Fig. S12–S14 (ESI†), where the MEM-lubricated steel substrate was demonstrated to exhibit an ultralow wear rate of 6.47 × 10⁻⁸ mm³ N⁻¹ m⁻¹ (equating to a wear volume around 10⁻⁴ mm³) and negligible abrasive scars after the friction tests. In comparison, other materials including n-hexane, deionized water, EG and MEM stabilized by DDAFe, DDAGd and DDACl all gave out several-fold larger wear rates/volumes and abrasive scars with a width of at least ∼200 μm. In addition, a steel substrate after lubrication with a 50 °C DDACe/EG/n-hexane/water mixture resulted in a nearly one order of magnitude larger wear rate of 6.38 × 10⁻⁷ mm³ N⁻¹ m⁻¹ and an apparent abrasive scar with a total volume and width of ∼1.1 × 10⁻³ mm³ and ∼325 μm, respectively, indicative of strongly lowered lubricity due to the MEM-to-EM transition.

2.4. Mechanism of superlubrication and switchable friction

Fig. 5a–f and Fig. S15–S18 (ESI†) illustrate the X-ray photoelectron spectroscopy (XPS) profiles of n-hexane, water, EG, the DDACe MEM and the 50 °C DDACe EM after the sliding friction tests, respectively. It can be seen that the MEM exhibited unique absorption bands at around 900, 882, 722 and 397 eV in XPS Ce 3d, Fe 2p and N 1s spectra, corresponding to the formation of CeO₂ and ferric nitrides on a steel substrate. The results suggest that the surfactant DDACe underwent tribochemical reactions to help effectively dissipate the mechanical energy generated during the frictional process,³² thus contributing to achieving superlubrication. In addition, given that surfactants without [CeCl₄]⁻ cannot result in ultralow friction under the same tribological conditions, and hence, the exceptional lubrication should mainly stem from reactions of the Ce(III)-containing coordination anion, which led to the formation of CeO₂ products having been testified to exhibit potent lubricity.^42–44 The absorption bands completely vanished at 50 °C, indicating that the tribochemical reactions were suppressed, which consequently gave rise to the significantly deteriorated lubrication of the DDACe/EG/n-hexane/water mixture.

We also measured the electrical contact resistance (ECR) of the oil, water, glycol and their mixtures, which can act as an indicator of the affinity of different materials to a metal substrate.^45,46 As shown in Fig. 5g, both n-hexane and water had a near-zero ECR value, corresponding to their poor adsorbability on steel surfaces. By contrast, EG and DDACe solution could provide a much higher ECR at 0.2 and 0.05 Ohm, respectively, but the values were still much lower than that of a MEM (∼0.5 Ohm). The results suggest that the MEM was likely to construct a sturdy “tribofilm” on steel surfaces via a synergistic effect between DDACe and EG. At 50 °C, the ECR of the DDACe/EG/n-hexane/water mixture fell prominently to a value close to that of the pure co-surfactant, which suggests that transition to the EM may lead to not only dissociation but also competitive adsorption between the surfactant and the co-surfactant. The higher adsorbability of EG, as indicated by its relatively higher ECR, in turn, blocked the formation of surfactant “tribofilms” on a steel substrate, which therefore inhibited the tribochemical reactions and remarkably enhanced the friction of a DDACe/EG/n-hexane/water mixture.

Fig. 5h illustrates a full Stribeck curve depicting the evolution of the CoF of the DDACe MEM and EM in a broad sliding velocity range between 0.005 and 0.5 m s⁻¹. At a velocity below 0.02 m s⁻¹ (i.e., region (i)), both the MEM and EM exhibited a relatively high and constant CoF, featuring a typical boundary lubrication where the lubricants were nearly completely absent from the contact area. At a velocity between 0.02 and 0.2 m s⁻¹ (i.e., region (ii)), the CoF generally reduced as a function of the sliding speed, indicative of mixed lubrication with liquid lubricants massively entrained into the contact zone. The superlubricity of the DDACe MEM could be achieved between 0.05 and 0.2 m s⁻¹. A further elevating sliding velocity led to the alteration of the lubricating nature into hydrodynamic lubrication (i.e., region (iii)), in which an increment in the thickness of the liquid film caused the CoF of both the MEM and EM to increase vs. sliding speed. The retention of the lubrication mechanism in mixed lubrication of the DDACe/EG/n-hexane/water complex upon changing the temperature from 20 to 50 °C suggests that the tribochemical reaction of the as-formed potent adsorbed DDACe/EG tribofilm should play a key role in the superlubrication of a MEM at optimal loads and velocity. The CoF (∼0.012) slightly beyond the range of superlubrication at −60 °C was likely due to the ultralow environmental temperature decelerating or slightly suppressing the tribochemical reactions.

Based on the experimental results above, we speculate that the mechanism for the DDACe MEM to attain superlubrication was likely because, on the one hand, the DDACe/EG mixture strongly bound to steel surfaces via physical adsorption to form a mixed lubrication film of ∼32 nm thickness calculated according to the Hamrock–Dowson formula (see details in the ESI†). The thickness–roughness ratio (λ) was determined to be 1.3, verifying the nature of mixed lubrication. On the other hand, the adsorbed magnetic surfactants underwent tribochemical reactions with the metal surface to effectively facilitate the dissipation of mechanical energy generated during the friction process and produce CeO₂ that has been reported to exhibit good lubricity.^42–44 The largely switched friction at elevated temperatures was likely owing to the competitive adsorption between DDACe and EG prohibiting the formation of a surfactant tribofilm and subsequent tribochemical reactions as well as an enlarged thickness of the lubrication film to ∼123 nm. The λ value at 50 °C was calculated to be 2.7, confirming no variations in the physical lubricating mechanism. We schematically illustrate the corresponding mechanism for the superlubricity and thermosresponsive lubrication of the DDACe MEM in Fig. S19 (ESI†).

Molecular dynamics (MD) simulations were performed to analyse in-depth the tribochemical reactions between [CeCl₄]⁻ and steel substrates that determined the superlubrication and competitive adsorption between DDACe and EG that dominated the thermally switchable friction. As shown in Fig. 6a and b, the Ce-containing complex anions could react with the Fe atoms in the substrate within 160 ps and produce a large number of CeO₂ and ferric nitrides on the metal surface after 650 ps at 20 °C. The results demonstrate the feasibility of the friction-induced chemical reactions that contribute to prominently facilitating dissipations of the mechanical energy and heat generated during the lubrication process and consequently attaining near-zero friction of an MEM. In contrast, as shown in Fig. 6c, such chemical reactions cannot be observed even after 1500 ps at 50 °C, which suggests that the approximately 25-fold elevation in the CoF of a DDACe EM was essentially a result of inhibition of the tribochemical reactions at increased temperatures.

The 1D and 2D density profiles in Fig. 6d and Fig. S20 (ESI†) show that although EG molecules had a much higher density than both DDA⁺ and [CeCl₄]⁻, corresponding to superior binding affinity to metal substrates, the tribochemical products still yielded the most concentrated density distribution in the distance range of 20–40 Å (i.e., 0–20 Å from the metal surface), which suggests that the competitive adsorption between the surfactant and the co-surfactant cannot suppress the chemical reactions between [CeCl₄]⁻ and Fe atoms during the friction process at ambient temperature. Nevertheless, at 50 °C, the co-surfactant enhanced its density distribution at relatively low distances while both DDA⁺ and [CeCl₄]⁻ improved their densities in the distance range between 80 and 100 Å as illustrated in Fig. 6e and Fig. S21 (ESI†). It indicates that an enlarged difference in adsorbability may therefore restrain the occurrence of the tribochemical reactions and result in strongly attenuated lubrication.

3. Discussion

So far, stimuli-responsive MEMs have found extensive applications in consumer products and other industrial fields, but not yet in achieving superlubrication for two contacting surfaces or two friction states in which the CoF can exhibit more than one order of magnitude difference. In addition, superlubrication for metallic materials has been discovered in van der Waals 2D materials and dispersions of lipids, nanoparticles and polymers,^46–48 but not in the MEM consisting of oil, water and surfactants. In this work, we report the first, to our knowledge, MEM capable of providing effective superlubrication with an ultralow wear rate of 6.48 × 10⁻⁸ mm³ N⁻¹ m⁻¹ even when the applied normal pressure reached as high as 2.2 GPa, on the basis of tribochemical reactions of a sturdy physically adsorbed mixed lubrication film of the DDACe/EG complex. The MEM was also able to retain its superlubricity in a broad temperature range from −30 to 20 °C and could abruptly and prominently convert its CoF by ∼25 fold as a consequence of the thermally reversible MEM–EM transition.

The advantages of employing the DDACe MEM over traditional liquid or solid superlubricants can be concluded as follows: (1) facile preparation and low cost. The superlubricative MEM can be prepared by directly mixing different compositions together under stirring, and the functional surfactant can be synthesized via one-step coordination. All the materials for obtaining a MEM including n-hexane, water, EG, DDACl and CeCl₃ are commercially available and relatively cheap. It makes the MEM superlubricant ideal for large-scale production and practical industrial applications; (2) high colloidal stability and magneto responsiveness. The MEM can be stored for at least 6 months without any changes in macroscopic appearance and internal size distribution, making it equal to serving as metalworking and drilling fluids that are usually continuously used until they completely lose their efficiency.^22,23 The magnetically responsive property may enable it to act as a smart liquid lubricant capable of rapidly filling up depressions on a rubber surface upon exposure to a low-strength external magnetic field, thus allowing continuous lubrication;^31,35 (3) environmental adaptivity and anti-freezing performance. Water-based lubricants or hydrogel lubricants are usually difficult to provide a low CoF at subzero temperatures due to the loss of their fluidities. Hence, various anti-freezing additives including salts, acids and small organic molecules are usually required to improve their lubricity at relatively low temperatures.^49–51 Herein, owing to n-hexane and ethylene glycol (EG) acting as efficient anti-icing agents and tribochemical reactions of DDACe contributing to strongly reducing the friction and wear, our prepared MEM was able to provide superlubrication for two contacting steel surfaces in a relatively wide low temperature range from −30 to 0 °C. An ultralow CoF of ∼0.012 could still be maintained even when the surrounding temperature declined to as low as −60 °C, thus making them ideal candidates for serving at extreme environments such as aerospace, polar industry, deep-sea exploration and liquid-propellant rockets;^37,38 (4) largely switchable friction. Materials with convertible lubrication can allow accurate manipulation of tribological interactions between two contacting moving parts, thereby being able to acquire precise control over the efficiency, accuracy and reliability of large equipment or small devices.^8,11–15 However, MEM-based lubricants whose CoF can be reversibly changed by more than one order of magnitude have seldom been reported. In this work, the DDACe MEM, which could abruptly and considerably switch its CoF by nearly 25 fold on the basis of a thermoreversible MEM–EM transition, was likely to have great potential in intelligent devices and controllable transportation systems that usually require highly switchable surface properties. With the above technical advances, we anticipated that our prepared novel smart and superlubricative MEM could find a diverse range of potential applications in materials science and chemistry.

In previous work, we have reported an EM-type lubricant whose CoF can be switched by nearly one order of magnitude upon exposure to alternate UV and visible light illuminations.²⁹ Nevertheless, the liquid lubricant can neither achieve an effective macroscale superlubrication nor provide a comparable ultra-high colloidal stability as the DDACe MEM (it can only be stored at room temperature without phase separation for about 1 month). Both its anti-freezing performance and low-temperature lubrication have not been testified. It also should be noted that although the DDACe MEM exhibited attenuated lubrication at elevated temperatures, it can still function as an effective liquid lubricant under warmer conditions, since it could provide a very low CoF of ∼0.014 at 30 °C and the CoF at 50 and 60 °C (∼0.15) was also comparable to those of many oil- or water-based lubricants obtained at room temperature.^52–56 Moreover, although some liquid superlubricants can display better tolerance to relatively high temperatures, they (especially water-based lubricants) may suffer the drawback of potentially declining their lubrication efficiency in cold environments, for instance, at temperatures far below zero in northern areas where the enviornmental temperature can even hardly attain 20 °C in summer. However, future works aiming at improving the high temperature tolerance of a MEM superlubricant is necessary to expand its range of potential applications. This can be achieved by either further rationally designing the chemical structure of the surfactant or using lubricating base oils suitable for working at high temperatures (e.g., ionic liquid base oils).^57,58 In addition, the use of evaporable, flammable and toxic n-hexane oil may also make the DDACe MEM not eco-friendly. Discharge or discard of the liquid superlubricant may result in certain environmental burdens, safety concerns and health risks. This important issue can be addressed in our future work by either using non-volatile, biocompatible and biodegradable lubricating base oils (e.g., vegetable oils) or further introducing a stimuli-responsive moiety such as the light-responsive azobenzene group in the surfactant structure to realize effective control over the on-demand separation and recycling of the MEM compositions on the basis of a reversible variation in the hydrophobicity of the surfactant.^29,36 Another important technical challenge that needs to be overcome in the future is to study the thermally responsive lubrication at controlled local in situ temperatures of a tribopair instead of controlled environmental or surrounding temperatures used in this study and many previous works.^{14,15,59–62} This may require further design and modification of the friction and wear testers as both our instrument and most commercially available tribometers can hardly realize direct measurement or control of the local in situ temperature of a sliding surface during the tribological tests in the current stage.

4. Conclusions

We have integrated both thermo- and magneto-responsiveness, superlubrication, considerably switched friction and anti-freezing performance into a MEM system constituted of water, n-hexane, surfactant DDACe and EG. Owing to the formation of a high-strength physically adsorbed film capable of undergoing tribochemical reactions to generate CeO₂ products, the MEM could provide near-zero friction and an ultralow wear rate for two contacting steel surfaces in a broad temperature range from −30 to 20 °C. On the basis of thermally induced reversible phase transition between an MEM and an EM, the liquid superlubricant was able to abruptly and repeatedly convert its CoF by 25 fold that can be utilized to control interfacial tribological interactions. Its good freezing resistance allowed it to maintain high lubricity even when the surrounding temperature reached as low as −60 °C. In combination with its inherent magnetically controlled migration showing potentials in achieving continuous lubrication, we envision that this novel smart and superlubricative MEM may find extensive applications in materials science and chemistry.

Author contributions

J. H., L. X. and W. L. supervised the research. S. C. and H. S. conducted the synthesis of surfactants and sample preparation and characterization. S. C. and J. L. performed the friction and wear tests. L. X. drafted the manuscript. J. W., H. L., J. H., L. X. and W. L. commented on and revised the manuscript.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was financially supported by the Talents Program of the Chinese Academy of Sciences (E30247YB), the Special Talents Program of Lanzhou Institute of Chemical Physics (E0SX0282), the National Natural Science Foundation of China (22202220), the Special Research Funds of State Key Laboratory of Solid Lubrication (O80216KJZ5) and the Innovative Research Funds of Shandong Laboratory of Advanced Materials and Green Manufacturing at Yantai (E1R06SXM07, E2R06SXM14).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3mh01978k

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