Mechanism of Biological Liquid Superlubricity of Brasenia schreberi

Mar 19, 2014 - In the present work, an excellent biological lubricant extracted from an aquatic plant called Brasenia schreberi (B.s) is reported. Wit...
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Mechanism of Biological Liquid Superlubricity of Brasenia schreberi Mucilage Pengxiao Liu, Yuhong Liu, Ye Yang, Zhe Chen, Jinjin Li, and Jianbin Luo* State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, People’s Republic of China

ABSTRACT: In the present work, an excellent biological lubricant extracted from an aquatic plant called Brasenia schreberi (B.s) is reported. With a rotary cylinder-on-ring tribometer, the lubrication properties of the B.s mucilage between quartz glass surfaces have been investigated under different rotation velocity, and an ultralow friction coefficient between 0.004 and 0.006 is obtained. It is observed that the ultralow friction coefficient is independent of the rotation speed, when it is less than 0.1 m/s. SEM images indicate that the mucilage surrounding B.s is composed of polysaccharide gels with a layered structure, which are called nanosheets in the following work. Moreover, it can be deduced that the liquid superlubricity is closely related to the B.s mucilage layer absorbed on the quartz glass surface by hydrogen bonds and the superlubricity behavior only occurs when the adsorption layer stably forms between the quartz glass surface and the B.s mucilage. It is also found that superlubricity is closely dependent upon the sheet structure of the B.s mucilage and water molecules in the mucilage. According to these results, a layered nanosheets lubrication mechanism has been revealed, i.e., the ultralow friction coefficient is due to the adsorption layer of polysaccharide on the quartz glass surface and the hydration layers of water molecules bonded on the polysaccharide nanosheets between the sliding surfaces. lubricated by water.16,17 In this system, the silica tribochemical layer is formed on the friction surface during the sliding process, which is dominated by tribochemistry wear instead of mechanical wear. Klein et al. achieved superlubricity with a superlow friction coefficient of 0.0006−0.001 using polymer brushes as a friction surface lubricated by water.18,19 They ascribed this superlubricity to the exceptional resistance to mutual interpenetration, fluidity of the hydration layer, and the counterion-swollen brushes. C. Matta et al. obtained a stable low friction coefficient below 0.01, using polyhydric alcohol as a liquid lubricant and pretreated steel as a friction pair.20 They attributed the superlow friction to the ease of gliding on a triboformed OH-terminated surface. In another research area, a few kinds of polysaccharide mucilage from plants are used as liquid lubricants.21,22 Although they were investigated by different friction methods, ultralow friction states are all achieved using the polysaccharide mucilage as a liquid lubricant.

1. INTRODUCTION Currently, with increasing shortages of energy, deteriorating trends of environmental pollution, and stronger demand for machine performance, lubricants in the mechanical industry require excellent lubricating properties. Therefore, since the appearance of the concept of superlubricity,1 it has attracted a great amount of attention among researchers in their search for lubricants, which would be one of the most effective strategies to partially solve energy issues. In the past few years, some solid lubricants have been found having ultralow friction, such as molybdenum,2,3 diamond-like carbon films,4,5 and highly oriented pyrolyticgrahite.6,7 This kind of superlubricity is generally attributed to incommensurate surface lattice structure, coulomb repulsion at the contact,8−10 or weak interfacial interaction that always requires special lubrication conditions, for instance, high vacuum or nitrogen protection. Nowadays, the liquid superlubricity, which is used to describe the liquid with an ultralow friction coefficient less than 0.005,11−15 becomes the research hotspot and attracts more and more attention. Research groups worldwide have found several water-based liquid lubricants that can reach the ultralow friction state. Koji Kato et al. found the ceramic materials © 2014 American Chemical Society

Received: January 20, 2014 Revised: March 19, 2014 Published: March 19, 2014 3811

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surface roughness of both the ring and cylinder was measured by a surface mapping microscope (ADE PHASE SHIFT MicroXAM). The characteristic of the ring surface after rubbing was investigated by AFM (Nanoman VS scan size: 2 × 2 μm, scan rate: 1.5 Hz) with tapping mode under ambient conditions. The friction sample used in AFM analysis was obtained by the method we used in our previous work ref 15, section 2.2. The difference is that in our present work, no additional load was added, and the test ended as the B.s sample slid to the other side of the bottom quartz glass. The bottom quartz glass surface that B.s mucilage slid across was the area we analyzed by AFM. 2.3. Friction Measurements. The friction tests were performed on a rotational cylinder-on-ring Universal Micro-Tribotester (UMT-3, CETR), which is demonstrated in Figure 1. A quartz glass cylinder and

Recent research on liquid superlubricity can be classified in two directions according to its measuring method. One is studied on the nanoscale and the other is on the macroscale. Klein’s group has also done much work on the friction properties of aqueous solutions at nanoscale.23−26 The excellent lubrication of the aqueous solution is caused by the forming of a hydration layer of absorbed cations that can keep the two friction surfaces apart and simultaneously acts as a highly efficient lubricant. But when the aqueous solutions are used as lubricant at a macroscale, the situation is much different from that in the nanoscale. In the previous work of our group,27−29 great attention has been paid to the water based liquid superlubricity at the macroscale, and a series of mixtures of acids and polyhydroxy alcohol in aqueous solution as lubricants that can lead to superlubricity has been reported. The lubrication mechanism is mainly attributed to the forming of a hydrogen-bond network between the hydroxyl alcohol and the water molecules on the stern layer after the running-in process. So far, reports on liquid superlubricity existing in natural plants are still scarce. In one of our previous works, an excellent biological lubricant Brasenia schreberi (B.s) has been reported.15 B.s is a kind of aquatic plant belonging to the family Nymphaeaceaeand. Chemistry and food industry researchers have revealed the composition of the B.s mucilage, which is composed of a polysaccharide consisting of D-galactose (34.1%), D-glucuronic acid (17.3%), D-mannose (13.4%), Lrhamnose (11.4%), L-fucose (10.9%), D-xylose (7.0%) and Larabinose (5.9%).30 The water content of the B.s mucilage is high, almost 98%.15 In our recent work, a novel measuring system based on friction lock to measure the friction coefficient was designed to investigate the friction properties of the B.s mucilage. A traditional tribometer (UMT-2) was also used to study the lubricating properties. However, it was found that the mucilage had no superlubricitive performance under a rotary ball-on-disk mode, and a friction coefficient of about 0.02 was obtained. Poor lubrication property was shown. It was explained that with the mucilage, it was difficult to form an effective lubricating layer to reduce friction because of the high hertz contact pressure (700 MPa) in the contact region. In the present work, the lubrication properties of the B.s mucilage are investigated by a traditional friction apparatus, Universal Micro-Tribotester (UMT-3) using a rotary cylinderon-ring mode. In contrast to our previous work, the B.s mucilage showed excellent lubricating behavior under experimental conditions. An ultralow friction coefficient between 0.004 and 0.006 is obtained, and the superlubricative mechanism is discussed in detail.

Figure 1. Demonstration of the UMT-3 test (cylinder-on-ring contact). ring, which were of an amorphous material, were selected as the friction pair. The diameter of the cylinder was 4 mm, and the length was 25 mm. The inside and outside diameter of the ring was 15 mm and 25 mm, respectively. The roughness of the quartz glass cylinder and ring was 10 nm and 5 nm, respectively. Before testing, all the quartz glass cylinders and rings were ultrasonically cleaned in ethanol and acetone for 10 min individually, followed by being washed in DI water and dried by compressed air. Then almost 1 mL B.s mucilage was dropped on the ring as lubricant. During the experiments, the cylinder was loaded on the ring with a constant load of 10 N (55 MPa) and a rotary speed of 20−100 rpm corresponded to an average linear sliding velocity of 0.02−0.1m/s. All the tests were performed under ambient conditions, with the temperature at 25 °C and relative humidity in the range of 25−35%.

3. RESULTS AND DISCUSSIONS 3.1. Details of the Mucilage. In order to investigate the chemical composition of the B.s mucilage, FTIR technology was carried out, and the results are displayed in Figure 2. It is acknowledged that the B.s mucilage is mainly composed of saccharine.30 Peaks of the main functional groups in saccharine

2. MATERIALS AND METHODS 2.1. Materials. The fresh B.s used in our work was picked from the West Lake of Hangzhou, China, and was preserved in acetic acid. Before testing, the B.s was flushed with deionized water until the pH value of the mucilage was about 6.8, the purpose of which was to remove the acetic acid. 2.2. Characterization and Morphology Analysis. The chemical composition of the B.s mucilage was investigated by Fourier transform infrared spectroscopy (Nicolet 6700FTIR) using the reflection mode at ambient pressure. The B.s mucilage was taken down from the leaves, dropped to the foil surface, and then dried in an oven at 80 °C. The morphology of the dry B.s sample, which was obtained by a vacuum-freeze-drying method (freezing in −50 °C for about 8 h, and then vacuuming for about 24 h) was tested by scanning electron microscope (QUANTA 200 FEG) under low vacuum conditions. The

Figure 2. Chemical composition of the B.s mucilage. 3812

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can be found in the FTIR spectrum. It can be found that the peaks at 1074 and 1732 cm−1 are the bending vibration adsorption of C−OH and stretching vibration adsorption of CO, respectively, and the peaks at 2945 and 3377 cm−1 are the stretching vibration adsorption of C−H and O−H, respectively. These results are nearly the same as the FTIR spectrum of a polysaccharide extracted in B.s in a previous study.15 To better understand the framework structure of B.s mucilage, the sample is investigated by SEM, as shown in Figure 3. The SEM images indicate that there are many

Figure 4. (a) Friction coefficient with time using B.s mucilage as lubricant under different rotation speed. (b) The relationship between friction coefficient and rotation speed.

Figure 3. SEM images of the B.s mucilage structure pretreated by vacuum-freeze-drying method.

nanosheets in the mucilage, which has also been found in our previous work.15 The nanosheets are the special network structure of cross-linked polysaccharides, which are known to be the main composition of B.s mucilage. The thickness of the nanosheet is about 80 nm. It can be seen that there are interspaces between these nanosheets, and the width of the space is nearly 35 μm. With the existence of spaces between the nanosheets and plenty of hydrogen bonds between the hydroxyl groups in the molecules and water molecules, the B.s mucilage can hold a large amount of water, which is almost 98% percent of B.s mucilage according to our calculation. 3.2. Results of Friction Experiments. The friction results of the B.s mucilage sample in the natural state are shown in Figure 4(a). It is found that the B.s mucilage presents an excellent lubricating behavior. When the linear velocity is below 0.1m/s, an ultralow friction coefficient between 0.004 and 0.006 can be achieved rapidly, about 30 s after the experiment started. Then, the friction coefficient tends to be stable for a long time until the mucilage dried between the two quartz glass friction pair surfaces. As a result, the ultralow friction coefficient is independent with the linear speed when it is less than 0.1m/s. However, when the linear speed is 0.12 m/s or more, the friction coefficient reduces from 0.1 to 0.03 rapidly, and then decreases gradually to 0.01 and will stay stable till the mucilage dries. This may be caused by the destruction of the B.s mucilage layered structure. When the average linear sliding velocity is 0.12 m/s, the layered structure of B.s mucilage will be destroyed under this high shear rate. The relationship between friction coefficient and rotation speed is shown in Figure 4(b). It can be clearly seen in this figure that below the

velocity of 0.1 m/s, the liquid superlubricity of B.s mucilage is stably achieved. Because of the absence of other lubricants, the good friction results imply that the excellent lubricating behavior is closely related to the structure of B.s mucilage and the water molecules in mucilage based on the observation of friction results. The interaction between quartz glass surface and B.s mucilage might play an important role in the lubrication procedure. In the following part, the origin of the ultralow friction state would be explored. After the friction tests, the contact surface of quartz glass is measured by a white light interfering profilometer (MICROXAM-3D), as shown in Figure 5. Before the test, the quartz glass surface is flushed with deionized water to remove the B.s mucilage residues. It can be seen that there are no obvious differences in the quartz glass surface before and after tests. The average surface roughness of the quartz glass is around 10 nm. This result shows the excellent lubricating behavior of the B.s mucilage, as there is no abrasive wear on the surface of the quartz glass after the friction tests. 3.3. Analysis of Friction Mechanism. In order to reveal the lubrication mechanism, the lubrication status is essential to know. The lubrication regime can be distinguished using the ratio of theoretical minimum film thickness to the combined surface roughness. The following formula can be used to calculate the ratio λ:31,32 λ=

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hmin = σ

hmin σ12 + σ2 2

(1)

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Figure 5. Topographical view of the quartz glass surface before (a) and after the friction test (b).

where hmin is the theoretical minimum film thickness, σ is the combined surface roughness, and σ1 and σ2 are the surface roughnesses of the quartz glass cylinder and ring, respectively.33 To determine the suitable film thickness calculation formula for our experimental conditions, according to the line-contact EHL state diagram proposed by Hooke,34 it is essential to know the elastic parameter ge and viscosity parameter gv. These two parameters can be calculated by the following formula: ⎛ α 2W 3 ⎞1/2 ⎟ g v = ⎜⎜ 3 2⎟ ⎝ η0UL R ⎠

(3) −1

where α(6.83 × 10 Pa ) is the viscosity-pressure coefficient, η0 (2 × 10−2 Pa·s) is the entrance viscosity of the bulk lubricating medium, U is the average linear velocity of the friction pair, W is the normal load (10N), and R (4 mm) and L (10 mm) are the radius and length of the cylinder, respectively. E′ is the effective elastic modulus; it can be expressed by the following formula: 2 1 − μ2 2 ⎞ 1 1 ⎛ 1 − μ1 ⎟⎟ = ⎜⎜ + E′ 2 ⎝ E1 E2 ⎠

(4)

where E1 and E2 are the elastic moduli of the friction pair, which is 7.5 × 1010 Pa, and μ1 and μ2 are the Poisson ratios of the friction pair. In our test, the friction pair is quartz glass, which is 0.17. After calculation by eqs 2, 3, and 4, it is known that ge and gv are 2.54 and 0.39, respectively. So the lubrication is in the regime of rigid-isoviscous according to the line-contact EHL state diagram proposed by Hooke. Thus, hmin can be calculated using Martin rigid-isoviscous formula,34 hmin =

η0URL W

linear velocity (m/s)

hmin (nm)

λ

0.02 0.06 0.1

0.8 2.4 4.0

0.07 0.21 0.36

To assess the interaction between the quartz glass surface and the B.s mucilage, a simple experiment was conducted, which is demonstrated in detail in the Materials and Methods. The area on the quartz glass surface contacted with and without the B.s sample is investigated with AFM separately, and the results are shown in Figure 6. It can be seen that an evenly spread layer is absorbed on the region contacted with the B.s mucilage in the experiment, which is polysaccharide based on the component of the B.s mucilage. In contrast, there is no absorption layer on the region without contacting the B.s sample. Furthermore, the average thickness of the adsorption layer is about 8 nm. It is concluded that the polysaccharide is easily absorbed on the quartz glass surface, as the B.s sample slipped on the quartz glass surface in a few seconds. Therefore, it is deduced that the friction occurred between the polysaccharide nanosheets in the mucilage. As it is known that there are plenty of hydroxyl groups in the molecules of the polysaccharide in the mucilage and hydroxyl groups in the quartz glass surface, the quartz glass should interact with the B.s mucilage with hydrogen bonding.35 As the friction results above implied, when the B.s mucilage is dropped on the quartz glass ring surface, a stable adsorption layer of B.s mucilage in quartz glass surface will form in a few seconds, and the shearing happened between the nanosheets interface and the outermost layer of mucilage adsorbing on the quartz glass surface. So the stable formation of the adsorbed layer is important for obtaining the ultralow friction state. With the exception of the AFM characterization, a designed experiment is carried out to verify the essential role of the layer adsorbed on the quartz glass surface to the stable liquid superlubricating state. First, the test running with the B.s mucilage as lubricating medium, a low friction coefficient about 0.006 is achieved. Second, 500 μL deionized water is evenly added to the B.s mucilage to destroy the interaction balance between the quartz glass surface and the B.s mucilage, while the load and rotation speed are kept constant in the whole process, and the results are shown in Figure 7. It is observed in Figure 7 that the friction coefficient jumps from 0.006 to about 0.02 in a few seconds after 500 μL

(2)

⎛ W 2 ⎞1/2 ⎟ ge = ⎜⎜ 2 ⎟ ⎝ η0URL E′ ⎠ −10

Table 1. Linear Velocity and Corresponding Theoretical Minimum Film Thickness, Ratio λ

(5)

The lambda ratio varies corresponding to the linear velocities, which are presented in Table 1. It is known that the lubrication is in the regime EHL and HD if the ratio λ is larger than 3, mixed lubrication is the ratio ranging from 1 to 3, and boundary lubrication occurs when the ratio is smaller than 1.11 The ratio λ we obtained were all smaller than 1 corresponding to the linear velocity ranging from 0.02 m/s to 0.1 m/s, meaning that the lubrication is in the regime of boundary lubrication. 3814

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Figure 6. Topographical and 3D AFM images of quartz glass surface. (a) Images of quartz glass surface without contacting the B.s mucilage. (b) Images of quartz glass surface after the B.s mucilage slid across.

superlubricating state could be achieved, otherwise the situation would be much different. 3.4. Lubrication Process to Achieve Superlubrication. According to all the experimental results and discussions above, the model of the lubrication process is proposed and illustrated in Figure 8. There are two main steps in the lubrication process, which lead to the liquid superlubricating state. The first step of the process is the stable absorption of the outer B.s mucilage on the surface of the quartz glass surface with hydrogen bonding, which supplies the probability of the second step. In the following step, a lot of water molecules are absorbed by the hydrophilic groups of the polysaccharide nanosheets with hydrogen bonds. As a result, a thin hydration layer forms around the polysaccharide nanosheet surfaces. The properties of the hydration layer have been demonstrated in detail in our previous work.15 During the experiment, the shearing actually happens between the hydration layers forming on the surface of the polysaccharide nanosheets and the adsorption hydration layer on the quartz glass surface, resulting in the liquid superlubrication.

Figure 7. Friction coefficient with time under the lubrication of B.s mucilage after 500 μL deionized water was added in the lubricant.

deionized water was added, and then fluctuated around 0.02 for about 1000 s, but no liquid superlubricity state appeared. This may be caused by the competing reaction between water molecules and hydroxyl groups in the saccharine with the quartz glass surface by hydrogen bonds. After the extra 500 μL deionized water is added to the B.s mucilage, which is used as a lubricant in the friction test, the water molecules will substitute the hydroxyl groups in the polysaccharide adsorbed on the quartz glass surface. So the interaction between the adsorption layer of B.s mucilage and the quartz glass surface will be weakened and cannot play the role in the friction process to reach an ultralow friction state. Therefore, it can be concluded that the interaction between the B.s mucilage and the quartz glass surface by hydrogen bonds to form an evenly spread adsorption layer is essential to achieve the ultralow friction state. If there is enough hydrogen bonds forming between the B.s mucilage and the quartz glass surface, a stable adsorption layer will be formed on the quartz glass surface, and a liquid

4. CONCLUSIONS In summary, an excellent biological lubricant is introduced, and its superlubrication properties are investigated using a traditional friction apparatus (UMT-3). Superlubricity is obtained with an ultralow friction coefficient between 0.004 and 0.006. A possible lubricating process to obtain an ultralow friction coefficient is proposed on the basis of a series of experimental results and analysis. The excellent lubrication properties may be affected by the following two factors: one is the stable formation of an adsorption layer on a quartz glass surface with hydrogen bonds; the other is the strong hydration effect of the hydrophilic groups in polysaccharide molecules, which can form the hydration layer between nanosheets in the mucilage.

Figure 8. Schematic illustration of the lubricating processes that lead to liquid superlubricating state. 3815

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Inspired by the excellent lubrication property of the B.s mucilage, a kind of saccharine polymer is expected to be synthesized or found for use as a capsule shell, in order to be easily swollen.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-10-62781385; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (51321092, 51335005), the National Key Basic Research Program of China (2013CB934200) and Tsinghua University Initiative Scientific Research program.

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REFERENCES

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