Excellent Lubricating Behavior of Brasenia schreberi Mucilage

Article pubs.acs.org/Langmuir

Excellent Lubricating Behavior of Brasenia schreberi Mucilage Jinjin Li, Yuhong Liu, Jianbin Luo,* Pengxiao Liu, and Chenhui Zhang State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China ABSTRACT: The present work reports an excellent lubrication property of an aquatic plant called Brasenia schreberi (BS). To investigate the lubrication characteristics of the BS mucilage, a novel measuring system is designed, and an ultralow friction coefficient about 0.005 between the mucilage and glass surface has been obtained. It is found that the ultralow friction is closely related to the structure of mucilage and water molecules in the mucilage. The microstructure analysis indicates that the mucilage surrounding BS forms a kind of polysaccharide gel with many nanosheets. A possible lubrication mechanism is proposed that the formation of hydration layers among these polymer nanosheets with plenty of bonded water molecules causes the ultralow friction. The excellent lubrication property has a potential application for reducing the friction between a glossy pill coated with such layer of mucilage and people’s throats.

1. INTRODUCTION In nature, there are some effective lubrication systems, which have become one of the forefronts of tribology,1 for example, the natural synovial joint lubrication,2−8 the mucous barrier lubricated by the gastric mucin,9,10 the eye lubricated with tears,11,12 and the motion of white blood cells along blood capillaries.13 The main difference between the general lubricant and the biological lubricant is that the former is usually oilbased while the latter is water based.1 Due to pressurizing water hardly increasing its viscosity at all,14 the conventional elastohydrodynamic lubrication theory does not adapt to the situation. Instead, some other mechanisms appear to describe the effective lubrication in nature, such as the brush layer formed by a charged polymer,15 the electrostatic double layer provided by a charged surface,16 and the hydration layer formed by bound water molecules,17 all leading to an ultralow friction. However, most research works on biotribology are concentrated upon the internal organs and outer surfaces of human, animals, and insects. At present, investigations on ultralow friction existing in the system of natural plants are not widely reported. Arad et al.18 reported a friction coefficient lower than 0.01, which was obtained by using polysaccharides extracted from red algae. Xu et al.19 reported a good tribological characteristic of aloe mucilage. Recently, we found that the Brasenia schreberi (BS) presented a very slippery feeling when drinking its soup (Figure 1) or rubbing it by fingers, which attracted us to investigate the lubrication property of BS. The BS is an aquatic plant belonging to the family Nymphaeaceaeand. Since it was first described in 1789, the investigation of this unique species has been restricted to anatomy, morphology, and physiology. The BS is found exhibiting some biological functions, such as antibacterial, antialgal, and allelopathic.20,21 It is also employed for health purposes in food and medicinal industries due to its hygienical © 2012 American Chemical Society

Figure 1. Schematic illustration and force analysis of eating BS and pill with a coating of mucilage. (a) BS sample in the natural state. (b) Schematic illustration of grafting such kind of mucilage onto the surface of a pill to form a natural edible superlubricant.

functions such as anti-inflammatory, anticancer, and hypolipidemic. Interestingly, its submersed organs including the young stems, buds, and the undersides of young leaves are all covered by a thick, clear mucilaginous substance (Figure 1). Research works on the BS mucilage have revealed that the mucilage is a mixture of polysaccharide consisting of L-arabinose (5.9%), Lfucose (10.9%), D-galactose (34.1%), D-glucuronic acid (17.3%), D-mannose (13.4%), L rhamnose (11.4%), and Dxylose (7.0%).22 In the present work, a novel system was designed to measure friction coefficient, which could keep the structure of BS mucilage from destroying in the test process. And an ultralow Received: December 6, 2011 Revised: April 29, 2012 Published: May 1, 2012 7797

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mm in thickness were used as substrates. First, the glass plates were cleaned in acetone and ethanol for 15 min separately in an ultrasonic bath, followed by washing in deionized water and drying by compressed air. And then the bottom glass plate was fixed horizontally. The left end of the top glass plate was connected with a force sensor through a string. The top glass plate tilted a small angle relative to the bottom one with the right end touching the bottom glass plate. The value of force sensor at the moment was defined as zero to eliminate the contribution of the gravity of the top glass plate. Finally, a part of stem (about 6 mm in length, 0.6 g in weight) cut from the fresh BS was used as the testing sample. The sample was confined between the two intersected glass plates, which was 120 mm from the joint line of two glass plates. And then the load increased gradually by moving down the force sensor at a very low speed of 0.01 mm/s, and there was no relative slide between the two glass plates during the whole test. The load and the angle between two glass plates changing with time were recorded simultaneously. All tests were performed at an ambient temperature about 25 °C, and the relative humidity was about 25%. 2.3. Surface Analysis. The dry BS sample structure was investigated by using a scanning electron microscope (SEM, QUANTA 200 FEG). The SEM image was obtained under low vacuum condition. The glass surface topographies contacting with or without the mucilage of BS after friction test were investigated by using an atomic force microscope (AFM, Nanoman VS). The images were obtained under an ambient condition operating in tapping mode. 2.4. Chemical Composition Analysis. The covering substance (mucilage) was taken down from the dry BS sample and investigated by Fourier transform infrared (FTIR) spectroscopy. The FTIR absorption measurement was performed at ambient pressure, by using a spectrometer (Nicolet 6700FTIR). 2.5. Water Content and State Analysis. The equilibrium water content of the BS sample was measured by vacuum-freeze-drying method. The weight change of the sample is the difference value between the initial weight and the dried weight. The percent equilibrium water content is determined by dividing the weight change over the initial weight. The water state in the BS sample was measured by using a Raman spectrometer (Jobin Yvon HR800). The laser with a wavelength of 613 nm was focused through a 50× objective of the microscope, which resulted in a laser power of approximately 12.7 mW on the sample. During the experiment, the sample was located in a small chamber, where the temperature fell down to −15 °C to freeze the sample and then rose to −4 °C gradually. At last, the Raman spectrum of the mucilage was measured successively at the temperature from −4 to 0 °C with an interval of 0.5 °C under ambient pressure. In addition, the Raman spectrum of free deionized water was measured in the same condition. 2.6. Viscosity Analysis. The rheological properties of the mucilage were determined at 25 °C in a standard rheometer (Physica MCR301, Anton Paar) operated in a cylinder-cylinder mode. The mucilage extracted from the BS sample by using a syringe was put in the cylinder. To evaluate the steady-shear viscosity as a function of shear rate, the steady-state shear stress was measured with the application of a given shear rate from 1 s−1 to 1000 s−1. 2.7. Compression Deformation Analysis. The pressure− deformation measurements of the BS were performed in the natural state and dry state. The BS sample in the natural state was cut from the fresh BS stem, while the BS sample in the dry state was cut from the BS stem that treated by vacuum-freeze-drying method, both in the scale of about 6 mm in length, 2.5 mm in diameter. The BS samples were confined between two parallel glass plates (50 mm × 50 mm), and the bottom glass plate was fixed horizontally. During the test, the top glass plate moved down with a constant speed of 0.01 mm/s. The pressure was monitored by the force sensor that fixed on the top glass plate.

friction coefficient between the mucilage and glass plate has been obtained, which is related to other recent achievements in superlubricity (friction coefficient is below 0.01) in our group.23,24 The mechanism of such low friction was discussed according to several experimental results. Considering the advantage of its excellent lubrication properties as well as its high nutrition, an application for the design of coating of pills or capsules to reduce the friction between them and the throat was suggested.

2. MATERIALS AND METHODS 2.1. Materials. The fresh BS used was collected in summer from the West Lake of Hangzhou, China, and was soaked in acetic acid for preservation. It was pretreated by flushing with water for about 5 min to wash off acetic acid. For obtaining the dry BS sample, it was treated by vacuum-freeze-drying method (freezing in −50 °C for about 6 h, and then vacuuming for about 24 h) to remove the water inside. To remove the mucilage surrounding the BS sample, it is placed in the ethanol solution for 24 h. 2.2. Friction Coefficient Measurements. In order to measure the lubrication property of the BS sample just in its natural state, we proposed a novel experimental strategy and designed an apparatus for measuring the friction coefficient, as schematically illustrated in Figure 2a. Two glass plates with 200 mm in length, 50 mm in width, and 3

Figure 2. Experimental apparatus and result. (a) Schematic illustration of the measurement system. The yellow region represents the friction lock region. The circle inset is force analysis at the moment that the sample just entered the friction lock region with a critical angle of θm, where G is the gravity of the BS sample, N1, N2 are the normal forces given by two glass plates separately, and F1, F2 are the friction forces between the sample and glass plates. (b) Friction measurement result based on the above measurement system. The first region is that the sample kept stationary at the beginning marked by pink color. The second region is that the sample moved slowly toward the −x direction, marked by cyan color. The last region is that the sample entered the friction lock region and was kept stationary, marked by yellow color.

3. RESULTS AND DISCUSSION 3.1. The Results of Friction Tests. The friction result of the BS sample in the natural state is shown in Figure 2b. At the 7798

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according to eq 5. However, after soaking the dried BS sample in deionized water for about 10 h to restore water-saturated state, its friction coefficient could be reduced to 0.005 again. These results indicate that the excellent lubricating behavior of BS sample is closely linked to the mucilage, and water plays a critical role in reducing friction. In the next section, the details of mucilage would be investigated to reveal the origin of ultralow friction in the natural state. 3.2. The Details of the Mucilage. As the mucilage is the key factor for obtaining ultralow friction, its microstructure of dry BS sample was first investigated via SEM, as shown in Figure 3a. It can be seen that there are many nanosheets in the

beginning of test, the BS sample was confined between two intersected glass plates and the angle between the two glass plates was 2.3°. With the load increasing, the sample kept stationary first (from 0 to 230 s), because the horizontal component of normal force N1 provided by top glass plate was less than the sum of friction forces along horizontal direction. When the load became high enough (>2.5 g) to overcome the friction, the sample would start to move along the −x direction. The sample moved slowly with the increasing load, accompanied by the angle reducing. The acceleration of the sample in this process is above zero according to the geometrical relationship, and its value is very small that can be neglected. In this stage, the force inequation in direction x and the force equation in direction y are established as follows: N1 sin θ > F2 + F1 cos θ

(1)

N1 cos θ + F1 sin θ + G = N2

(2)

where F1 and F2 are the friction forces between BS sample and two glass plates, respectively, N1 and N2 are the normal forces given by two glass plates separately, G is the gravity of BS sample, and θ is the angle between two glass surfaces, as defined in Figure 2a. When the sample moves, F1 = μN1 and F2 = μN2 can be obtained, where μ is the friction coefficient between the sample and glass plate. Combining with the ineq 1 and eq 2, we can get ineq 3 as follows: sin θ > 2μ cos θ + μG /N1 + μ2 sin θ

(3)

It should be noted that the ineq 3 can be valid only as θ is larger than a critical value θm, at which the BS sample would stop moving even though the load increases further (it is called “friction lock”). As the angle between two glass plates reduced to about 0.6°, the sample stopped moving and kept stationary even though the load increased to 50 g, which means that the sample just entered the friction lock region. In this case, eq 4 can be obtained according to ineq 3 as follows: sin θm = 2μ cos θm + μG /N1m + μ2 sin θm

(4)

where N1m is the critical normal force when the sample just stops moving, which is about 44 g according to the applied load. Since the values of μ, θm, and G/N1m are quite small at the moment, the values of the last two items in the right part are much less than that of first item. Therefore, they can be neglected and the friction coefficient can be obtained by eq 5 approximately. μ=

1 tan θm 2

Figure 3. Microstructure and chemical composition of mucilage. (a) SEM image of the mucilage after treating by vacuum-freeze-drying method. The figure inside is a high resolution SEM image of the section of nanosheet. (b) FTIR spectra of the mucilage after treating by vacuum-freeze-drying method.

(5)

Calculated according to Figure 2b, the friction coefficient between the BS sample and glass plate is about 0.005, which is less than one-tenth of that of the traditional oil lubricants. In order to know the factors leading to such low friction, the friction tests with the dried BS sample and the BS sample without mucilage were carried out in our designed apparatus. The initial angle (θ) between two glass plates was 3.5°, and the load increased from 0 to 50 g gradually. The friction behavior of the dried BS was similar to that of BS without mucilage. Both kept stationary with the load increasing until the end of test. This result indicates that they have already located in the friction lock region from the beginning of test. Therefore, it can be concluded that the initial angle is smaller than the critical angle, and the friction coefficient must be larger than 0.03

mucilage. These nanosheets are solid and their thickness is measured about 75 nm from the section of nanosheets, as shown in Figure 3a inside. It is also found that there are some interspaces between these nanosheets in the dry mucilage, which is favorable to hold a large amount of water in the mucilage. The SEM image indicates that the mucilage has a special network structure, which is probably linked to the excellent lubrication property. The chemical composition of these nanosheets investigated by FTIR is shown in Figure 3b. It is found that the absorption 7799

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peaks at 3386 and 2935 cm−1 are the stretching vibration absorption of O−H and C−H bonds; the peaks at 1724, 1608, and 1074 cm−1 are the adsorption of CO, adsorption of amide, and bending vibration adsorption of C−OH, respectively. The result is almost the same as the FTIR spectrum of polysaccharide extracted from the BS in ref 25, which suggests that the nanosheets are formed by the polysaccharide cross-linking. The water content in the BS mucilage in the natural state is measured at about 98%. The water state in the mucilage is investigated by Raman spectroscopy, as shown in Figure 4. It is

been widely detected for explaining the rheological behaviors.29 It has been recognized that the cross-linking among long polymer chains is broken during the shearing process, resulting in the shear-thinning performance.30 Figure 6 shows the relationship between pressure and compression deformation for the BS sample in the natural state

Figure 6. Pressure−compression deformation curve for the BS sample in the natural state and dry state under uniaxial compression.

Figure 4. Raman spectra of mucilage from temperatures −4.0 °C to −2.5 °C with an interval of 0.5 at ambient pressure.

and dry state. It is seen that the pressure in the dry state increases very slowly with increasing compression deformation. It indicates that there are large interspaces in the network structure of mucilage in the dry state. When the pressure is supplied on the mucilage, the network structure formed by polysaccharide is easy to deform without water. However, when it is in the natural state, it needs much bigger pressure to produce the same deformation, suggesting that the water in the mucilage plays an important role in preventing the deformation of mucilage under pressure. In addition, the pressure increases faster when the deformation is bigger. It is inferred that hydrogen bond effect in the mucilage can result in a hydration layer on the network structure to support pressure. The contact area is measured about 10 mm2 at the end of compression process in the natural state, and the maximum contact pressure is about 0.23 MPa. 3.4. Friction Surface Analysis. To further investigate the lubrication mechanism, the regions on the glass surface contacted with the BS sample or not were characterized via AFM after the friction test, as shown in Figure 7. It is found that a residual layer of polysaccharide is absorbed on the region in contact with the BS sample during the test, while there is no absorbed layer on the other regions of glass surface. It can be seen that many small ring-shaped polysaccharide stacks

a typically spectrum of ice when the temperature of the mucilage is at −4.0 °C and −3.5 °C, but it becomes the spectrum of free water when the temperature rises to −3.0 °C. It indicates that the melting temperature of water in the mucilage is between −3.5 °C and −3.0 °C, which is lower than the melting temperature of free deionized water (0 °C) by the same measuring method. Therefore, it can be concluded that there are some hydrated water molecules in the mucilage in addition to free water molecules.26,27 Because of large amounts of hydrophilic groups in the polysaccharide molecules, such as hydroxyl group and carboxyl group, water molecules can be adsorbed on the polysaccharide network firmly by hydrogen bond to form a hydrated layer.28 3.3. The Physical Properties of the Mucilage. The rheologcial performance of mucilage is shown in Figure 5. It is found that the mucilage is a kind of non-Newtonian fluid with shear thinning behavior. The apparent viscosity keeps being reduced from 0.33 to 0.025 Pa·s with increasing the shear rate from 1 to 1000 s−1. The structures of the polysaccharide have

Figure 7. Topographical AFM images of glass surface. (a) Image of glass surface region without contacting with the BS sample. (b) Image of glass surface region in contact with the BS sample.

Figure 5. Shear stress and the viscosity of mucilage as a function of shear rate. 7800

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is very low under the test condition, of which the average value is only about 0.05 MPa at the maximum load. At the condition of such low pressure, the hydration layer would be very effective in lubrication.17 Therefore, it can be deduced that the lubrication of BS mucilage is actually lubricated by itself. In other words, when the sample moves with respect to the substrate, the shearing occurs between the polymer nanosheets in the mucilage, leading to an ultralow friction. The high water content and the fluid hydration layer among nanosheets provide a favorable condition to realize ultralow friction. In addition, the adsorption of mucilage on the substrate is probably independent of the surface properties due to the friction force between a surface and nanosheet being much more prominent than the friction force between nanosheets and a layer of mucilage would be left on the surface consequently. This result indicates that the ultralow friction of BS mucilage is independent of the properties of the substrate material. It provides the evidence to explain the slippery feeling when drinking the BS soup or rubbing it by fingers. According to these results above, if the mucilage could be transferred onto the surface of other objects, it would greatly reduce the friction coefficient regardless of the original surface properties. There is a fact that, when people eat a pill or capsule, it is difficult to swallow because of the high friction between the pill/capsule and throat. Seriously, it can become lodged in people’s throats, especially for children. Considering the excellent lubrication properties of the BS mucilage as well as its high-plane in nutrition, we suggest grafting this kind of mucilage onto a pill/capsule surface, which would greatly reduce the friction between the pill/capsule and throat as illustrated in Figure 1.

distributed evenly on the glass surface with a height of 5 nm and a diameter of approximately 80 nm. It indicates that the mucilage is easy to adsorb on the glass surface. Therefore, it is concluded that the shearing should take place between the polymer nanosheets in the mucilage, which is the origin of friction force. It is a fact that there is a hydrogen bond effect between the polysaccharide molecule and the glass surface, which is a favorable factor for the adsorption of mucilage.31 However, there is also the other factor contributing to the adsorption. As implied by the friction result, the friction force between nanosheets is very small, which is much smaller than the friction force between nanosheet and substrate, resulting in the shearing happening at the nanosheet interface and the outermost layer of mucilage being left on the glass surface. In other words, the friction force between the nanosheet and substrate would become a main factor for the adsorption of mucilage due to the friction force between nanosheets being too small. Therefore, when moving relatively, a layer of mucilage would be left on the surface, even if there is no hydrogen bond effect between the surface and mucilage. 3.5. Lubrication Model and Application. In terms of these results above, it is thought that the mucilage surrounding BS is a kind of hydrogel with high content of water. It is particular compared to other published work on mucilage as lubricant.18 It is also different from the other common hydrogels, for instance, polyvinyl alcohol hydrogel as articular cartilage substitution,32 because the mucilage of BS has an ultralow friction. Therefore, it is important to understand its excellent lubrication mechanism for further application. According to the experimental data and the analysis presented above, it is concluded that the mucilage plays a critical role in reducing friction, and a physical lubrication model is proposed as shown in Figure 8. When the BS sample

4. CONCLUSIONS In summary, a novel method for measuring the friction coefficient of a biosurface in its natural state is proposed to investigate the lubrication properties of BS mucilage. It shows that the mucilage of BS has an excellent lubrication property with an ultralow friction coefficient about 0.005 between its surface and a glass surface. And a possible physical model for such low friction is proposed according to a series of relative experimental analysis; that is, the excellent lubrication property is mainly attributed to the strong hydration effect between nanosheets in the mucilage. Taking advantage of the excellent lubrication property, it will probably have a wide application prospect for the manufacturing of the glossy pill coating, which could make people take medicine more comfortably and safely.

Figure 8. Molecular schematic representation of mucilage surrounding the BS sample during lubrication. The nanosheet structure is formed by polysaccharide which is composed of eight kinds of monosaccharide. Between the nanosheets, it is filled with water molecules to form the hydration layer.

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is moving on the glass plate, the outermost layer of mucilage can be adsorbed on the glass surface under pressure, which prevents the BS sample from directly contacting with the glass plate. In the mucilage, large numbers of water molecules are adsorbed by the hydrophilic groups through hydrogen bonding, which can form a thin hydration layer on the polymer nanosheet surfaces. Due to their strong adsorption, the water molecules of the hydration layer are stable and hard to be squeezed out under pressure.33 When the BS sample moves on the glass surface, the shearing actually occurs between the hydration layers formed on the polymer nanosheets as illustrated in Figure 8. The fluidity of the hydration layer,34 as well as the strong adsorption of mucilage on the substrate, results in the ultralow friction. In addition, the contact pressure

AUTHOR INFORMATION

Corresponding Author

*Telephone: 8610-62781385. 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 (51021064, 51075227), the Basic Research Program of Shenzhen (0021539012100521066).

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