AFM Studies on Liquid Superlubricity between Silica Surfaces

May 18, 2016 - Supercharged Bleach Powers Greener Oxidations. Run-of-the-mill liquid bleach, aqueous NaOCl, is an attractive green option for industri...
4 downloads 13 Views 3MB Size
Download PDF
Article pubs.acs.org/Langmuir

AFM Studies on Liquid Superlubricity between Silica Surfaces Achieved with Surfactant Micelles Jinjin Li,* Chenhui Zhang, Peng Cheng, Xinchun Chen, Weiqi Wang, and Jianbin Luo* State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: By using atomic force microscopy (AFM), we showed that the liquid superlubricity with a superlow friction coefficient of 0.0007 can be achieved between two silica surfaces lubricated by hexadecyltrimethylammonium bromide (C16TAB) solution. There exists a critical load that the lubrication state translates from superlow friction to high friction reversibly. To analyze the superlow friction mechanism and the factors influencing the critical load, we used AFM to measure the structure of adsorbed C16TAB molecules and the normal force between two silica surfaces. Experimental results indicate that the C16TAB molecules are firmly adsorbed on the two silica surfaces by electrostatic interaction, forming cylinder-like micelles. Meanwhile, the positively charged headgroups exposed to solution produce the hydration and double layer repulsion to bear the applied load. By controlling the concentration of C16TAB solution, it is confirmed that the critical load of superlow friction is determined by the maximal normal force produced by the hydration layer. Finally, the superlow friction mechanism was proposed that the adsorbed micellar layer forms the hydration layer, making the two friction surfaces be in the repulsive region and meanwhile providing excellent fluidity without adhesion between micelles. lubricity.11−15 Their superlubricity mechanism is mainly attributed to the extremely low shear strength of confined aqueous solution or weak interaction between two friction surfaces.16−19 Klein’s group made a systematic study on liquid superlubricity at nanoscale by using surface force balance (SFB). They found that the hydrated counterions bound to mica surfaces immersed in one salt solution resulted in an ultralow friction coefficient of less than 0.001 due to the formation of fluid hydration layer.16,20 Meanwhile, they also proved that the charged or zwitterionic groups of synthetic or biological macromolecules immersed in water can form the hydration layer to reduce friction coefficient down to the level of 0.0001.21,22 In addition to SFB, atomic force microscopy (AFM) is also a sensitive and versatile tool for studying liquid superlubricity at nanoscale.23 Spencer’s group found that the grafted poly(ethylene glycol) could be extended to the aqueous solution to form molecular brushes, leading to a large reduction in the friction coefficient.24 Feiler et al. confirmed that a friction coefficient of approximately 0.0003 could be achieved in cyclohexane between a gold sphere and Teflon surface due to the formation of repulsive van der Waals force.25 Li et al. achieved an ultralow friction coefficient of 0.001 between the silica−graphite interfaces with the lubrication of one ionic

1. INTRODUCTION Friction and wear are two major obstacles to improving energy consumption in the mechanical systems,1 especially in the nano- and micromachines owing to their large surface-tovolume ratio. Attempts to reduce friction between sliding surfaces have been made by researchers in many fields.2 One of the most effective methods is to introduce liquid lubricants between two sliding surfaces to reduce the friction force, by forming either a fluid lubricating film or a boundary layer with adsorbed molecules.3−5 For example, cationic surfactant solutions,6 plant mucilage,7 joint synovial fluid,8 and glycerol mixtures9,10 all enable a large reduction in the friction force when the two sliding surfaces are immersed in these solutions. Once the friction coefficient reduces to the level of 0.001 or less by these methods, it can be referred as liquid superlubricity.2 Relative to the common lubrication, the liquid superlubricity can significantly reduce energy dissipation in sliding systems, such as bearing in the nano- and micromachines. Considering that the annual cost of energy losses due to friction is estimated to be about 5% of the gross national products in most developed nations,1 the achievement of liquid superlubricity is possible to bring a large economic benefit. Much progress has been made in achieving liquid superlubricity both at nano- and macroscales during the recent years. Several studies have shown that the formations of selfassembled monolayers (SAMs), polymer brush, hydration layer, or hydrogen bond network between two friction surfaces are the most effective methods to achieve liquid super© XXXX American Chemical Society

Received: March 31, 2016 Revised: May 17, 2016

A

DOI: 10.1021/acs.langmuir.6b01237 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir liquid by applying a positive potential on AFM.26 Oncins et al. found that the friction force reduced to nearly zero when the AFM tip slid on the self-assembled phospholipid bilayers under a low pressure.27 Recently, we have found that the surfactant solution could also exhibit superlubricity properties (the friction coefficient reduced to about 0.0007) between two silica surfaces by controlling the contact pressure on AFM, and the superlubricity showed good reversibility with the applied pressure. Therefore, in the present work, the superlubricity behavior of surfactant solution was studied in detail by AFM to reveal its superlubricity mechanism and establish its superlubricity condition.

3. RESULTS AND DISCUSSION The lateral force, FL, versus normal load, FN, under the lubrication of C16TAB solution (3·CMC) was measured between two silica surfaces when the scanning rate was set as 2 Hz (5000 nm/s), as shown in Figure 1. It was found that the

2. EXPERIMENTAL SECTION Materials. The hexadecyltrimethylammonium bromide (C16TAB) with a purity of >99% was purchased from Aladdin Industrial. A silicon wafer with a 300 nm thick coating of chemical-vapor-deposited silica was supplied by Zhogjingkeyi, Co., Ltd. The monodispersed spherical silica particles with a diameter of 23 μm were purchased from NanoMicro, Co., Ltd. The pure water for the experiments was taken from a Nanopure water purification with total organic carbon (TOC) monitoring, resistivity of 18.2 MΩ·cm−1, and 0.1) when the load exceeds the critical load. However, whether the process is reversible is not clear. From Figure 2, when the friction force was measured in the B

DOI: 10.1021/acs.langmuir.6b01237 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 2. Ratio of lateral force to normal load as a function of circles under a high load (FN = 224 nN) and a low load (FN = 61 nN). The scanning speed was set as 5000 nm/s. Shown are data from runs at three different positions with five steps (from increasing load to reducing load as one step).

same scanning area, it is interestingly found that the ratio of FL to FN (approximately equal to the friction coefficient) reduced from the level of 0.2 to 0.001 with the normal load decreasing from 224 to 61 nN. It strongly suggests that the superlubricity can be restored when the normal load reduces to less than the critical load. Even when the steps (from increasing load to reducing load) were repeated many times, the superlubricity could always be restored as long as the normal load reduced to a low value, which indicates that the liquid superlubricity is reversible with the applied load. In addition, we also found that the ratio of FL to FN was always maintained at the level of 0.001 under a load of less than 100 nN even if the sliding lasted for 3 h, showing that the superlubricity has very good stability with time. To determine the lubrication state of superlubricity, the lateral forces versus different normal loads with C16TAB solution (3·CMC) were measured under three different scanning speeds (5, 9.8, and 19.5 μm/s), as shown in Figure 3a. It is found that the friction behaviors under these different scanning speeds were almost the same. The lateral force remained superlow until the load increased to 183 nN. When the normal load exceeded 183 nN, the lateral force suddenly increased to more than 100 times. The result shows that the critical loads under different speeds were the same, which means that the critical load has no correlation with speed. In addition, it is also found that the friction coefficients in the superlow friction region were almost the same under these three different speeds (Figure 3a, inset). The further investigation on the relationship between friction coefficient and speed is shown in Figure 3b when the normal load was set as 41.8 nN (in the superlow friction region). It can be seen that the lateral force always fluctuated within the range of 0.02−0.09 nN (noise level) with sliding speed variation from 630 to 97660 nm/s (the effect of hydrodynamic drag is negligible, as shown in the Supporting Information), indicating that the friction coefficient in the superlubricity state is almost independent of the sliding speed. From this result, it can be inferred that the lubrication state of C16TAB is boundary lubrication when superlubricity occurs. These above results confirm that the superlubricity of C16TAB can be achieved in boundary lubrication when the applied load is less than the critical load, but the superlubricity

Figure 3. (a) Lateral force as a function of normal load between two silica surfaces immersed C16TAB solution (3·CMC) under three different scanning speed (5, 9.8, and 19.5 μm/s). The inset shows in detail the relationship between lateral force and normal load when the normal load is less than 150 nN. (b) Relationship between lateral force and scanning speed when the normal load is set as 41.8 nN.

mechanism and the factors influencing the critical load are still not clarified. To solve these two questions, the structure of C16TAB molecules adsorbed on the silica surface was first investigated, as presented in Figure 4. It is found that the surfactant aggregate was organized by the C16TAB molecules on the silica surface, in the form of long and meandering cylinder-like micelles. It indicates that the micelles can be firmly attached to the silica surfaces through the electrostatic

Figure 4. AFM image of the structure of C16TAB molecules adsorbed on the silica surface (immersed in C16TAB solution, 3·CMC). The figure inside shows the height profile of the adsorbed micellar layer on the position of the white line. C

DOI: 10.1021/acs.langmuir.6b01237 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

corresponding to a surface charge density of one charge per 8.2 nm2. When the two surfaces were at a shorter separation (7.0− 9.0 nm), the van der Waals attractive force was not observed; instead, there was still a repulsive force, which increased more steeply in magnitude than the double-layer repulsion. It originates from the short-range hydration force on the headgroups of C16TAB molecules.35 The adsorbed micellar layer has many headgroups of C16TAB exposed to the solution, which can firmly attach water molecules to form the hydration layer, ensuring the hydration repulsion overcome the van der Waals attraction. The hydration force increased until the separation between two surfaces further reduced to about 7.0 nm, a critical separation where the repulsion collapsed and the silica ball pushed through the adsorbed micellar layer and fell into contact with the silica substrate. As shown above, the diameter of C16TAB micelle is less than 5 nm; thus, it can be inferred that the critical separation (7.0 nm) corresponds to the thickness of two contact micellar layers (one layer adsorbed on the silica substrate and the other one adsorbed on the silica ball). From the normal force profile, it can be inferred that the two adsorbed micellar layers can bear a normal force no more than 218 nN. Once the applied force exceeds the maximal normal force (Fmax), the adsorbed micellar layer would collapse and the hydration force would disappear. Compared with Figures 1 and 5, the critical bearing load (Fcrit) of superlow friction is less than the maximal normal force (Fmax) produced by the adsorbed micellar layer, which indicates that the superlubricity can only be achieved in the repulsive region (hydration or double repulsion). It is therefore inferred that Fcrit is determined by Fmax according to the inequation Fcrit < Fmax. To confirm this inference, the relationship between Fcrit and Fmax was investigated under different Fmax, which was controlled by the variation of concentration of C16TAB solution.6 The friction behaviors of C16TAB solution with seven different concentrations were first measured between two silica surfaces (scanning speed was set as 5000 nm/s), as shown in Figure 6a. It is seen that the friction behaviors under these different concentrations were almost the same as that shown in Figure 1; that is, the friction force remained superlow until the applied load reached the critical load, and then it abruptly increased to a very high value. However, the critical loads were different under different concentrations. When the concentration was less than 1·CMC, Fcrit increased with concentration increasing, but when the concentration exceeded 1·CMC, Fcrit became constant (Fcrit = 183 nN) with concentration increasing. The result indicates that Fcrit can be controlled by changing the concentration of C16TAB solution. Meanwhile, the maximal normal forces of adsorbed micellar layer under these seven different concentrations were measured by the method shown in Figure 5. We found that the normal forces versus separation between two silica surfaces under different concentrations were almost the same as that shown in Figure 5. It suggests that there exists a repulsive hydration force under all concentrations due to the adsorbed C16TAB molecules on the two silica surfaces (it forms bilayer structure when the concentration is less than 1·CMC36 and micelle structure (Figure 4) when the concentration is greater than 1· CMC). The maximal normal forces under these different concentrations are shown in Figure 6b. It can be seen that Fmax increased with concentration increasing when the concentrations were not more than 1·CMC, which indicates that the coverage of bilayer increased with concentration increasing. When the concentrations exceeded 1·CMC, Fmax became

interaction between the positively charged headgroups of C16TAB and the initial negative charges on the silica surface.32 The distance of cylinder to cylinder axes is approximately 7.5 nm with a small variation over the scale of the image, as shown in the height profile of aggregate in Figure 4. This value is larger than the maximal diameter of C16TAB micelle, which is twice the fully extended molecular length of C16TAB (l ≈ (0.3 + 0.15 + 0.1265n) nm, consisting of the headgroup thickness, the radius of terminal methyl group, and the length of fully extended chain).28 Thus, the diameter of the C16TAB micelle should be less than 5 nm. The difference between the cylinder to cylinder distance and the micelle diameter indicates that the micelles are slightly spaced rather than closely packed on the silica surface due to the existence of separation-dependent repulsive electrostatic forces between micelles, which favors a constant separation.33 Second, the interaction force between two silica surfaces across C16TAB solution (3·CMC) was measured, as shown in Figure 5. When the silica ball approached the silica substrate,

Figure 5. Normal force/R (R is the radius of silica ball) as a function of separation between two silica surfaces across C16TAB solution (3· CMC) when the silica ball approaches the silica substrate with a velocity of 400 nm/s. The force was fitted by the DLVO theory with parameters kB = 1.38 × 1023, T = 298 K, e = 1.6 × 10−19 C, and AH = 0.83 × 10−20 J.

there appeared a long-range electrostatic double-layer repulsive force, which exhibited an exponentially decaying with increasing the separation between two surfaces. It indicates that the micellar layer adsorbed on the silica surfaces has a net charge to form the double electrical layer. The charges originate from the positively charged headgroups of adsorbed C16TAB molecules, which lead to the formation of a positively charged plane on the micellar layer. According to the DLVO theory, the double-layer force can be described by eq 134 Fn = 128πckBTκ −1 tanh(eψ0/4kBT )e−κD − AH /6D2 R

(1)

where kB is the Boltzmann constant (1.38 × 10 ), T is the room temperature (298 K), c is the ion concentration, κ−1 is the Debye screening length, ψ0 is the effective surface potential at far field, e is the electronic charge, and AH is the Hamaker constant (0.83 × 10−20 J).28 By fitting the nonlinear force curve according to eq 1, the parameters of Debye length and surface potential can be obtained. Here, the fitting parameters give a Debye length of κ−1 = 5.9 nm, which is consistent with the value in the literature,33 and a surface potential of 124 mV, 23

D

DOI: 10.1021/acs.langmuir.6b01237 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 7. Normal force as a function of Z-sensor position across C16TAB solution (3·CMC) when the maximal contact force was set as 305 and 152 nN, respectively. The full line is the normal force curve when the silica ball approaches the silica substrate with a velocity of 400 nm/s, and the dashed line is the normal force curve when the silica ball retracts from the silica substrate with a velocity of 400 nm/s.

which was less than Fmax shown in Figure 5, the two silica surfaces were always in the repulsive region and the adsorbed micellar layer did not collapse with the probe approaching the surface (the final contact surface was the micellar layer). When the probe retracted from the contact surface, it can be seen that the retracting force curve was completely in coincidence with the approaching force curve. It indicates that the normal forces are reversible upon their separation, which means that no adhesion exists between the two micellar layers adsorbed on the silica surfaces. Compared with these results, it can be concluded that there is no adhesive force when the two silica surfaces slides in the superlow friction region, but there is a large adhesive force when sliding in the high friction region. It indicates that the absence of adhesion between two friction surfaces is also a requirement for superlubricity, in addition to the existence of a repulsive force between two friction surfaces. According to these results, the lubrication model of C16TAB solution was proposed, as shown in Figure 8. When the

Figure 6. (a) Lateral force as a function of normal load between two silica surfaces immersed in C16TAB solutions with seven different concentrations. The scanning speed was set as 5000 nm/s. (b) Maximal normal forces and critical bearing loads under seven different concentrations. The blue region is a superlow friction region where the applied loads are less than the critical bearing loads. The scanning speed was set as 5000 nm/s.

constant (Fmax = 214 nN), indicating that the coverage of micelles remained constant. Compared with the critical bearing loads at the same concentration, it is found that the variation rule of Fmax was consistent with that of Fcrit, which indicates that there exists a positive correlation between Fmax and Fcrit. Moreover, it is easy to find that Fcrit was always less than Fmax at every different concentration, which confirms our inference that Fcrit is determined by Fmax (Fcrit < Fmax). It is also confirmed that the superlubricity can only be achieved in the repulsive region where the adsorbed micellar layer does not collapse. If the applied load exceeds the maximal normal force, the adsorbed micellar layer would be destroyed by compression, leading to a much higher friction force (100 times) than superlubricity. In addition to maximal normal force, the adhesive force is also an important factor influencing friction force. To investigate the relationship between adhesion and superlubricity, the normal force curve versus Z-sensor position at a surfactant concentration of 3·CMC was measured when the maximal contact forces were set as 305 and 152 nN, respectively, as shown in Figure 7. When the maximal contact force was set as 305 nN, which was greater than Fmax shown in Figure 5, the repulsion would collapse and the silica ball would finally jump into contact with the silica surface with the probe approaching the surface. When the probe retracted from the contact surface, it can be seen that there existed a strong adhesive force (Fa = 210 nN) between two silica surfaces. However, when the maximal contact force was set as 152 nN,

Figure 8. Lubrication model of C16TAB solution (>1·CMC) between two silica surfaces when the applied load is less than the critical load (in the superlow friction region). The right part is the illustration of the shear layer between two micelles.

concentration is greater than 1·CMC, the C16TAB micelles are firmly adsorbed on the two silica surfaces by electrostatic interaction between Si−O− (negative charge on silica surfaces) and C16H33(CH3)3N+ (positive charge on headgroup), which can lead to a reversal of the initial negative charges on silica surface. After that, the positively charged headgroup on the micellar layer (exposed to the solution) can firmly attach water molecules to form the hydration layer, which would produce hydration repulsion to bear the applied load. When the applied E

DOI: 10.1021/acs.langmuir.6b01237 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

force and the double electrical layer force to bear the applied load. When the two friction surfaces slides with each other, the shear occurs in the hydration layer if the applied load is in equilibrium with the repulsive hydration force, which is the origin of superlow friction. When the applied load exceeds the limit hydration force, the micellar layers would be destroyed by compression, leading to a much higher friction force. The limit repulsive force (maximal normal force) determines the critical load (Fcrit < Fmax), and therefore the boundary condition of liquid superlubricity can be forecasted by the normal force profile. This work demonstrates that the cationic surfactants have excellent superlubricity properties between two silica surfaces, which are possible to be widely used in the nanomachine lubrication, as long as the load capacity of superlubricity is improved by enhancing the maximal normal force.

load is less than the maximal normal force, the hydration force can always be in equilibrium with the applied load. Thus, no direct contact occurs between two adsorbed micellar layers, and instead, they are separated by the hydration layer. When the two friction surfaces slide with each other, the shear actually occurs in the hydration layer owing to its excellent fluidity and the absence of adhesive force. Because the shear strength is extremely low in the hydration layer,16 it is the origin of the superlow friction force. Similarly, when the concentration is less than 1·CMC, the C16TAB molecules are adsorbed on the two silica surfaces in the form of bilayer structure through electrostatic interaction. The positively charged headgroup exposed to the solution can form the hydration layer, which would produce hydration repulsion and provide extremely low shear strength to achieve superlow friction, as shown in Figure S5. However, when the applied load exceeds the maximal normal force, the two adsorbed micellar layers or bilayers would collapse due to compression, and thereby the two silica surfaces would contact with each other, which would cause a very high friction force. It indicates that the superlubricity can only occur in the repulsive region because the load has to be balanced by the repulsive force; otherwise, the two silica surfaces would jump into contact with each other. Therefore, the limit repulsive force (maximal normal force) determines the critical load (Fcrit < Fmax). Usually, the maximal normal force is influenced by surface charge density, adsorption site type, concentration of surfactant, length of surfactant molecule, and so on.32,33,35 Figure 6b shows the direct evidence that the maximal normal force is influenced by the C 16 TAB concentration until reaching the saturated state (CMC), and as a result the critical load is also influenced by the surfactant concentration. Therefore, to improve the load capacity of superlubricity, the maximal normal force needs to be enhanced by adjusting the relative parameters. From the lubrication model, it can be inferred that the superlubricity is not limited to C16TAB solution. Other surfactant solutions can also achieve superlubricity as long as the following three conditions are satisfied. First, the surfactant molecules can be adsorbed on the silica surfaces in the form of micelles or bilayer structure through electrostatic interaction. Second, there are positively charged headgroup on the surfactant molecules to produce the repulsive hydration force. Finally, the applied load should not exceed the maximal normal force produced by the hydration layer. To confirm this inference, we measured the friction forces between two silica surfaces under the lubrication of C12TAB and C14TAB solution with a concentration above CMC, finding that both the friction coefficients were in the level of 0.001 when the applied loads were less than the critical loads (the critical loads were different due to different maximal normal forces), as shown in Figure S6. According to the results, we believe that most cationic surfactants have excellent lubrication properties (superlubricity) between silica surfaces or other negatively charged surfaces, which have a potential application in nanomachine or microelectromechanical systems lubrication.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01237. SEM image of silica particle attached to the cantilever end, SEM image of special wedge for lateral force calibration, representative friction loop in the superlow friction and high friction region, effect of hydrodynamic drag on the lateral force, lubrication model of C16TAB solution (