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Effect of Surface charge on the nanofriction and its velocitydependence in an electrolyte based on lateral force microscopy Dalei Jing, Yunlu Pan, Dayong Li, Xuezeng Zhao, and Bharat Bhushan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04332 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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Rev. Jan. 10, 2017; Dec. 1, 2016

Effect of Surface charge on the nanofriction and its velocity-dependence in an electrolyte based on lateral force microscopy Dalei Jing1, Yunlu Pan2*, Dayong Li3, Xuezeng Zhao2 and Bharat Bhushan2,4* 1 School of Mechanical Engineering, University of Shanghai for Science and Technology, Shanghai, 200093 2 School of Mechanical Engineering, Harbin Institute of Technology, Harbin, 150001 3 School of Mechanical Engineering, Heilongjiang University of Science and Technology, Harbin 150022, China 4 Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLB2), The Ohio State University, 201 W. 19th Avenue, Columbus, OH 43210-1142, USA Abstract: The nanofriction between a silicon nitride probe and both a silicon wafer and an octadecyltrichlorosilane (OTS) coated surface is studied in saline solution by using lateral force microscopy (LFM). The effects of surface charge on the nanofriction in an electrolyte, as well as its velocity-dependence, are studied while the surface charge at the solid-liquid interface is adjusted by changing the pH value of the electrolyte. The results show that the nano-frictional behavior between the probe and the samples in an electrolyte depends strongly on the surface charge at the solid-liquid interface. When the probe and the sample in the electrolyte are charged with the same sign, a repulsive electrostatic interaction between the probe and the sample is produced, leading to a reduction of the nanofriction. In contrast, when the two surfaces are charged with the opposite sign, nanofriction is enhanced due to the attractive electrostatic interaction between the probe and the sample. The velocity-dependence of nanofriction in an electrolyte is believed to be tied to charge regulation referring to a decreasing trend of surface charge densities for the two approaching charged surfaces in an electrolyte. When the probe slides on the sample at a low velocity, charge regulation occurs and weakens the electrostatic interaction between the probe and the sample. As a result, nanofriction is reduced for surfaces charged with the opposite sign, while it is enhanced for surfaces charged with the same sign. When the sliding velocity between the probe and the sample is high, there is insufficient time for charge regulation to occur. Thus, the friction pair shows a larger nanofriction when the surfaces are charged with the opposite sign, and a smaller nanofriction when the surfaces are charged with the same sign when comparing with the case of a lower sliding velocity. Keywords: Lateral force microscopy, Nanofriction, Charge regulation, Electrolyte

*[email protected], [email protected] 1

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1. Introduction Two solid surfaces in sliding or rolling contact exhibit friction, the significance of which has been recognized since ancient times1-5. Recently, mechanical applications at the micro/nanoscale have become more and more important. To understand thoroughly the frictional behavior of the friction pair in such mechanical applications, a series of in-depth studies of friction at the micro/nanoscale or molecular/atomic level is needed5, 6. Fortunately, with the emergence and development of the atomic force microscope (AFM)7 and other relevant techniques, systematic studies of micro/nanotribology have been realized5, 8. With the aid of single asperity contacts between an AFM tip and solid surface, AFM, especially lateral force microscopy (LFM), is a desirable tool to study the frictional behavior over the micro/nanoscale. Scientists have performed wide studies on the micro/nanofriction between various friction pairs8-10. Being different from friction in air or in a vacuum, for the friction pair in an aqueous environment, some new phenomena are interesting and should be considered. For example, surface charge at the solid-liquid interface can be an important factor affecting frictional behavior, especially over the micro/nanoscale11-14. For solid surfaces contacting with an electrolyte, they can be spontaneously charged through different mechanisms such as the dissociation or ionization of surface groups, binding or adsorption of free ions, and charge exchange15, 16. The sign of surface charge at the solid-liquid interface can be positive or negative. Some scientists have found that the magnitude and sign of surface charge at the solid-liquid interface, which can be adjusted by the pH of the liquid, affect micro/nanofriction through changing the electrostatic interaction between the charged AFM tip and the charged sample11, 12. On the other hand, it also was found that the effect of surface charge on micro/nanofriction can be realized as a result of hydration lubrication. Hydration lubrication occurs when water molecules surround the counter-ions (ions having opposite sign surface charge from the charged solid surface) adsorbed on the charged solid surface, which acts as a lubricant and reduces the friction13, 14. In the meantime, the velocity-dependence of nanofriction is an important property, and has been widely studied based on LFM13, 17-22. However, the velocity-dependence of various friction pairs can be completely different and complex, which can strongly depend on the chemical nature of the interface19, adhesion at interface18, thermal fluctuations22, structure of ions within the confined 2


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space between the fiction pairs in an electrolyte13, and so on. Moreover, the mechanisms to explain the velocity-dependence of nanofriction are still unclear, and the effect of surface charge on the velocity-dependence of nanofriction in an electrolyte has been less studied and deserves wide investigation. In this paper, the nanofriction between an AFM probe and both a silicon wafer or an octadecyltrichlorosilane (OTS) coated surface in an electrolyte environment is measured by using LFM. The surface charge density at the AFM probe-electrolyte interface and the sample-electrolyte interface is adjusted by changing the pH value of the electrolyte. By doing so, the effect of surface charge at the solid-liquid interface on the nanofriction between the friction pair consisting of AFM probe and sample is studied. Furthermore, the velocity-dependence of nanofriction in an electrolyte environment is studied and the underlying mechanisms are analyzed. 2. Experimental Section 2.1 Samples and chemicals The samples used in the present nanofriction experiments were a silicon wafer with a thermally-grown silicon oxide layer, and the newly-prepared OTS surface coated on the silicon wafer. The preparation of the OTS sample was based on the same procedure mentioned in our previous study23. The root mean square roughness of the silicon wafer and OTS surface, respectively, were measured to be 0.15 ± 0.02 nm and 0.12 ± 0.02 nm over a scanning area of 1 µm × 1 µm by an Innova AFM (Bruker) operating in tapping mode. The usage of these two smooth surfaces effectively can reduce the influence of the surface topography. To study the effect of surface charge at the solid-liquid interface on nanofriction, chemicals with different pH values were used to achieve different surface charge densities. The background electrolyte used is saline solution (NaCl) with an ionic concentration of 0.15 M. The pH value of the electrolyte was adjusted by adding 0.01 M hydrocholoric acid (HCl) to achieve the acid solution and 0.01 M sodium hydroxide (NaOH) to achieve the alkaline solution. The target pH of the chemicals were 3.0 ± 0.1, 6.0 ± 0.1, and 9.0 ± 0.1. Using the background electrolyte of 0.15 M NaCl keeps a nearly constant ionic concentration during the adjustment of pH values. 2.2 Quantification of surface charge density using a colloidal probe AFM 3


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The electrostatic force between the sample surface and AFM probe in an aqueous environment was measured by an Innova AFM (Bruker) operating in contact mode to quantify the surface charge density at the solid-liquid interface. To keep the effectiveness of the electrostatic force and reduce the influence of force applied on the cantilever, a colloidal probe was used to measure the electrostatic force. This probe was prepared by attaching a silicon oxide sphere with a diameter of about 20 µm to a rectangular AFM cantilever (ORC8, the B cantilever, Bruker) using epoxy (Araldite, Bostik, Coubert). When the sample and the colloidal probe approach each other in an electrolyte, the force applied on the colloidal probe includes hydrodynamic force and electrostatic force in a large range of separation distances (larger than 10 nm) under the assumption of neglecting van der Waals force. When the approaching velocity between the sample and colloidal probe was low enough, for example 0.5 µm/s, the hydrodynamic force applied on the colloidal probe was sufficiently small as compared with the electrostatic force that it can be neglected. Under this assumption, the force applied on the colloidal probe at the low velocity was electrostatic force24. Here, the electrostatic force was measured based on this low velocity method. When the electrostatic force was measured, the surface charge density at the solid-liquid interface can be quantified based on the relationship between the electrostatic force and the surface charge density. The details to quantify the surface charge densities of the silicon wafer and OTS surface immersed in electrolytes by using the electrostatic force was mentioned in our previous work25. 2.3 Measurement of nanofriction by using an LFM The nanofriction was measured by an Innova AFM (Bruker) operating in lateral force mode in an aqueous environment. A silicon nitride AFM probe (ORC8, the A cantilever, Bruker) with a typical length of 110 µm and a typical normal spring constant of 0.73 N/m was used. The real normal spring constant of the AFM probe is calibrated as 0.67 N/m using our previous method24. In the lateral force mode, the AFM probe first was pressed against the sample at a fixed applied normal load, and then the sample was driven to carry out a reciprocating linear motion in a direction perpendicular to the length direction of the cantilever. The signal of friction force was recorded during the scanning motion over a range of 10 µm to obtain the friction force by the following equation. ‫ܨ‬௙ =

ܸ௅ ܵ௅ ‫ܭ‬௅ 2‫ܪ‬ 4


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where VL is friction force signal in a single scanning cycle, SL is the lateral sensitivity of the detector, KL is the lateral spring constant of the AFM probe, and H is the distance between the bottom of the AFM tip to the midpoint of the AFM cantilever. The lateral sensitivity is calibrated using the method employed by Choi et al26 in every group of experiments. For a rectangular AFM cantilever, its lateral spring constant can be determined by the following equation27. ‫ܭ‬௅ =

2‫ܭ‬ே ‫ܮ‬ଶ 3(1 + ߥ)

where KN is the normal spring constant, L is the length of the AFM cantilever, and ν is Poisso’s ratio of the silicon nitride AFM probe. To study the pH-dependence of nanofriction, experiments were performed in electrolytes with different pH values. To study the velocity-dependence of nanofriction, the scanning rate was adjusted from 0.1 Hz to 20 Hz under a constant scan length of 10 µm to change the scanning velocity.

3. Results and Discussion 3.1 The surface charge densities of silicon wafer and OTS Figure 1 shows the electrostatic force applied on the SiO2 AFM colloidal probe when it approached both the silicon wafer and the OTS surface immersed in the electrolytes with different pH values. From Fig. 1, it can be found that both for the silicon wafer and OTS surface, the electrostatic forces on the AFM colloidal probe increased with increasing pH value of the electrolyte. This trend of the electrostatic force is related to the surface charge at the colloidal probe-electrolyte interface and the sample-electrolyte interface. Based on the electrostatic force between the sample and the SiO2 AFM colloidal probe in an aqueous environment and its relationship with surface charge densities at the two solid-liquid interfaces, the surface charge densities of silicon wafer and OTS can be quantified. Figure 2 shows the surface charge densities of silicon wafer and OTS immersed in electrolytes with different pH values. Although our method to quantify the surface charge density at the solid-liquid interface cannot provide the sign of the surface charge directly, the silicon wafer with a layer of SiO2 is believed to be negatively charged in the electrolytes with pH = 3, 6, and 9 because the isoelectric point of SiO2 is about 228, 29. According to the repulsive force between the AFM colloidal probe and OTS surface shown in Fig. 2, the OTS surface also is negatively charged. 5


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Furthermore, the absolute values of their surface charge densities increase with the increasing pH value. For the silicon wafer, its negative surface charge is generated by the dissociation of silanol groups at the SiO2-electrolyte interface30, 31. The increasing pH of the electrolyte can promote the dissociation of silanol groups, resulting in an increasing absolute value of surface charge density. For the OTS surface, its origin of surface charge is different from that of the silicon wafer. The surface charge of OTS in saline solution is generated by the adsorption of free ions from the electrolyte, including hydrogen ions, hydroxyl ions, and chloride ions. Furthermore, these three ions have different activities to be adsorbed on the OTS surface. Preferential sequence of these three ions to be adsorbed on the OTS is hydroxyl ion > chloride ion > hydrogen ion32. Based on this mechanism, the OTS surface is negatively charged. With the increasing pH value of the electrolytes, the ionic concentration of the hydroxyl ions increases, which means many more hydroxyl ions are adsorbed on the OTS surface, leading to the increasing absolute value of surface charge density for OTS surface. In order to analyze the dependence of nanofriction on surface charge, the surface charge of the silicon nitride AFM probe used to measure nanofriction is also needed. Although the surface charge of the silicon nitride AFM probe is not quantified in the present work, it can be derived from the literature. Based on the relevant literature, the isoelectric point of silicon nitride is about 6 ± 0.433. This means the silicon nitride AFM probe is positively charged in the electrolyte with pH = 3 and negatively charged in the electrolyte with pH = 9. In the electrolyte with pH = 6, the surface charge density of silicon nitride AFM probe can be assumed to be zero.

3.2 Surface charge-dependence of nanofriction Figure 3 shows the friction forces between the silicon nitride AFM probe and both the silicon wafer and the OTS surface in electrolytes with different pH values as a function of the applied normal load. There is a linear dependence between the friction force and the normal load, which is in good agreement with Amonton’s law. The slope of the friction force-normal load curve reflects the nano-frictional response of certain friction pairs, which is the so-called frictional coefficient. From the results shown in Fig. 3, it can be found that both the AFM probe-silicon wafer pair and the AFM probe-OTS pair show a decreasing frictional response with the increasing pH value of 6


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the electrolytes. That is, the frictional coefficient decreases with the increasing pH value of the electrolytes. The results presented here are similar to previously reported results11, 12. The variation of the nano-frictional response with the pH can be explained in terms of the electrostatic interaction between the AFM probe and the sample under the aqueous environment11, 12, which is related to the surface charge of the two solid-liquid interfaces, as the schematic cartoon shown in Fig. 4. According to our analysis of surface charge at the sample-electrolyte interface and AFM probe-electrolyte interface in Section 3.1, both the silicon wafer and the OTS surface are negatively charged in electrolytes with pH = 3, 6, and 9. However, the silicon nitride AFM probe is positively charged, nearly zero charged, and negatively charged, respectively, when the pH of the electrolytes is 3, 6, and 9. This means the electrostatic interaction between the AFM probe and sample shows a trend changing from attractive to zero and then to repulsive when the electrolyte pH increases from 3 to 6 and then to 9. This variation of the electrostatic interaction between the AFM probe and sample leads to a decreasing frictional response of the friction pair consisting of AFM probe and sample in an electrolyte environment. Additionally, when considering the potential mechanism of hydration lubrication13,14 on nanofriction, the attractive electrostatic interaction between the AFM probe and sample, which are charged with the opposite sign (in pH = 3), can compress the lubrication layer consisting of water molecules. However, the repulsive electrostatic interaction between the AFM probe and sample, which are charged with the same sign (in pH = 9) enhances the thickness of the lubrication layer. A thicker water lubrication layer has a better lubrication effect; thus, the case of pH = 9 can lubricate more effectively than the case of pH = 3, leading to a smaller nanofrictional coefficient, which is consistent with the results shown in Fig. 3. Nevertheless, it should be noted that although the effect of surface charge on hydration lubrication13,14 can be used to analyze the effect of surface charge on the nanofrictional behavior between an AFM probe and sample shown in Fig. 3, here, the effect of water lubrication layer on the nanofrictional behavior is weak. This is because the radius of the sharp tip on the AFM probe is small, meaning the water lubrication layer formed between the AFM probe and sample is small and can be neglected. It is believed that the effect of the water lubrication layer on nanofriction can be strong when a colloidal probe with a larger dimension is used because of the formation of effective water lubrication layer between the colloidal probe and the sample, as reported in the 7


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analysis of Donose et al13.

3.3 Velocity-dependence of nanofricion Figure 5 shows the velocity dependence of nanofriction between the friction pair consisting of sample and AFM probe in electrolytes with different pH values as a function of scanning velocity. From the results shown in Fig. 5, when the pH value of the electrolytes is equal to 3 and 6, the friction forces between both the silicon wafer-AFM probe pair and OTS-AFM probe pair show an increasing trend with increasing scanning velocity. When the pH value of the electrolyte is 9, there are different results for the silicon wafer-AFM probe pair and OTS-AFM probe pair. The friction force of silicon wafer-AFM probe pair shows a decreasing trend with the increasing scanning velocity, however, the friction force of OTS-AFM probe pair shows an increasing trend with the increasing scanning velocity. Although some previous literature20 studied the velocity-dependence of nanofriction based on LFM, their experimental setups were different from ours. In the work of Taran et al20, a colloidal probe, not a common AFM probe with a sharp tip, was used. Nevertheless, it can still be found that their results are similar with ours. They found that the nanofriction force decreases with the increasing scanning velocity when pH = 5.6 or pH = 10.6. Considering the materials of the colloidal probe and the sample are same, the surface charges at the colloidal probe and the sample keep the same sign when pH = 5.6 or pH = 10.6. Thus, their results are similar to the velocity-dependence of nanofriction force between the AFM probe and silicon wafer, which are charged with the same negative sign when pH = 9, presented in this work. It is believed that these phenomena are related to the variation of surface charge at the two solid-liquid interfaces when the AFM probe slides on the sample. Although the surface charge density at the solid-liquid interface for a solid-electrolyte-solid system is widely assumed to be constant, this assumption is not always valid. The surface charge at the solid-liquid interfaces is related to the separation distance between the two charged solid surfaces in practice when they are driven to approach each other. For the two charged surfaces with surface charge in the same sign, when they are forced into molecular contact, the counter-ions near the solid-liquid interface are forced to re-adsorb onto their original surface sites. This leads to decreasing surface charge densities at the solid-liquid interfaces, especially when the separation distance between the two charged solids is close to zero. This phenomenon is so-called charge regulation15, as shown in Fig. 6(a). For the two charged surfaces with surface charge in 8


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opposite sign, similar phenomenon can also be found because of the re-adsorption of the counter-ions or charge neutralization of the two approaching surfaces with surface charge in opposite sign. This charge regulation is the basis to analyze the velocity-dependence of the nanofriction. When the pH value of the electrolyte is equal to 3, the surface charge at the AFM probe-electrolyte interface and the sample-electrolyte interface are in opposite sign. This leads to a larger nanofriction because of the attractive electrostatic interaction between the AFM probe and the sample, as shown in Figs. 3 and 4. When the AFM probe slides on the sample at a low scanning velocity, the molecular contact between the AFM probe and samples means decreasing surface charge densities at the two solid-liquid interfaces from charge regulation as mentioned above. These decreasing surface charge densities result in a smaller attractive interaction between the AFM probe and the sample, leading to a smaller nanofriction. When the scanning velocity is higher, there is not enough time for the occurrence of charge regulation. That is, the surface charge densities at the solid-liquid interface do not have enough time to decrease, leading to a larger nanofriction as compared with the case of the lower scanning velocity. Thus, the nanofriction between both the silicon wafer-AFM probe pair and OTS-AFM probe pair shows an increasing trend with the increasing scanning velocity when the pH value of the electrolyte is equal to 3, as shown in Fig. 6(b). Similar results can be explained for the nanofriction between these two friction pairs when the pH value is equal to 6. When the pH value of the electrolyte is equal to 9, the surface charge at the AFM probe-electrolyte interface and the sample-electrolyte interface have the same sign, leading to a smaller nanofriction arising from the repulsive electrostatic interaction between the AFM probe and the sample, shown in Figs. 3 and 4. When the AFM probe is forced into contact with the sample at a molecular level and performs the scanning motion at a low velocity, the surface charge densities at both of the solid-liquid interfaces have enough time to decrease due to charge regulation. The decreasing surface charge densities at both of the solid-liquid interfaces induces a decreasing repulsive electrostatic interaction between the AFM probe and the sample, leading to an increasing nanofriction. However, for the case of high scanning velocity, the surface charge densities at the two interfaces do not have enough time to decrease and remain at a relatively high status. Thus, the repulsive electrostatic interaction between the AFM probe and the sample is larger for the case 9


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of higher scanning velocity. Therefore, the nanofriction shows an decreasing trend with increasing scanning velocity when the pH value of the electrolyte is equal to 9, as the results of silicon wafer shown in Fig. 5 and the schematic shown in Fig. 6(c). When considering the effect of water lubrication layer on the nanofriction, if the AFM probe and the sample are charged in the opposite sign (pH = 3), charge regulation at a lower sliding velocity results in a decreasing surface charge density and a smaller attractive electrostatic force between the probe and sample. This smaller attractive electrostatic force leads to a thicker water lubrication layer and a smaller nanofriction force. Similarly, when the AFM probe and the sample are charged in the same sign, a lower sliding velocity results in a smaller repulsive electrostatic force between the probe and sample by decreasing the surface charge density due to the occurrence of charge regulation. The smaller repulsive electrostatic force leads to a thinner water lubrication layer and a larger nanofriction force. This explanation regarding the effect of charge regulation on the water lubrication layer is consistent with our experimental results shown in Fig. 5. However, practically speaking, this effect can be neglected considering the small size of the sharp tip on the AFM probe. The main mechanism to explain the results presented in Fig. 5 is the effect of charge regulation on the electrostatic interaction between the AFM probe and the sample. Based on this analysis, the OTS surface in the electrolyte with pH = 9 should also show a similar result as the silicon wafer. However, the OTS surface shows a surprisingly opposite trend. This opposite result can be explained as follows. First, the surface charge density of the OTS-electrolyte is smaller as compared with the silicon wafer, as shown in Fig. 2. This may lead to a smaller degree of decrease in the surface charge density for the OTS surface due to charge regulation. The decreasing trend of nanofriction with the increasing velocity due to charge regulation is not obvious enough. On the other hand, the increasing fluidic drag on the AFM probe with the increasing scanning velocity counteracts the decreasing nanofriction with the increasing scanning velocity due to charge regulation. In addition, the fluidic drag on the AFM probe in the case of the OTS surface is larger considering the enhancement of boundary slip24, 25 at the OTS-electrolyte interface on the relative velocity between the sample and AFM probe as compared with the silicon wafer. This further reduces the effect of charge regulation on the nanofriction. Thus, the nanofriction of the OTS-AFM probe pair shows an increasing trend with the increasing opposite scanning velocity, which is opposite with the results of silicon wafer, and 10


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this is possible and reasonable. Thus, it is summarized that the velocity-dependence of nanofriction between a solid-electrolyte-solid system, which is affected by the decreasing surface charge densities arising from charge regulation, shows a different trend for the different surface charge status at the two solid-liquid interfaces. For the two charged solid surfaces with surface charge in the same sign, the system shows a negative correlation between the velocity and the nanofriction. That is, a decreasing nanofriction varies with the increasing velocity. However, for the two charged solid surfaces with surface charge in the opposite sign, the nanofriction of the system is positively correlated with the velocity; that is, an increasing nanofriction with increasing velocity.

4. Conclusions In this paper, the nanofriction between a silicon nitride AFM probe and both a silicon wafer and an OTS surface in an electrolyte is measured based on the LFM technique. The effects of pH on nanofriction and its velocity-dependence are studied, and the underlying mechanisms are analyzed regarding surface charge at the solid-liquid interface. The results presented in this study show that surface charge at the solid-liquid interface is an important factor affecting the nanofriction between an AFM probe and the sample immersed in an electrolyte. When these two solid-liquid interfaces have a surface charge in the same sign, a repulsive electrostatic interaction is generated, reducing the nanofriction of the friction pair in the electrolyte environment. In contrast, for the two solid-liquid interfaces with surface charge in the opposite sign, an induced attractive electrostatic interaction between the AFM probe and the sample will enhance their nano-frictional response. The velocity-dependence of nanofriction in an electrolyte shows a strong correlation with charge regulation, which is a phenomenon indicating decreasing surface charge densities for two approaching charged solid surfaces to a degree of molecular contact in an electrolyte. When the sliding velocity between the AFM probe and sample in molecular contact is low enough, charge regulation occurs. The surface charge densities of the AFM probe-electrolyte interface and the sample-electrolyte interface reduce, leading to a weaken electrostatic interface between the AFM probe and the sample. The reduction of electrostatic interface due to charge regulation results in a smaller nanofriction for the two charged interfaces with surface charge in opposite sign, but a 11


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larger nanofriction for the two charged interfaces with surface charge in same sign. When the sliding velocity between the AFM probe and the sample is high, there is not enough time for charge regulation to occur. Thus, the friction pair with surface charge in the opposite sign shows a larger nanofriction. However, the friction pair with surface charge in the same sign shows a smaller nanofriction when comparing with the case of a lower sliding velocity.

Acknowledgement The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51505292 and 51505108) and the Young Teachers Training Program of Shanghai, China (No. ZZsl15025).

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(17) Gnecco, E.; Bennewitz, R.; Gyalog, T.; Loppacher, Ch.; Bammerlin, M.; Meyer, E.; Güntherodt, H.-J. Velocity Dependence of Atomic Friction. Phys. Rev. Lett. 2000, 84, 1172-1175. (18) Tambe, N.; Bhushan, B. Friction model for the velocity dependence of nanoscale friction. Nanotechnology 2005, 16, 2309-2324. (19) Chen, J.; Ratera, I.; Park, J. Y.; Salmeron, M. Velocity Dependence of Friction and Hydrogen Bonding Effects. Phys. Rev. Lett. 2006, 96, 236102. (20) Taran, E.; Kanda, Y.; Vakarelski, I.U.; Higashitani K. Nonlinear friction characteristics between silica surfaces in high pH solution. J. Colloid Interf. Sci. 2007, 307, 425-432. (21) Granato, E.; Ying, S. C. Non monotonic velocity dependence of atomic friction. Tribol. Lett. 2010, 39, 229-233. (22) Braun, O.M.; Peyrard, M. Dependence of kinetic friction on velocity: Master equation approach. Phys. Rev. E 2011, 83, 046129. (23) Pan, Y.; Bhushan, B. Role of surface charge on boundary slip in fluid flow J. Colloid Interf. Sci. 2013, 392, 117-121. (24) Pan, Y.; Bhushan, B.; Zhao, X. (2014), Study of Surface Wetting, Nanobubbles and Boundary Slip with an Applied Voltage: A Review. Beilstein J. Nanotechnol. 2014, 5, 1042-1065. (25) Jing, D.; Bhushan, B. The coupling of surface charge and boundary slip at the solid-liquid interface and their combined effect on fluid drag: a review. J. Colloid Interf. Sci. 2015, 454, 152-179. (26) Choi, D.; Hwang, W.; Yoon, E. Improved lateral force calibration based on the angle conversion factor in atomic force microscopy. J. Microscopy 2007, 228, 190. (27) Gibson, C. T.; Watson, G. S.; Myhra, S. Lateral force microscopy-a quantitative approach. Wear 1997, 213, 72-79. (28) Eugène, P. Adsorption on Silica Surfaces; Marcek Dekker: New York, 2000. (29) Adamczyk, Z.; Nattich, M.; Wasilewska, M.; Zaucha, M. Colloid Particle and Protein Deposition-Electrokinetic Studies. Adv. Colloid Interf. Sci. 2011, 168, 3-28. (30) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (31) Behrens, S. H.; Grier, D. G. The charge of glass and silica surfaces. J. Chem. Phys. 2001, 115, 6716-6721. (32) Tian, C. S.; Shen, Y. R. Structure and Charging of Hydrophobic Material/water Interfaces 14


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Studied by Phase-sensitive Sum-frequency Vibrational Spectroscopy. Proc. Natl. Acad. Sci. 2009, 106, 15148-15153. (33) Lin, X.; Creuzet, F.; Arribart, H. Atomic force microscopy for local characterization of surface acid-base properties J. Phys. Chem. 1993, 97, 7272-7276.

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Figure Captions Fig. 1 The electrostatic force applied on the AFM colloidal probe that is approaching (a) silicon wafer and (b) OTS surface in an electrolyte with different pH values. Fig. 2 The effect of pH on the surface charge densities of silicon wafer and OTS in an electrolyte. Fig. 3 The friction forces between the AFM probe and (a) the silicon wafer and (b) OTS surface in electrolytes with different pH values as a function of the applied normal load. Fig. 4 Schematic showing the effect of surface charge on the nanofriction between AFM probe and the sample in an electrolyte with different pH values. Fig. 5 The velocity-dependence of nanofriction between the AFM probe and samples in electrolytes with different pH values. Fig. 6 Schematic of charge regulation (a) and its effect on the velocity-dependence of nanofriction between the AFM probe and the sample charged with the opposite sign (b) and charged with the same sign (c) in an electrolyte.

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Graphical Abstract 85x40mm (300 x 300 DPI)


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Fig. 1 The electrostatic force applied on the AFM colloid probe that is approaching (a) silicon wafer and (b) OTS surface in an electrolyte with different pH value 152x68mm (300 x 300 DPI)


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Fig. 2 The effect of pH on the surface charge densities of silicon wafer and OTS in an electrolyte 76x64mm (300 x 300 DPI)


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Fig. 3 The friction forces between the AFM probe and (a) the silicon wafer and (b) OTS surface in electrolytes with different pH values as a function of the applied normal load 149x69mm (300 x 300 DPI)


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Fig. 4 Schematic cartoon showing the effect of surface charge on the nanofriction between AFM probe and the sample in an electrolyte with different pH value 76x25mm (300 x 300 DPI)


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Fig. 5 The velocity-dependence of nanofriction between AFM probe and samples in electrolytes with different pH values 76x205mm (300 x 300 DPI)


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Fig. 6 Schematic cartoon showing the effect of surface charge on the velocity-dependence of nanofriction between AFM probe and the sample in an electrolyte 131x140mm (300 x 300 DPI)


Effect of Surface Charge on the Nanofriction and ... - ACS Publications

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