Friction Reduction at a Superhydrophilic Surface: Role of Ordered

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Friction Reduction at a Superhydrophilic Surface: Role of Ordered Water Chunlei Wang,† Binghai Wen,†,‡ Yusong Tu,§ Rongzheng Wan,† and Haiping Fang*,† †

Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P.O. Box 800-204, Shanghai 201800, China ‡ College of Computer Science and Information Engineering, Guangxi Normal University, Guilin 541004, China § College of Physical Science and Technology, Yangzhou University, Yangzhou 225001, China S Supporting Information *

ABSTRACT: Low-friction but superhydrophilic materials are urgently needed in biomedical and engineering fields because of their nonfouling property and biocompatibility, particularly when the surfaces are definitely superhydrophilic, such as metal or TiO2 as the surface coatings of the intravascular stents. However, generally, there is a higher friction coefficient on the superhydrophilic surfaces than on the hydrophobic surfaces. On the basis of molecular dynamics simulations, we show that the friction on the superhydrophilic surface with appropriate charge patterns is evidently reduced, where the lower friction is similar to that of a rather hydrophobic surface with a contact angle of water droplet of ∼44°. This reduction is attributed to the existence of an ordered water monolayer on the superhydrophilic surface with appropriate charge patterns, and the friction between this ordered water monolayer and the water molecules above is small.

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INTRODUCTION The nature of surface boundary friction,1−14 related with the development of microfluidic and nanofluidic systems,15−19 the surface lubrication and nanotribology,3−7 and the motion of biological molecules,20 has long been a subject of interest. One main challenge is how to effectively reduce the friction between materials and water, which is important in various applied fields, such as self-clearing materials with superhydrophobic porous nanostructures,13 the biomedical materials,7,9 and nanocoating materials of ships.21 It is generally accepted that there is relatively low friction on hydrophobic surfaces but relatively high friction on hydrophilic or superhydrophilic surfaces.15,22−26 However, hydrophobic materials are deficient in terms of biocompatibility and their nonfouling properties, which are exactly important for applications, such as the coatings of marine or ship surfaces to prevent fouling with nonspecific proteins.9,21 Thus, when surfaces are definitely superhydrophilic, such as the TiO227 or metal Ta28 as the surface coatings of the intravascular stents, low-friction materials in merit of nonfouling are of urgent need in applications.9 For example, implanted intravascular stents and catheters with high biocompatibility require low friction so they can be easily maneuvered within the patient’s vasculature;9,29 the surfaces coatings of marine or ship that can prevent the fouling with nonspecific proteins also require the low surface friction to save energy in the navigation of the ships.21 In 2009, we predicted a unique wetting phenomenon with a water droplet on a water monolayer on a superhydrophilic surface with a water layer completely spreading over the surfaces with rather high surface water interactions at room temperature, © 2015 American Chemical Society

which was termed an ordered water monolayer that does not completely wet water.30−35 A similar phenomenon has also been observed on many real solid surfaces in experimental27,36−38 or numerical studies.39−41 However, whether this ordered water on the solid surface with charges results in friction reduction is still unknown. In fact, Huang et al. proposed that there is no one-to-one correspondence relationship between the wetting property and surface friction.42 This indicates that the relationship between surface friction and surface hydrophobicity/hydrophilicity is complicated. In the previous work by Ho et al.,43 a slip length indicating a friction reduction on a hydrophilic model surface with contact angle of 30° has been found. The mechanism resulting in the large slip lies that the water molecule easily migrate from one adsorption site to the neighboring sites but without necessarily leaving the contact layer. However, this mechanism may not be transferred to the superhydrophilic surface due to fact that the water molecules at the superhydrophilic surfaces are usually strongly bound, such as on the mica, talc, hydroxylated SiO2, and Al2O3 surfaces. In this article, using molecular dynamics simulations, we show that the friction on the superhydrophilic surface is greatly reduced even when the water molecules are strongly bound by the appropriate charge patterns, where the low friction is similar to that of a rather hydrophobic surface with a contact angle of water droplet of ∼44°. This friction reduction is attributed to Received: March 2, 2015 Revised: May 2, 2015 Published: May 4, 2015 11679

DOI: 10.1021/acs.jpcc.5b02024 J. Phys. Chem. C 2015, 119, 11679−11684

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The Journal of Physical Chemistry C

the space). The molecular dynamics (MD) simulations were performed for 30 ns, and the data from the last 20 ns were collected for analysis. The data are averaged by running three independent systems for each surface type. In a second set of simulations, we used the systems with 1000 water molecules on each surface to study the wetting behavior. We followed the work by Vanzo et al.46 for calculating the contact angle of a cylindrical water droplet, and other settings were the same as our previous work.30,34 The method for calculating the contact angle of water droplet on the water monolayer is the same as in our previous work,30 and the contact angle of a water droplet directly appearing on solid surface is the same as in the previous work by Guo et al.47 All of the MD simulations used a time step of 1.0 fs, and the periodic boundary conditions in three dimensions with a constant volume and temperature (NVT) were used. The Berendsen thermostat48 was used to maintain the temperature at 300 K. The Lennard-Jones parameters of the solid atoms were εss = 0.105 kcal/mol and σss = 3.343 Å.30,35 The Lorentz−Berthelot rule was used, and the SPC/E model was used for water. The particle-mesh Ewald method with a real space cutoff of 10 Å was used to treat long-range electrostatic interactions, and a 10 Å cutoff was applied to all van der Waals interactions.

the existence of the ordered water structures on the superhydrophilic surface with appropriate charge patterns, and the friction between this ordered water monolayer and the water molecules above is small. In practical applications, the ordered water monolayers have been observed on several real solid surfaces,27,36−41,44 suggesting that there is a high probability that materials associated with unique friction properties existing.

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SIMULATION METHODS As shown in Figure 1A, we have implemented the nonequilibrium Couette flow to investigate the surface friction

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RESULTS AND DISCUSSION Generally, when the strength of the surface−water interactions increases, the surfaces become more hydrophilic and the friction of water molecules flowing across the surface increases. Figure 1C shows the surface−water interaction energy per unit area (E) near the upper plates, where the interactions are calculated summing the van der Waals interaction and electrostatic interactions between all the water molecules and the surfaces per unit surface area. Because of the large surface charge values (q = 0.80 e), the surface−water interaction energy per unit area of the type 2 and 3 solid surfaces are expected to be larger and these two surfaces are expected to be much more hydrophilic than the type 1 surface with q = 0.45 e.49 The calculations of the surface−water interaction energy per unit area show that the energy for the type 2 surface (E2 = 200.7 kJ mol−1 nm−2) is very close to that of the type 3 surface (E3 = 201.9 kJ mol−1 nm−2), and both are more than twice that of the type 1 surface (E1 = 84.8 kJ mol−1 nm−2), i.e., E1 < E2 ≈ E3. To measure the surface friction when pulling the upper plate, we calculate the frictional stress σ in these Couette fluid systems with the same velocity of the upper plate. Here, σ is calculated by σ = F/s, where F is the friction force that is equal to the shearing force and s is the surface area. It is expected that the order of friction stress for the three types of surfaces would be σ1 < σ2 ≈ σ3, that is, consistent with the order of the surface− water interaction energy per unit area of E1 < E2 ≈ E3. To our surprise, the frictional stresses of the Couette fluid systems for the three types of surfaces are in the order σ1 ≈ σ2 < σ3; that is, σ2 = (2.08 ± 0.08) × 107 Pa is 11% less than σ3 = (2.34 ± 0.11) × 107 Pa, but very close to σ1 = (2.09 ± 0.09) × 107 Pa, as shown in Figure 1C. This result shows that there is a clear friction reduction on the superhydrophilic surface when comparing type 2 and the type 3 surfaces, despite the very close surface−water interaction energies. To further understand the mechanism of friction reduction at the superhydrophilic surface, we analyze the friction coefficients of the solid/liquid interfaces of these three systems. The analysis show that the thickness of the first water layer in contact with the solid surface was 0.4 nm, which is consistent

Figure 1. (A) Side-view snapshot of the simulation system with v = 100 m/s in the zigzag direction of the upper surface. (B) Hexagonal lattice of the solid surface showing charged pairs. Red and blue spheres represent solid atoms of opposite charge, and the green spheres represent neutral solid atoms. Magenta arrow shows a crystallographic direction. (C) Frictional stress σ at the solid surface of Couette fluid systems and surface−water interactions per unit area E on the three different surfaces (types 1−3). (D) Friction coefficient λ and apparent contact angles 44°, 74°, and 0° of the three types of surfaces. λsurface (red solid bar) is the surface friction coefficient that couples the solid/ liquid friction coefficient λsolid/liquid (blue solid bar) and the friction coefficient of the first−second water layers λ1,2 (green solid bar).

based on molecular dynamics simulations. The simulation systems consisted of two solid plates with a 4 nm thick water slab. Each solid surface contained 1664 atoms with a hexagonal structure similar to the graphene surface, and the hexagonal nanoscale charge patterns with positive and negative charges of same magnitude q were assigned to atoms located diagonally in neighboring hexagons. We have marked neighboring bond lengths denoted l the planar hexagonal structure (see Figure 1B). Overall, the model solid surface was neutral. The MD simulation software Gromacs45 was used to study the nonequilibrium Couette flow for the purpose of investigating the surface frictions. During the simulations, the solid surfaces were fixed and the top plate moved with a constant velocity v = 100 m/s along the zigzag direction (marked with red arrow in Figure 1B) of the surface. For comparison, we designed three types of surfaces with different length l and charge values q: type 1 (l = 0.170 nm and q = 0.45 e), type 2 (l = 0.142 nm and q = 0.80 e), and type 3 (l = 0.170 nm and q = 0.80 e). For the systems with three surface types 1, 2, and 3, the number of water molecules was 7166, 5103, and 7166, respectively, under the same pressure (∼1 atm) in the confined space (see PS3 in the Supporting Information for the density of water confined in 11680

DOI: 10.1021/acs.jpcc.5b02024 J. Phys. Chem. C 2015, 119, 11679−11684

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The Journal of Physical Chemistry C with our previous results.30,35 We adopt the method proposed by Falk et al.16 to evaluate the friction coefficient λsolid/liquid between the surface and the first water layer (see PS3 of the Supporting Information for more discussions on the role of the first water layer in surface−water interactions):

λ = F /(AΔv)

Interestingly, this relationship is similar to the classical law to calculate the electrical resistance of the parallel circuit. As shown in Figure 1D, λsurface of the type 2 surface is (2.7 ± 0.2) × 106 N s/m3, which is 25% less than that on the type 3 surface ((3.5 ± 0.2) × 106 N s/m3) but quite close to that on the type 1 surface ((2.3 ± 0.3) × 106 N s/m3). This agrees with the previous frictional stress order σ1 ≈ σ2 < σ3 and indicates that unless combining the friction coefficient λsolid/liquid with λ1,2, the friction reduction effect cannot be appropriately explained. To the best of our knowledge, the mechanism that λsolid/liquid and λ1,2 jointly determine the surface friction coefficient has not been previously reported. Note that the friction reduction from the type 2 to the type 3 surface mostly originates from λ1,2 (95%) rather than λsolid/liquid because λsolid/liquid is 1 order of magnitude larger than λ1,2. This friction reduction might be universal for different pulling velocities (see PS1 of the Supporting Information for more discussion when v = 50 m/ s). In Figure 1D, we show the wetting behavior of the three types of surfaces. A water droplet with a contact angle of θ1 = 44° forms on the type 1 surface, which is quite close to the contact angle of water on the Si surface.51 However, the wetting behavior on the type 2 and 3 surfaces is completely different, despite the close surface−water interaction energy per unit area. A water droplet with a contact angle of 74° forms on the water monolayer on the superhydrophilic type 2 surface with a water layer completely covering the surfaces, which is called an ordered water monolayer that does not completely wet water.30,35 However, no clear water droplet but thin water film forms on the type 3 surface, also indicating a superhydrophilic solid surface. Despite the much larger surface− water interaction energy near the type 2 surface (E2 = 200.7 kJ mol−1 nm−2) than near the type 1 surface (E1 = 84.8 kJ mol−1 nm−2), the contact angle on the type 2 surface (θ2 = 74°) is unexpectedly larger than that on the type 3 surface (θ1 = 44°), and the surface frictions of the two surfaces are similar. Our work is similar to the previous results showing that the friction coefficient on boron nitride is much larger than that on graphene on the very similar structure between the graphene and boron nitride sheet.52 However, these results are found on the hydrophobic surfaces of graphene and boron nitride, different from our simulation results on the superhydrophilic surfaces. Our results also show that the contact angle, surface− water interaction energy, and surface friction are not one-toone corresponding each as expected intuitively in theory.42 Why is the friction coefficient on the type 2 surface significantly less than that on type 3 surface but similar to that on type 1 surface? To answer this question, we first compare the type 2 and type 3 surfaces. Considering that the friction reduction from the type 2 surface to the type 3 surface mostly originates from λ1,2 (95%) rather than λsolid/liquid, we investigate the first water layer and calculate the number of hydrogen bonds NHB formed per water molecule in the first layer with the water molecules above. As shown in Table 1, NHB

(1)

where λ is the friction coefficient of the two contacting surfaces, F is the friction force of the two contacting surfaces that is equal to the shear force, A is the contact area, and Δv is the velocity difference of the solid and the first water layer. As shown in Figure 1D, for the type 1 surface λsolid/liquid = (1.0 ± 0.1) × 107 N s/m3. When the charge q is increased to 0.80 e for the type 2 and 3 surfaces, the solid surfaces are expected to be more hydrophilic and λsolid/liquid is expected to be greater than that of the type 1 surface. Indeed, the λsolid/liquid values of these two surfaces are as large as (5.0 ± 1.5) × 107 and (7.0 ± 1.9) × 107 N s/m3 for the type 2 and 3 surfaces, respectively, which are several times larger than that on the type 1 surface, indicating the very strong binding of water molecules. This seems to be consistent with the general view that when a surface becomes more hydrophilic with stronger surface−water interactions, the surface friction also increases. However, this result is in sharp contrast to the data shown in Figure 1C, where the friction stress near the type 2 surface is less than that near the type 3 surface but almost equal to that near the type 1 surface, i.e., σ1 ≈ σ2 < σ3. This contradiction motivates us to further consider the roles of the friction coefficients between the first water layer and the water layers above. We choose the thickness of the second water layer to be 0.4 nm and thus divide the water along the z-axis into 10 layers each with a thickness of 0.4 nm. We then uniformly measure the local internal friction between the nth and (n + 1)th (1 ≤ n ≤ 9) water layers analogous to eq 1. To our surprise, when we calculate the friction coefficient at the first/second water layer interface, we find that the friction coefficient λ1,2 near the type 2 surface is (2.8 ± 0.1) × 106 N s/ m3 (see Figure 1D). This is 7% and 24% less than that the corresponding values of the type 1 and 3 surfaces ((3.0 ± 0.1) × 106 N s/m3 near the type 1 surface and (3.7 ± 0.2) × 106 N s/m3 near the type 3 surface), respectively. Because of the symmetry of the systems, the friction coefficient near the upper surface is almost the same as that near the bottom surfaces (see Figure S1 in Supporting Information). We also calculate the friction coefficients λn−1,n (3 ≤ n ≤ 8) between the (n − 1)th and nth water layers along the z-axis and find that the friction coefficients λn−1,n (3 ≤ n ≤ 8) are almost constant at (1.8 ± 0.2) × 106 N s/m3 (see also Figure S1 in Supporting Information) for all three systems with surface types 1−3. Clearly, this result shows that the friction coefficient between water layers in the bulk water is different from that between first water layer and the second water layer λ1,2. This may account of the nonlinear effect of velocity profiles close to the surface when the shear rate is large (see Figure S5 in Supporting Information), which have also been found in previous work by Netz et al.50 When comparing the friction coefficients of the three systems, we only consider the friction coefficient λsolid/liquid and λ1,2 due to the constant λn−1,n (3 ≤ n ≤ 8). Here, we propose a new parameter λsurface to describe the surface friction coefficient that combines λsolid/liquid and λ1,2 (see Supporting Information PS1 for more details about the derivation of this relationship): 1/λsurface = 1/λsolid/liquid + 1/λ1,2

Table 1. Number of Hydrogen Bonds per Molecule between the First Monolayer and Water Molecules above (NHB) and the Corresponding Attractive Interactions Eww for the Three Types of Surfaces

(2) 11681

surface type

type 1

type 2

type 3

NHB Eww (kJ mol−1)

1.09 −29.1

0.76 −20.0

1.01 −28.2

DOI: 10.1021/acs.jpcc.5b02024 J. Phys. Chem. C 2015, 119, 11679−11684

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The Journal of Physical Chemistry C = 0.76 for the type 2 surface with l = 0.142 nm, which is 25% less than that of the type 3 surface with l = 0.170 nm (NHB = 1.01). This fewer number of hydrogen bonds leads to an 8.2 kJ mol−1 decrease in the attraction energy Eww (electrostatic energy plus van der Waals energy) between each water molecule in the water monolayer and the water molecules above, which is consistent with the 24% lower friction coefficient λ1,2 between the monolayer and the contacting water of the type 2 surface compared with the type 3 surface. Consequently, this fewer number of hydrogen bonds should be the physical origin of the friction reduction of the type 2 surface compared with the type 3 surface. Furthermore, this fewer number of hydrogen bonds formed per water molecule in the first layer with the water molecules above near the type 2 surface can be also attributed to the well-ordered water structure. As shown in Figure 2, a hexagonal pattern (blue

the large slip on the hydrophilic surface indicating the friction reduction is attributed to the water molecules that easily migrate from one adsorption site to a number of neighboring others without necessarily leaving the contact layer. This fewer number of hydrogen bonds is also the physical origin of the water droplet with the unexpected large contact angle of 74° on the ordered water monolayer on the type 2 surface.30,35 We then compare the type 1 and type 2 surfaces. As shown in Table 1, NHB = 0.76 for the type 2 surface, which is 29% less than that of the type 1 surface (NHB =1.09). This can be attributed to that there is no ordered water configuration on the type 1 surface (see Figures 2). However, a water droplet with a contact angle of 44° forms because of the weaker surface−water interaction E1 = 84.8 kJ mol−1 nm−2 for the type 1 surface. However, interestingly, the λsurface values are similar for the type 1 and type 2 surfaces (see Figure 1D). This can be attributed to that the counterbalance of the larger λ1,2 value of the type 1 surface because the larger interlayer hydrogen bond number NHB (see Table 1) and the smaller λsolid/liquid value because of the smaller solid−water interaction of the type 1 surface. This clearly demonstrates the importance of combining λsolid/liquid and λ1,2, which jointly determine the surface friction coefficient λsurface.

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CONCLUSIONS Contrary to the general viewpoint that friction on hydrophilic or superhydrophilic surfaces is always high, we show that the friction on the superhydrophilic surface with appropriate charge patterns can be evidently reduced, which is similar to that of a rather hydrophobic surface with a contact angle of a water droplet of ∼44°. This friction reduction is attributed to the existence of the ordered water structures on the superhydrophilic surface with appropriate charge patterns, and the friction between this ordered water monolayer and the water molecules above is small. We also highlight the importance of combining the friction coefficient λsolid/liquid between the surface and the first water layer and the friction coefficient λ1,2 between the first water layer and the water layers above, which jointly determine the surface friction coefficient λsurface. This highlights the role of water molecules themselves, which are usually neglected, in determining the solid/water interfacial frictions. Our work provides a new way to reduce surface friction by introducing ordered water on the superhydrophilic surfaces. In practice, room temperature ordered water exists on a variety of materials surfaces, including the sapphire,37 titania (TiO2),27 talc,39 hydroxylated SiO2 and Al2O3,40 the Pt(100),41 and mica.53 We believe that the results in this study will prompt more extensively experiments and may be of great help for designing superhydrophilic materials requiring low surface friction and other possible engineering fields. We noted that the TiO2 is used as the coatings of the biomedical intravascular stents. Very recently, it has been also found that the similar phenomenon of “ordered water that does not completely wet water” exists on the TiO227 surface at room temperature, which may indicate that the friction reduction on the superhydrophilic surface indeed exists but is still unrevealed before.

Figure 2. (A) Snapshots of the structures of the first water layer near the three types of solid surfaces. The blue hexagonal pattern of ordered water near the type 2 surface is shown as a guide for the eye. (B) Angular probability distribution of angle φ between the xy plane projection of one water molecule’s dipole orientation in the first water layer and a crystallographic direction for the three different types of solid surfaces.

lines) of ordered water formed near the type 2 surface while the water structure is disordered near the type 3 surface. We then calculate the probability distribution of the water dipole orientation angle φ,30,35 which is the angle formed between the projection of a water molecule dipole orientation onto the xy plane and a crystallographic direction (see Figure 1B of magenta arrow showing this direction). As shown in Figure 2B, there is a clear orientation preference of the water dipole on the type 2 surface with three peaks at 30°, 150°, and 270°, indicating hexagonal ordered water. However, the peaks are much smaller for the type 3 surface with l = 0.170 nm, even though the charge of the atoms is the same (q = 0.80 e). This shows that the arrangement of the charge dipoles on the solid surface is important for the formation of the ordered water structure on the superhydrophilic surface as well as the number of hydrogen bonds formed per water molecule in the first layer with the water molecules above. Clearly, compared with disordered water, these ordered hexagonal configurations lead to an increase of the number of hydrogen bonds within the monolayer and consequently a decrease of the number of hydrogen bonds30 and a smaller friction coefficient λ1,2 between this ordered monolayer and the second water layer. This mechanism is greatly different from the previous work43 that

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ASSOCIATED CONTENT

S Supporting Information *

More discussion on the analysis of the friction coefficient in the confined space, effect of pulling velocity on surface friction, role of the first water layer in surface water interactions, and density profiles of the water confined in the space. The Supporting 11682

DOI: 10.1021/acs.jpcc.5b02024 J. Phys. Chem. C 2015, 119, 11679−11684

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Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02024.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully thank Drs. Guosheng Shi, Wenpeng Qi, Bo Zhou, Jingye Li, and Yi Gao for the helpful discussions. This work was supported by the NSFC (grants 11290164, 11204341, and 11362003), the Key Research Program of Chinese Academy of Sciences (grant KJZD-EW-M03), the Knowledge Innovation Program of the Chinese Academy of Sciences, the Youth Innovation Promotion Association CAS, the Shanghai Supercomputer Center of China, the Deepcomp7000 and ScGrid of the supercomputing Center, and the Computer Network Information Center of the Chinese Academy of Sciences.

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DOI: 10.1021/acs.jpcc.5b02024 J. Phys. Chem. C 2015, 119, 11679−11684