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Interstitial Water Enhances Sliding Friction Adrian P. Defante, Alex Nyarko, Sukhmanjot Kaur, Tarak Nath Burai, and Ali Dhinojwala Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00100 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Interstitial Water Enhances Sliding Friction Adrian P. Defante, Alex Nyarko, Sukhmanjot Kaur, Tarak N Burai, and Ali Dhinojwala∗ Department of Polymer Science, The University of Akron, Akron, OH E-mail: [email protected]

Abstract

This study examines how surfaces with different water contact angles (wettability) affects dry and underwater adhesion and friction. These studies were conducted by bringing a deformable hydrophobic poly(dimethyl siloxane) (PDMS) lens in contact with surfaces of gradient wettability. Based on our adhesion and friction results, we divide the results in three regions. In Region I (water contact angles greater than 80◦ ), the dry adhesion is lower than underwater adhesion. In contrast, in Region III, (water contact angles less than 50◦ ), the dry adhesion is higher than underwater adhesion. For surfaces with water contact angles between 50◦ to 80◦ (Region II), the dry and wet adhesion values are comparable. Interestingly, in this Region II, the underwater coefficient of friction (COF) values are higher than those in Regions I and III. We have used surface sensitive sum frequency generation spectroscopy (SFG) to probe if the contact interface in static conditions and during dynamic sliding is dry or wet. The SFG results reveal that the contact is dry in Region I. If this dry contact is maintained, the underwater COF follows the trend of adhesion hysteresis in dry conditions (adhesion hysteresis decreases with an increase in water contact angles). In Region ∗ To

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III, the contact is wet and the underwater COF follows the trend for adhesion hysteresis in wet conditions (adhesion hysteresis increases with an increase in water contact angles). By knowing whether the contact interfaces are dry or wet, we can relate the trends in COF with trends in adhesion hysteresis. For conditions where the contact interfaces have both dry and wet patches (Region II), the COF values are higher than those in completely dry conditions, suggesting that a partially lubricated system can exhibit a higher COF.

Introduction Fundamental studies that investigate the role of water in interfacial interactions are described mostly by three different scenarios. In one scenario, two hydrophobic surfaces are expected to drive out water at the contact interface promoting strong adhesion. 1–3 In an opposite scenario, water is present between two contacting hydrophilic surfaces which reduces adhesion by preventing contact and provides a lubricating film for facile sliding. 4–6 In the third scenario, hydrophobichydrophilic contact, water is also pervasive in the contact area and its presence does not necessarily eliminate adhesive contact as in the case for two contacting hydrophilic surfaces. 7,8 These types of contact scenarios only explain a limited number of cases. What is the impact on adhesion and friction underwater when a hydrophobic surface makes contact with a surface of interfacial energy in between the two extremes of hydrophobic and hydrophilic contact? One could interpolate between these two extremes and assume that water should infiltrate the contact zone between these two limits. If water is present at the contact interface, then what is its influence on friction and adhesion? Understanding these scenarios have a broader impact on prevalent and more practical underwater contact problems. 8–11 Molecular simulations have investigated the role water plays in underwater adhesion between two surfaces that differ in wettability. 12,13 They present an adhesion map as a function of water contact angles for two surfaces and the results are divided into three regions; hydration repulsion, dry adhesion, and cavitation-induced attraction. They predict that an underwater contact between

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a hydrophobic and hydrophilic surface can lead to hydration repulsion, which is consistent with the previous observation of confined water between a hydrophobic polydimethylsiloxane (PDMS) lens and a hydrophilic sapphire substrate. 7 In the other extreme, where a contact is made between two hydrophobic surfaces, the simulation results predict a cavitation-induced attraction. This is consistent with the dry contact observed between a PDMS lens and an OTS-coated sapphire substrate. 3 The simulation results predict that the contact between a surface with a water contact angle of 105◦ and surfaces with variable wettability undergoes a transition from hydration repulsion to cavitation-induced attraction for a surface with a water contact angle of around 60◦ to 75◦ . Under these conditions, the type of contact is described as dry adhesion. These types of contacting surfaces, also described as asymmetric contacts, are the most prevalent in many applications, yet experimentally understudied. In this study, we explore the role wettability plays in underwater adhesion, and friction. We have fabricated surfaces with gradient wettability by exposing a pristine hydrophobic monolayercoated surface to a gradient oxygen plasma chemistry. We have measured adhesion and friction between a PDMS lens and this gradient surface with water contact angles from 0◦ to 110◦ in both dry and wet conditions. We have explored the question whether there is a relationship between adhesion and friction or adhesion hysteresis and friction as expected from earlier studies. 14–16 The answers to these fundamental questions are relevant to a broad range of contact related phenomena that are encountered in consumer applications including those observed for animal adhesion. This basic understanding will help in designing more efficient surfaces with higher friction and adhesion properties for underwater applications. 8,17–21

Results and discussion Preparation and characterization of surfaces Figure 1a shows the static water contact angles, θ , for a self-assembled monolayer of octadecyltrichlorosilane (OTS) silanized on glass. The contact angles measured across the length of the 3


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substrate are 110◦ ± 2◦ and indicates a well-ordered OTS monolayer was formed. 22 To alter the surface wettability, the pristine monolayer is treated with air plasma, a method used to increase the wettability of the surface by generating polar molecules on the surface. The set-up used for plasma treatment is shown in Figure 1b. This strategy was chosen because it minimizes the need to use thiol chemistry on gold substrates to alter surface chemistry 23 and can be adopted for applications in industrial processes. 24 The spacing between a glass shield and an OTS-coated substrate controls the severity of the oxygen plasma. Since the spacing between these two plates changes monotonically, we obtain surfaces with a continuous gradient of wettability. This gradient can be tuned by adjusting the exposure time of oxygen plasma or by changing the tilt angle. This treated glass slide is cut along lengthwise into pieces measuring between two cm to three cm to test adhesion and sliding friction. The changes in contact angles in Figure 1a and Figure 1c suggests the chemistry of the OTS film can be altered in a controlled manner. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection infrared spectroscopy (ATR-IR) provide direct evidence of the completion of OTS silanization process and the influence of oxygen plasma on these hydrophobic monolayers. The XPS results in Figure 2a show an increase in the intensity of the C1s peak for an OTS film (θ = 110◦ ) compared to the bare substrate (θ = 0◦ ). Changes in the high resolution C1s spectral data (Figure 2b-g) are observed for surfaces with water contact angles between 0◦ and 110◦ . With no plasma treatment, OTS has the highest percentage of C-C bonds and the intensity of this carbon peak decreases with increasing wettability. See Supporting Information for quantitative results from XPS analysis (SI1). As the contact angle increases, there is an increase in the contribution from peaks corresponding to C − O −C and O −C = O, suggesting the formation of polar groups as a product of oxygen plasma chemistry. The intensity of the O1s peak was not analyzed because this peak could originate from the underlying SiO2 layer on top of the silicon wafer. The XPS results also measure the silicon peak from the supporting substrate because it has a penetration depth of around 10 nm. This is larger than the 5 nm thick OTS layer expected based on monolayer coverage. 25

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Increased we+ability Figure 1: a.) The water contact angles of a glass surface coated with an OTS monolayer and after exposure of this OTS monolayer to an air plasma for two or four seconds. The volume of the water drop size was two µL. The wettability of the surfaces is controlled by the spacer height and exposure time of the plasma chemistry treatment. b.) Schematic of the surface preparation set-up, where the spacer height was kept constant in these experiments. The dimensions of the shield, length (l) and width (w) are 75 mm by 25 mm. c.) The water contact angles can be visualized along the distance of the glass substrate and are higher when the surfaces are exposed to less plasma either through a small shield spacing or a shorter exposure time. All uncertainties in this manuscript are expressed as standard uncertainty, n = 3.

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Figure 2: The elemental composition of the treated surfaces are characterized by XPS shown in the a.) survey scans and b-g.) high resolution scans of the C1s region.

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Figure 3: The vibrational spectra of the treated surfaces are shown by ATR-IR in the a.) hydrocarbon and b.) carbonyl region. The two peaks in the hydrocarbon region correspond to 2850 cm−1 CH2,sym , and 2935 cm−1 - CH2,asym . The bulk OTS sample is for OTS in the solution state used for surface functionalization. The 1-minute plasma sample refers to an OTS treated sample exposed to plasma treatment for one minute. In Figure 3a, the ATR-IR results show the hydrocarbon peaks (2850 cm−1 , 2935 cm−1 ) for an OTS functionalized surface. These two peaks correspond to the methylene symmetric (2850 cm−1 ) and asymmetric stretching (2935 cm−1 ) vibrations, consistent with the chemical structure of OTS. These results along with evidence from XPS and water contact angles indicate the bare surfaces are functionalized with a well-ordered OTS monolayer. With an increase in wettability, the methylene signal decreases and after exposure to one minute of plasma, the hydrocarbon signals are lost due to the removal of the OTS monolayer. Figure 3b shows an increase in the intensity of the carbonyl functional groups (1720 cm−1 , 1640 cm−1 ) upon exposure to oxygen plasma indicating increase in polar groups. The ATR-IR results are consistent with the observations of polar groups in the high resolution C1s peak in the XPS data. The carbonyl peaks are absent in ATR-IR scans for an OTS monolayer or a surface treated with plasma for one minute. The infrared spectroscopy and XPS data support the introduction of polar groups due to plasma treatment and this results in lower water contact angles. The surface characterization measurements demonstrate that the alteration of the OTS mono7


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layer using oxygen plasma process is a simple method to modulate the surface wettability. The plasma treatment does not contaminate the OTS-coated surfaces with other elemental groups. The oxygen plasma treated surfaces are composed of a mixture of chemical groups with terminal methyl and oxygen (carbonyl) containing functional groups. These surfaces in contact with a hydrophobic PDMS surface corresponds to an asymmetric contact. 12,13 From here on we will refer to these oxygen plasma treated surfaces by their static water contact angles to describe the adhesion and friction results.

Adhesion and friction measurements Figure 4a shows the thermodynamic adhesion energy, Wa , measured by zero load Johnson-KendallRoberts (JKR) approach in dry and underwater conditions. The contact area of a gently placed PDMS lens atop a surface is measured under no externally applied normal load (SI2). As the wettability decreases from 0◦ to 110◦ in dry conditions, there is a slight decrease in the Wa from 40 mJ/m2 to 30 mJ/m2 that occurs around θ 60◦ . As the wettability increases, the underwater Wa decreases from θ 110◦ to θ 60◦ . Below θ 60◦ the Wa is not measurable because the lens does not adhere and make contact. Adhesion energy was also calculated using the Young-Dupré equation by measuring the contact angles of liquid PDMS on gradient substrates. This method yields similar adhesion results as the zero load JKR measurements in dry and wet conditions (SI3, Figure SI3). From here on, our discussion focuses on the Wa measured by the JKR method Based on the adhesion measurements, the results can be divided into three different regions. In Region I (θ ≥ 80◦ ), the underwater adhesion values are higher than dry adhesion. In Region III (θ ≤ 50◦ ), the dry adhesion is higher than wet adhesion. There exists a transition zone, Region II (50◦ ≤ θ ≤ 80◦ ), where both dry and wet adhesion are comparable. The definition of our three regions are consistent with the molecular dynamics (MD) simulation results where the Region III is similar to the region dominated by hydration repulsion and Region I that is dominated by cavitation-induced attraction. 12 The expected transition from hydration repulsion to cavitationinduced attraction for PDMS (water contact angle of around 105◦ ) from the MD results is 75o , 8


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Figure 4: A comparison of the adhesion and friction measurements for dry and wet conditions as a function of water contact angle: a.) Work of adhesion, Wa , measured by zero load JKR contact mechanics approach. b.) Pull-off forces, F p , measurements. The pull-off velocity was set to 0.4 µm/s. c.) Coefficient of friction (COF) measurements. The dashed lines are used to guide the eye for the COF measurements. The sliding velocity was set to 5 µm/s for the friction experiments. d.) Adhesion hysteresis was calculated by the difference in the Wa measured from zero load JKR and the Wa calculated from F p , measurements using the JKR equation. The shaded blue and red regions represent the calculated uncertainty for adhesion hysteresis. The measurements are categorized into three regions. The highlighted green area bounds Region II.

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which overlaps with our definition of Region II. Because the MD results do not provide quantitative values of adhesion energies, we are unable to directly compare the experimental and simulation results. In Figure 4b, adhesive strength was also quantified by measuring pull-off force, F p . In dry conditions the F p increases with increasing wettability. For PDMS in contact with blank glass or sapphire substrate, the pull-off forces are extremely high and we would expect a cohesive failure, consistent with previous observations on glass or sapphire substrates. 7,26 The values of F p measured underwater decrease with an increase in wettability, similar to the zero load JKR measurements underwater. The Regions I, II, and III used to categorize the Wa data in Figure 4a can also be used to categorize the pull-off data in Figure Figure 4b. The coefficient of friction (COF) for sliding PDMS lenses on gradient surfaces in dry and submerged in water are shown in Figure 4c. The COF, was calculated from the slope of the measured shear force as a function of normal load (SI4, Figure SI4). The friction between an elastomeric lens and solid substrate could be characterized by a single value of COF. This could be due to an almost linear dependence of the normal load and the actual contact area. In Region III the COF measurements in dry conditions are higher than wet conditions. In the transition zone, Region II, the COF measurements are similar in magnitude in dry and wet conditions. Surprisingly in Region I the underwater COF is lower than dry COF, even though the wet adhesion is higher than dry adhesion. This anomaly leads to the maximum underwater COF in Region II. Because friction can be influenced by roughness, Atomic Force Microscopy (AFM) was used to measure the surface roughness (SI5, Figure SI5). No statistical differences were observed in the values of root mean square height as a function of surface wettability (SI5). A comparison of adhesion and friction measurements leads to both expected and unexpected results about the role surface wettability plays in these interfacial measurements. As expected, the underwater Wa decreases with increasing wettability. In Region III, the increase in surface polarity reduces underwater contact between the PDMS lens and the substrate due to the presence of strongly bound water molecules leading to a loss of adhesion and a lower COF. 6,27 The increase

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in polar molecules confirmed by ATR-IR and XPS measurements explains the higher pull-off force in dry conditions. Unexpectedly, the strong adhesion in wet conditions in Region I, does not lead to high underwater COF values, suggesting that these results are perhaps related to adhesion hysteresis as proposed before. 14,15,28 Figure 4d plots the calculated values of adhesion hysteresis in dry and wet conditions. Adhesion hysteresis is calculated by taking the difference between the work of adhesion during pull-off 2F

(Wa = - 3Rπp ) and the work of adhesion during loading (Wa ). Because we are unable to measure pull-off forces and the work of adhesion during loading for surfaces with identical contact angles, we fitted the data in Figure 4a and Figure 4b using a weighted linear regression (SI7, Figure SI7). The results of this linear regression was then used to calculate adhesion hysteresis. The adhesion hysteresis trend as a function of wettability is completely opposite in dry and wet conditions. The adhesion hysteresis in dry conditions increases with an increase in wettability. In contrast underwater, the adhesion hysteresis decreases with an increase in wettability. Since the COF has been shown to relate with adhesion hysteresis rather than adhesion, it appears that the increasing trend for the COF in dry conditions is similar to the increase in dry adhesion hysteresis with an increase in wettability. Underwater, the COF does not follow underwater adhesion hysteresis completely. Instead, the COF follows the wet adhesion hysteresis for water contact angles less than 60◦ to 70◦ . For water contact angles greater than 60◦ to 70◦ , the COF measured underwater follows the trend for dry adhesion hysteresis. Because there is no quantitative model at this time to relate the COF measured underwater with adhesion hysteresis, the observations comparing the trends of underwater COF and adhesion hysteresis are discussed qualitatively. Qualitative trends in the COF measured underwater explained by adhesion hysteresis suggests the nature of contact (dry or wet contact). The COF measured underwater trends with dry adhesion hysteresis in Region I, where the contact is expected to be dry. In Region III, the contact is expected to be wet because the COF measured underwater trends with wet adhesion hysteresis. The absence or presence of water for the two wettability extremes has been shown in our previous results using

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surface sensitive sum frequency generation spectroscopy (SFG). 3,6,7 However, if the nature of the contact interface for each Region I and III remains true, why then is the COF measured underwater lower than dry conditions in Region I? Also, what is the nature of the contact interface in Region II if the COF in dry and wet conditions are comparable? Direct confirmation is needed about the state of the contact interface for the different Regions to provide a better understanding for underwater COF.

SFG spectroscopy to study the static and dynamic contact interface Figure 5a shows the SFG data for surfaces in Regions I, II, and III. The nonlinear nature of SFG spectroscopy provides details about molecular structure and orientation of surface molecules, not measurable by linear vibrational spectroscopy (ATR-IR) or elemental bond energies (XPS). The SFG data shows no distinguishing peaks for the untreated sapphire surface, which indicates that this surface with a contact angle of 0◦ is free from contamination, similar to the Si crystal used for ATR-IR measurements. For a surface with a contact angle of 110◦ (θ OT S,110◦ -air), the SFG spectra show two peaks at 2871 cm−1 and 2931 cm−1 , assigned to vibrational modes of terminal methyl groups, which is a signature of a well-ordered OTS monolayer. 29 For surfaces with a contact angle of θ 70◦ -air, the SFG spectra has two peaks assigned to symmetric and asymmetric methylene vibrations 2857 cm−1 and 2924 cm−1 . 30 The SFG data for θ 60◦ -air are different than θ OT S,110◦ -air and θ 70◦ -air with spectral features of methylene and methyl groups (2850 cm−1 , 2872 cm−1 , 2919 cm−1 ) 30,31 . For θ 50◦ -air, we observe a combination of weak methylene and methyl peaks (2853 cm−1 , 2877 cm−1 , 2929 cm−1 , 2943 cm−1 ). The observations of methylene peaks signify the presence of gauche defects and increased disorder. Figure 5b shows the SFG data for the same surfaces in Figure 5a in contact with water. At the OTS-water interface (θ OT S,110◦ -H2 O) the OTS film maintains a well-ordered monolayer structure underwater with a small concentration of gauche defects indicated by a weak peak at 2865 cm−1 . 3,32 This methylene peak is more prominent for θ 70◦ -H2 O, but this interface is well ordered compared to the other treated surfaces with a lower contact angle. The changes in SFG intensity 12


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Figure 5: The effect of plasma treatment is shown in the static SFG spectra collected in SSP polarization (s-polarized SFG, s-polarized visible, p-polarized infrared) at the a.) air interface for surfaces of different wettability. Spectra are offset for clarity to compare changes in spectral features. The same surfaces are exposed to water and the SFG spectra were collected for the b.) hydrocarbon and c.) water region. d.) The full spectrum (2800 cm−1 to 3800 cm−1 ) of the sapphire-H2 O interface is shown for clarity. These spectral features are different than the ATR-IR measurements due to the sensitivity of SFG to both concentration and molecular orientation. The solid lines are fit to a Lorenztian equation and the parameters are summarized in SI8. SFG spectra are multiplied in some cases for the comparison of spectral features. Regions I, II, III are marked in reference to the regions described in the macroscopic measurements in Figure 4.

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of the methyl and methylene peaks for θ 60◦ -H2 O indicate restructuring of the surface and the blue shift in the spectral assignments shows the influence of the interaction of water at this treated interface. 33 The SFG spectrum of θ 50◦ -H2 O shows a dramatic loss in hydrocarbon signal compared to θ 50◦ -air indicating a highly disordered monolayer film underwater. The SFG spectra collected in the water region (3000 cm−1 to 3800 cm−1 ) are shown in Figure 5c. The peak assignments for strong, weak, and non-bonded vibrational peaks for hydrogen bond stretches of water occur at 3150 cm−1 , 3400 cm−1 , and 3700 cm−1 , respectively. 34 Observations for water are focused on the strongly coordinated water peak near 3150 cm−1 because the 3700 cm−1 peak could also be assigned to OH groups on the sapphire substrate. 35 This strongly coordinated water peak is observed for all of the surfaces (SI8 and SI9, Figure SI9). In addition to this strongly coordinated water peak the SFG spectra for the sapphire-water interface (θ sapphire,0◦ -H2 O) show a weakly coordinated water peak and the absence of any hydrocarbon contamination. The SFG data in the hydrocarbon region are shown for underwater contact between PDMS and the treated OTS surfaces in Figure 6a for Regions I, II, and III. For Region I, the θ OT S,110◦ -PDMS interface shows the lens is in contact with a well-ordered monolayer composed of terminal methyl groups (2872 cm−1 , 2927 cm−1 ). This is similar to the spectra observed for the θ OT S,110◦ -air interface containing minimal gauche defects. A similar spectra is also observed for the θ 70◦ -PDMS interface (2882 cm−1 , 2938 cm−1 ) which also returns to a trans-like structure under contact. For θ 60◦ -PDMS, a change in the contact interface is observed by the appearance of methylene groups (2857 cm−1 , 2927 cm−1 ). Methyl and methylene peaks (2854 cm−1 , 2936 cm−1 ) are observed for θ 50◦ -PDMS. For θ sapphire,0◦ -PDMS contact, the hydrocarbon signal originates from the SiCH3,asym groups of PDMS (2955 cm−1 ) (SI10). 7,36 In the water region, the absence of a peak near 3150 cm−1 indicates dry contact. This is observed for Region I, θ OT S,110◦ -PDMS, and partly in Region II, θ 70◦ -PDMS (SI9, Figure SI9). In Region II with a water contact angle less than 60◦ and Region III, confined water is present at the contact interface. In Region I, the dry contact established underwater supports the higher underwater adhesion (Wa and F p ) in comparison to dry adhesion. This dry contact also supports a higher COF for

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II II III Figure 6: The static SSP SFG spectra for PDMS in contact with surfaces of different wettability underwater in the a.) hydrocarbon and b.) water region.

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θ OT S,110◦ -PDMS compared to θ sapphire,0◦ -PDMS. In Region III, water is present in the contact region leading to poor adhesive contact, that promotes easier sliding compared to Region I due to a lubricating water film. In Region II, the contact remains dry at θ 70◦ -PDMS and then becomes partially wet from θ 60◦ -PDMS onwards to Region III (SI9, Figure SI9). In this transition region the SFG results indicate heterogeneous contact due to the presence of both hydrocarbon and water signals. Surprisingly, the SFG results for θ 60◦ -PDMS show the presence of water in the contact zone while the adhesive contact is still maintained. This patchy contact between two substrates, has been observed in MD simulations. 12 This transition from dry to wet confinement around θ = 60◦ to 75◦ , 12 similar to our transition region from dry to wet contact. For θ ≥ 60◦ , the contact remains dry during contact and the MD simulations predict a cavitation induced adhesion in this region. Overall, the SFG data corroborates the nature of contact suggested by adhesion hysteresis data and how these hysteresis measurements are useful in explaining the trends in underwater COF in Regions I and III. Although a dry or wet contact interface measured underwater can be used to explain the qualitative trends in the COF, there are several experimental observations that are not clear. For example in Region II, confined water is present at the contact interface while the COF in dry and wet conditions are comparable. This indicates that the presence of water alone does not automatically lead to a lower underwater COF compared to dry contact. The static SFG spectroscopy also does not explain why the magnitude of COF measured underwater in Region I is lower than compared to dry conditions despite the fact that contacts in both conditions are dry. It is possible that the SFG spectroscopy in static conditions may not capture the molecular structure during sliding. The SFG data during sliding conditions are needed to better understand the COF results. To address the question concerning the differences at the contact interface between static and sliding contact underwater, the peak intensity ratio for both conditions, Rθ ◦ = Iλ1 / Iλ2 , are compared in Figure 7. To obtain the Iλ during sliding, we measured the changes in SFG signals at a fixed wavenumber as the lens overlaps with the laser spot. The Iλ1 is the peak intensity from water (3100 cm−1 ) and Iλ2 is the peak intensity from a hydrocarbon peak at the static contact interface (2940

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cm−1 or 2950 cm−1 ). Details about the acquisition of SFG sliding data are described elsewhere. 6

Rθ = I3100 cm-1/I2950 cm-1

Rθ = I3100 cm-1/I2940 cm-1

Figure 7: a.) A comparison of the SFG intensity ratio, R, for Regions I, II, and III. Rθ ◦ = Iλ1 / Iλ2 where Iλ1 is the intensity for the wavenumber. The Iλ1 is the peak intensity from water (3100 cm−1 ) and Iλ2 is the peak intensity from a hydrocarbon peak (2940 cm−1 or 2950 cm−1 at the static contact interface). R0◦ is normalized with respect to the methyl peak of PDMS at 2950 cm−1 . The 8 other Rθ values are normalized from the hydrocarbon peaks of the monolayer film. Comparing the values for R for static and sliding contact is a measure to compare changes of interfacial structure for the different contact modes underwater. In Region I, Rθ ◦ for static contact is close to zero indicating dry contact as expected by the adhesion measurements. This ratio does not change during sliding and indicates that dry contact is maintained for θ OT S,110◦ -PDMS and θ 70◦ -PDMS. In Region II, the ratio also does not change for R60◦ , but the contact is wet instead of dry in static and sliding contact. Moving from Region II after a water contact angle of 60◦ towards Region III, the water is still present during sliding. An increase in R50◦ and R0◦ during sliding compared to static contact indicates an increase in water signals, which could be due to multiple reasons. Since the SFG signals are a function of both orientation and concentration of surface groups, it is difficult to draw any conclusions for the reason behind these differences in SFG intensity. Overall, the SFG results during sliding show that the conditions of static contact whether dry or wet is maintained during sliding. SFG probed the contact interface and confirmed whether the contact measured underwater was

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dry or wet suggested by the adhesion and friction data. The dry contact during static and sliding conditions explains the correlation between the COF measured underwater and the dry adhesion hysteresis in Region I. Similarly, the wet contact during static and sliding contact explains the decrease in the COF measured underwater. This wet contact results in a decrease in wet adhesion hysteresis with an increase in surface wettability in Region III. Figure 8 summarizes several possible mechanisms either on its own or in combination used to explain the trends in the adhesion hysteresis data and the COF measured underwater when either the contact is dry or wet. Dry Contact: In Region I, the SFG spectra of the treated monolayers prior to contact shows that these films rearrange when in contact with water and further rearrange back when in contact with PDMS. These changes could be interpreted as changes in surface mobility, monolayer packing, or chain interdigitation of the monolayer with the PDMS lens. These factors could give rise to an increase in dry adhesion hysteresis and the COF measured underwater as the surface wettability increases. 14,28,37–39 These large differences in the SFG spectra outside and inside of the contact region underwater also could result in more energy dissipation as the surfaces slide past one another where the molecules have to switch from one orientation to another. 33 An increase in the COF and adhesion hysteresis could also be explained by increase in polar interactions within the dry contact patches due to increase in the concentration of polar groups (XPS and ATR-IR results). Wet Contact: The SFG results show that the wet contacts are patchy and that these patches increase in size causing the fraction of interstitial water to increase with an increase in surface wettability (Region II to III). The changes in the quantity of this interfacial water could result in a decrease in the wet adhesion hysteresis and COF measured underwater in two possible ways. First, as the wettability of the contacting surface increases, the intercalated film of nanometer thin water becomes thicker. Confined water in the boundary lubrication regime predicts the viscosity of water to depend on the film thickness. This would cause the viscosity to trend from a solidlike behavior to a more liquid like bulk behavior leading to a decrease in wet adhesion hysteresis and COF measured underwater. 7,40,41 The second possibility is that the isolated patches of trapped water in Region II could lead to a higher COF underwater due to the increased energy required

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Possible mechanism(s)

II

III

I Dry Contact

Wet Contact

Top View Side View Wettability Figure 8: A schematic of the contact area for each Region is used to explain the potential mechanism(s) for the COF measured underwater pictured as a top and side view of the contacting PDMS lens. In Region I the contact is in a dry state (white shaded area) and the COF is controlled by the state of the monolayer. In Region II, a small amount of interstitial water is present between the PDMS lens and the surface. Emphasis of Region II and III is focused on envisioning interstitial water. Visual representation of the treated films in these Regions is difficult to diagram from the SFG results. This boundary layer of trapped water is relatively thin compared to the trapped water in Region III and is arranged as unconnected islands. In Region III, the islands of trapped water create a percolated network of water that promotes smooth sliding. This water film exhibits more fluid like properties compared to Region II because it is thicker. The guide to the eye in red indicates dry contact and wet contact in blue.

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to drag the trapped water across hydrophobic patches during sliding. These islands of trapped water are sealed by the solid-solid contact between PDMS and the solid surface. As the wettability increases, these patches begin to percolate into a continuous network of water thus creating a stable lubricating film to lower the COF that is measured in Region III. A consequence of the increase in the quantity of water is a reduction in the contact area that could decrease the COF from Region II to Region III. These results are intriguing because the COF measured underwater in Region II remains high even with the presence of interstitial water. It is possible that not one single proposed mechanism contributes to the observed sliding friction phenomena, but could be a combination of the possibilities described above. The absolute values of the COF in OTS measured in dry and underwater contact are difficult to explain based on the spectroscopy results. In both the dry and wet case, there is the absence of strongly or weakly coordinated water. In addition, the static contact SFG spectra for both wet and dry conditions are similar as shown in our earlier work. 3 This should have suggested that the COF values should have been similar as well. There is the possibility of the presence of non-bonded water within the spectral region between 3500 cm−1 to 3700 cm−1 when the PDMS lens slides across the surface. Because this region has an overlapping peak from the sapphire surface, 35 we are unable to comment about the influence of this unbound water on the COF during sliding. This should be explored in future experiments using deuterated water (D2 O) instead of water. Despite these complications, it is clear that water is playing a unique role in sliding friction and its mere presence does not necessarily lead to facile sliding. 42–44

Conclusions The main objective of this study was to understand underwater adhesion and friction between a hydrophobic PDMS elastomer and surfaces of different wettability. It was known that the contact of two hydrophobic surfaces in water will result in stronger wet adhesion than dry adhesion and the contact of a hydrophobic PDMS and hydrophilic surfaces would result in patchy contact with no adhesion. Here for surfaces with different wettability we divide our results in three regions. In 20


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Region I, the wet adhesion is higher than dry adhesion. In Region III, the wet adhesion is lower than the dry adhesion and in Region II, the wet and dry adhesion are comparable. We find that the trends in adhesion hysteresis, and not adhesion, match the trends in the COF measurements. In Region I, the underwater COF measurements and dry adhesion hysteresis decreases with increase in water contact angles. In Region III both underwater COF measurements and adhesion hysteresis increases with increase in water contact angles. Surface sensitive SFG spectroscopy was used to probe the nature of the contact interface in static and sliding conditions and the spectroscopy results confirmed the conclusions reached from the adhesion and friction experiments that the contact interface in water is dry in Region 1 and patchy in Regions II and III. Even though SFG data showed the presence of interstitial water in Region II, the COF measurements in dry or wet conditions were comparable. This indicates that just the presence of confined water does not necessary lead to a lower COF. Surprisingly, in Region I the underwater COF measurements were lower than dry COF, while the SFG data revealed that underwater contact did not show trapped liquid-like confined water. However, we were not able to rule out the presence of non-hydrogen bonded confined water which could also alter the mobility and friction between a PDMS lens and monolayer surfaces. Regardless of the origin of the mechanism, water is playing a unique role in these interfacial processes. 45–47 From a broader perspective, this work provides insight into how a small amount of trapped water may actually lead to higher, not lower friction. First, it may explain why adhesive pad structures are more successful for wet adhesion used by variety of animals such as geckos, frogs, and ants. 17–20,48,49 In these cases, a thin layer of interstitial water does not cause slippage at the interface for a hydrophobic surface in contact with a surface that is partially wettable, but instead provides a stronger frictional adhesion. On a practical level, this work provides guidance for tuning wettability to fabricate surfaces that provide strong grip in both dry and wet conditions and this optimum condition occurs for surfaces where the water contact angles are between 60◦ to 70◦ . The oxygen plasma treatment used here provides a simple, scalable, and inexpensive processing method to tune surface wettability to mass-produce a variety of structural adhesive materials for

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underwater applications.

Materials and Methods OTS monolayer preparation and poly(dimethylsiloxane) lens formation has been described previously. 3 Prior to silanization, all substrates used (glass, silicon dioxide, sapphire) were cleaned by soaking in hot Piranha solution (7:3 H2 SO4 and H2 O2 ) for one hour and rinsed thoroughly with copious amounts of DI water. Specifically, for silicon substrates, a 5 wt. % of hydrofluoric acid (HF) in DI water was used to remove the native oxide layer prior to cleaning. CAUTION must be taken when using Piranha and HF solution. Elemental surface analysis was measured by X-ray Photoelectron Spectroscopy (XPS). The spectrometer used was a PHI 5000 Versaprobe III Surface Analysis Instrument from Physical Electronics. The chamber pressure during analysis was maintained to about 2 x 10−6 Pa. The spectra were recorded with micro-focused Al Kα X-ray radiation (25 W, 15 kV, 100 µm). The take off angle of the photoelectron was fixed at 45◦ . Survey scans and high resolution N1s, O1s and C1s spectra were collected with an analyzer pass energies of 117.4 eV and 11.75 eV respectively and step sizes of 0.5 eV and 0.1 eV respectively. High resolution survey scans were measured in the C1s region in order to monitor changes in atomic bonding for the carbon atoms due to plasma treatment. Vibrational infrared spectra were collected using an iS50 FTIR system equipped with a mercurycadmium-tellerium (MCT) detector manufactured by Thermofisher. A PIKE HATR (Attenuated total reflection) accessory was used to collect multibounce (10 bounces) ATR spectra from a surface functionalized silicon crystal. To collect the spectra of a plasma treated monolayer, the control sample of an OTS monolayer was first measured. If the OTS monolayer measured a similar spectrum to that of bulk OTS, the crystal was then plasma treated in a similar manner as the procedure described in the main text. Unlike the surface generated for macroscopic measurements, the silicon crystal was not exposed to the gradient on its long axis. This would generate a gradient across the silicon crystal rendering the surface inhomogeneous. Instead, the crystal was placed in differ22


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ent positions underneath the shield. After each exposure to plasma treatment, the wettability was measured using static water contact angle measurements, dried with N2 gas, and then the ATR infrared spectrum was collected. After the last plasma treatment, the crystal was thoroughly cleaned as described earlier and used as the background spectrum. This ATR infrared spectrum from this pristine crystal was used as background subtraction for the previous measurements. The spectrum collected was average of 100 interferograms with a 4 cm−1 step scan. Adhesion energy was measured three ways. The first method uses contact angle measurements. This is not to be confused with the static water contact angles probed by DI water to characterize surfaces of different wettability generated by the plasma treatment of the OTS monolayer. For the adhesion measurements, PDMS liquid was brought into contact onto the substrate of interest in either dry or underwater conditions. The adhesion energy was estimated using the Young-Dupré equation. These measurements were done three times on the same substrate on different spots. The second method uses contact mechanics to measure adhesion energy. A PDMS lens was gently placed on the substrate of interest in both dry and wet conditions. The contact area was measured using an optical microscope. These measurements were done three times on the same substrate by removing the lens and placing on a different contact spot. From JKR theory, the adhesion energy was calculated from the zero-load equation, Wa = Ka3 /6πR2 , where a, R, and K are the contact area, measured radius and modulus of the lens, respectively. The modulus of the PDMS lens was estimated from the JKR equation by a measuring a loading cycle of a PDMS lens to a glass substrate. The third method uses the JKR equation to estimate adhesion energy from a pull-off force described in the main text and SI7, Figure SI7. Friction measurements were performed using a homebuilt biaxial friction cell as previously described. The sensors for both normal and shear force were calibrated using known weights in the range of 1 to 98 mN. 6 A PDMS lens was brought in contact with the SAM substrate first at different normal loads (1 to 30 mN). The substrates were first tested dry then wet. The lens and substrate were completely submerged and brought into contact underwater. These loads correspond to pressures of 10 to 100 kPa based on adhesion energies estimated using the Hertzian and JKR theories.

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An independent measurement using a force sensor and optical microscope supported our calculations for contact area. The COF was obtained from the slope of the applied normal force versus the measured shear force. The shear force was normalized with respect to the calculated contact area from JKR theory. The trend in the COF as a function of surface wettability yielded similar results. R The lens was slid at a rate of 5 µm/s, using the Newport Optics Picomotor . Measurements were

repeated at least three times on different substrates and contact spots. Statistical analysis of the measured samples are shown in SI6, Figure SI6. Roughness measurements were conducted using atomic force microscopy (AFM) on prepared substrates using a Dimension Icon III AFM (Bruker, Massachusetts, USA) with a silicon nitride tip (Tap 300-G, Ted Pella, California, USA), Tapping mode in air. Topography images were taken using Nanoscope v8.10 and analyzed with Nanoscope Analysis v1.5. Roughness values were reported as root-mean-square (RMS) roughness. Three various locations on the sample were measured using a scan size of 1 µm x 1 µm to obtain a standard error value. A power spectra density plot was also obtained using Nanoscope analysis v1.5, calculated from each topography file. Sum frequency generation (SFG) spectroscopy is an interface specific nonlinear optical technique used to obtain information about the chemical makeup of the surface. SFG signal is generated when two beams of different frequencies are overlapped spatially and temporally at the interface. To probe the molecular groups present at the interface, a tunable infrared (IR) beam is overlapped with the visible beam and the SFG intensity is enhanced when the IR frequency matches with the vibrational stretch of the interrogated molecular groups. SFG spectra are collected at room temperature using a picosecond laser system of Spectra Physics having a tunable IR beam (2000 cm−1 to 3800 cm−1 , 1 ps pulse width, 1 KHz repetition rate) and 800 nm wavelength visible beam (1 ps pulse width, 1 KHz repetition rate). The spot area where the beams are overlapped is approximated to be 2 mm2 to 3 mm2 . The spectra were collected in both SSP and PPP polarization combination. The OTS monolayer is coated on an equilateral sapphire prism which is then plasma treated to obtain the desired wettability corresponding to the desired water contact angle. As a proxy for a hydrophilic surface bare sapphire is used in place

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of glass or silicon as both surfaces contain similar surface chemistries and are completely wetted by water. The critical angle used to probe the particular contact angle surface against air, water and PDMS contact are 42◦ , 16◦ and 8◦ respectively. SFG data collected to understand dynamic contact is done in a similar fashion to friction experiments measured by the biaxial friction cell. The spectrometer to record SFG intensity is held at a specific wavelength of interest such as 2940 cm−1 . The laser position is also fixed on the sample as a polydimethoxysiloxane (PDMS) lens slides across the surface of interest at 5 µm/s moving in and out of contact. The SFG signal intensity is recorded as a function of time.

Acknowledgement The authors thank Mr. Ed McLaughlin for construction and fabrication of instrumental devices. In addition, we thank Mr. Michael Wilson, Mr. Siddhesh Dalvi, Ms. Amanda Stefin, Mr. Yang Zhou, Mr. Mario Echeverri, Dr. Katherine Vorvalakos, Dr. Manoj Chaudhury for discussions. We thank the financial support of National Science Foundation (AD), Coloplast (TNB) and the Akron Functional Materials Center (Matthew L. Becker).

Supporting Information Available The Supporting information provides the XPS analysis, procedure used for calculating adhesion energy using zero load method, adhesion energy calculated using the Young-Dupré equation, friction data as a function of normal load, root mean square roughness measured using AFM, calculation of adhesion hysteresis, and results of the parameters used to fit the SFG spectra. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Coefficient of Friction (COF)

Graphical TOC Entry

Wettability

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Interstitial Water Enhances Sliding Friction - Langmuir (ACS

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