How Slippery are SLIPS? Measuring Effective Slip on Lubricated

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How Slippery are SLIPS? Measuring effective slip on lubricated surfaces with colloidal probe AFM Liam Ronald John Scarratt, Liwen Zhu, and Chiara Neto Langmuir, Just Accepted Manuscript • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Figure 1. Schematic illustration of (a) the no-slip boundary condition, and (b) the partial slip boundary condition with corresponding slip length b. (c)-(f) Optical micrographs of static contact angle in air of (c) silicone oil on OTS-Si, with value of < 5°, (d) water on OTS-Si infused with excess silicone oil, with wetting ridge visible; (e) silicone oil droplet under water on OTS-Si, with contact angle value of 8°; (f) silicone oil droplet under water on Si with contact angle of 133°. 27x33mm (300 x 300 DPI)

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Figure 2. AFM approach and withdraw force curves on OTS-Si in air (a) non-infused, and in sucrose solution on OTS coated silicon (k = 2.9 N/m), (b) non-infused (k = 3.6 N/m), and (c) infused with 300 ± 2 nm silicone oil (k = 2.1 N/m). In (c) force curves were taken in the space between dewetted oil droplets. AFM cantilevers used had spring constants k = 2 – 4.5 N/m. 104x186mm (300 x 300 DPI)


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Figure 3. (a) – (b) Hydrodynamic approach force curves using colloid probe AFM in sucrose solution at an approach rate of 40 µm/s on (a) OTS-Si, (b) OTS-Si infused with silicone oil. (c) Average values of effective slip length over multiple experiments. Colloid probes used had radius 8.5 – 9.5 µm, and spring constant k = 0.18 – 0.9 N/m. 116x198mm (300 x 300 DPI)


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How Slippery are SLIPS? Measuring effective slip on lubricated surfaces with colloidal probe AFM Liam R. J. Scarratt, Liwen Zhu, Chiara Neto* School of Chemistry and the University of Sydney Nano Institute, The University of Sydney, New South Wales 2006, Australia. Keywords: Slip Length, Hydrodynamic Drainage Force, SLIPS, Colloidal Probe AFM, Lubricant Infused *[email protected] Abstract Lubricant-infused surfaces have attracted great attention recently and are described as slippery (SLIPS). Here we measured hydrodynamic drainage forces on SLIPS by colloid probe atomic force microscopy (AFM) and quantified the effective slip length over a nano-thin silicone oil layer on hydrophobized (OTS-coated) silicon wafers. The thickness of a stable silicone oil film on OTSSi under sucrose solution was determined to be 1.8 ± 1.3 nm, and found to induce an average effective slip length of 29 ± 3 nm, very close to that of an uninfused OTS substrate. These relatively low values of effective slip are confirmed by the relatively large macroscopic roll-off angle values of water droplets on the same substrates. Both the nano- and macro-scale results reflect the immobilized nature of a silicone oil layer of thickness around 2 nm within an underlying


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monolayer. These results have important implications for the design of drag-reducing coatings using lubricant infusion. Introduction Slippery liquid infused porous surfaces (SLIPS) have attracted much attention in the past 5 years for their anti-fouling,1-2 anti-icing,3 and potential drag-reducing properties.4-5 They rely on microand nano-scale surface topography to trap a lubricant film by capillary wicking, coating the surface and forming a liquid interface.6-8 It has been shown that surface feature size and morphology influence retention of lubricant under different shearing conditions,9 with the initial wetting configuration informed by the interfacial surface energy of solid-lubricant-liquid combinations.1011

Recently, it has been shown that the properties of SLIPS can be achieved also on smooth, non-

structured surfaces infused with lubricants,2, 12-13 and liquid-like grafted brush layers.14-15 The dragreducing properties of SLIPS have received little attention,16-19 recent work showed that in turbulent flows lubricant infusion can lead to as much as 35% reduction in drag.20-21 The stability of the lubricating film, crucial to drag reduction and other SLIPS functions, is under intense scrutiny due to depletion by exposure to high shear flows,16 and minimal solutions to this issue have been proposed.22-24 The amount of lubricant covering SLIPS is crucial to its function, and is expected to determine the degree of drag reduction. The minimum thickness of lubricant needed to provide drag reduction or the effectiveness of a partly-dewetted lubricant film in inducing interfacial slip are not known. Techniques with nanoscale resolution are needed to answer this question. Atomic force microscopy (AFM) is ideally suited to study laminar flows on liquidinfused surfaces, as it can determine liquid film thickness,25 and interfacial slip with nanoscale accuracy.26-27


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The flow of liquids over solid surfaces can be described by identifying the boundary conditions (Figure 1).26-27 The no-slip boundary condition (Figure 1(a)), whereby the relative velocity of a liquid flowing over a surface becomes zero at the surface, correctly describes liquid flow at the macroscale for most surfaces. The partial slip boundary condition (Figure 1(b)) allows a liquid flowing over a surface to have a finite velocity at the interface, and defines slip length b as the distance inside the surface at which the velocity of the liquid extrapolates to zero.26 Colloidal probe AFM is among the most accurate techniques to establish the boundary conditions of flow, by the measurement of the hydrodynamic drainage force acting on a microsphere vertically approaching a surface, as the liquid is squeezed out between them.26 The hydrodynamic drainage force Fh acting on a sphere perpendicularly approaching a wall is described by Brenner’s solution of the NavierStokes equations28 in the lubrication approximation Fh = [(6πηr2υ)/h]f*, where η is the viscosity of the liquid, υ is the velocity of the approaching microsphere, and r is the radius of the microsphere (r >> separation distance h, making the surfaces locally parallel). The correction factor f* allows for the presence of partial slip on the surfaces, and depends only on slip length b, and separation distance h.29 The correction factor f* reduces Fh acting on the approaching microsphere, and enhances flow velocity at the interface. Over the past 15 years it has been established that solvophilic surfaces have lower slip length than solvophobic surfaces.30-31 In the case of lubricantinfused surfaces, the “effective” slip over a surface can be measured to quantify the reduction of hydrodynamic drag due to the presence of the lubricant on the solid surface.27 In colloid probe AFM experiments, the effective slip length b is found as the only fit parameter for experimental Fh to the partial-slip equation. Measuring the effective slip is of great practical interest as it can inform the design of drag reducing coatings, and allows meaningful results to be collected even in the presence of molecular grafted layers, roughness, and lubricant layers.27, 32 In this paper, the


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effective slip length was quantified for a thin film of silicone oil (10 cSt) on a smooth hydrophobised (OTS-coated) silicon wafer, by measuring hydrodynamic forces in a sucrose solution with viscosity of approx. 43 cSt, used to increase the signal/noise ratio.32 This work provides insight into the drag reducing properties of an ultra-thin residual lubricant film left behind after dewetting; macro-scale sliding angle experiments revealed a dependence of slippery properties on lubricant film thickness.

Figure 1. Schematic illustration of (a) the no-slip boundary condition, and (b) the partial slip boundary condition with corresponding slip length b. (c)-(f) Optical micrographs of static contact angle in air of (c) silicone oil on OTS-Si, with value of < 5°, (d) water on OTS-Si infused with excess silicone oil, with wetting ridge visible; (e) silicone oil droplet under water on OTS-Si, with contact angle value of 8°; (f) silicone oil droplet under water on Si with contact angle of 133°.


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Materials and Methods All samples and glassware were cleaned via sonication in ethanol, acetone and blown dry with a high purity nitrogen stream. Before conducting experiments, Si wafer samples were cleaned by a CO2 snow-jet to remove any particulate matter. Surface tension and contact angles were collected using a KSV CAM 200 goniometer, and contact angles were measured by hand using Image J and Young-Laplace fitting. The thickness of OTS and silicone oil films were measured by spectroscopic ellipsometry (J.A. Woollam Co. Inc., M2000 V). The thickness of silicone oil films was also measured by force approach curves in air (Bruker Multimode 8 AFM) and in water and sucrose 60% w/w (MFP-3D Asylum Research AFM), with standard rectangular cantilevers (Multi75Al-G, Budget Sensors, Bulgaria), with spring constants in the range of 2 – 4.5 N/m. Silicone oil (10 cSt, Mw ≈ 1250 g/mol, Sigma Aldrich) was applied to surfaces via spin coating and drop coating, with thickness controlled via varying spin coating speed and shearing with nitrogen stream respectively. For example, a speed of 8000 rpm for 5 mins produced a 154 nm thick film. An octadecyltrichlorosilane (OTS) self-assembled monolayer was prepared on a Si wafer using standard procedures33 via immersion in a 3 mM solution in toluene. Prior to silanisation, Si wafers were plasma cleaned in an air plasma (PDC-32G-2 Harrick Plasma). A MFP-3D Asylum Research AFM was used to perform colloid probe hydrodynamic force measurements. The colloid probe employed was a smooth borosilicate sphere (Duke Scientific, Palo Alto, CA) of ∼20 μm diameter glued to the terminal point of one of three silicon rectangular cantilevers (HQ:CSC37/tipless/Al_BS-15, Mikromasch, Bulgaria) with a spring constant in the range of 0.18 – 0.9 N/m (Figure S4). The microsphere radius R and surface features were measured by scanning electron microscopy (SEM) and inverse imaging contact mode AFM.34 The cantilever


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spring constant was measured using a modification of the hydrodynamic method,35 in cases where the Sader method36 and thermal method37 were unsuccessful. The colloid probe was cleaned just prior to an experiment by plasma treatment. Sucrose 60% w/w solution was used as a viscous liquid. The viscosity of sucrose 60% w/w is approx. 43 mPa s at 25 °C, with the viscosity varying slightly in different experiments depending on the cell temperature.38 The temperature of the sucrose 60% w/w in the AFM liquid cell was constantly monitored using a thermocouple inserted into the liquid cell. The equilibrium contact angle of sucrose 60% w/w solution is 110° on an OTSSi surface and approximately 33° on Si. Hydrodynamic force measurements were taken over 4000 nm at an approach rate of 40-80 μm/s, with average compliance values taken from approach rates of 1-10 μm/s. For all force measurements, the zero of separation was treated as hard contact with the OTS-Si and Si wafer. The average piezo driving velocity was calculated using the raw data on piezo position and time provided by the AFM software. When performing hydrodynamic force experiments, two samples were placed in the same liquid cell and measurements were performed with the same colloid probe. The typical pairings used were clean Si and OTS-Si, and OTS-Si and infused OTS-Si. For the latter case, we expected some creep of silicone oil across to the clean OTS-Si in the same liquid cell,39 and so slip measurements on these surfaces have been excluded from the slip length averages for OTS-Si. For additional statistical details regarding hydrodynamic force experiments see the Supplementary Information.

Results and Discussion Octadecyltrichlorosilane (OTS) is a hydrophobic silane that covalently binds to silicon oxide via polycondensation reactions with hydroxide groups,33 and has been used to imbibe lubricants such as silicone oil in structured SLIPS surfaces.11 A silicon wafer (Si, coated with a native oxide layer)


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was coated with an OTS layer (OTS-Si) of thickness approx. 2.8 nm and an RMS roughness of 0.35 nm (Figure S1). OTS-Si was coated with silicone oil at different thicknesses (infused OTSSi). The sliding angle of water droplets was used to define macroscopic slippery properties.6, 11, 15 A summary of the water contact angle values in air on Si, OTS-Si, and infused OTS-Si is provided in Table 1. Upon silanization, the hydrophilic silicon wafer (static water contact angle ≈ 33° and high droplet adhesion) became hydrophobic (water contact angle ≈109°); the water droplet sliding angle of approx. 15° was used as a benchmark for an uninfused OTS-Si surface. Silicone oil spreads entirely (static contact angle < 5°) in air on both OTS-Si and Si (Figure 1(c)). When submerged in water, silicone oil had a static contact angle ≈ 8° on OTS-Si (Figure 1(e)) and ≈ 133° on Si (Figure 1(f)). This high contact angle implies that in the presence of water and sucrose solution, a silicone oil film is displaced on hydrophilic surfaces, but may be stable on hydrophobic surfaces, depending on the exact interface potential.13 Indeed, on excess infused Si, a water droplet displaced the silicone oil in air, contacting the hydrophilic surface beneath, producing a high sliding angle (> 90º, Table 1). Upon excess infusion of OTS-Si, the silicone oil layer reduced the droplet sliding angle from 15° to < 1°, forming a ‘slippery surface’.2, 11 The exact contact angle was obscured by a large wetting ridge formed around the droplet base (Figure 1(d)). Water droplet sliding angles remained very low, approx. 2° degrees, a signature of a slippery surface, for thickness values down to 30 ± 2 nm. At a silicone oil film thickness of 22 ± 3 nm, the water sliding angle increased to 7°. Once the silicon oil film was sheared down to 0 nm, the water static contact angle and sliding angle values returned to those of OTS-Si prior to infusion. Silicone oil films were unstable and dewetted from OTS-Si when covered with water or sucrose solution (Figure S2), and the initial lubricant thickness influenced dewetted droplet size. This could be predicted based on the van der Waals forces across the lubricant film and the spreading


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coefficient. Firstly, a positive disjoining pressure (П(h) = -A132/(6πh3),40

where A132 is the

non-retarded Hamaker constant for medium 1 and 2 separated by layer 3 with thickness h, calculated via Lifshitz theory41) indicates a repulsive van der Waals force across layer 3, leading to a stable thin film of medium 3. Secondly, a positive spreading coefficient (S = γol (cosθo - 1), where 𝛾𝑜𝑙 is the surface tension of silicone oil in the liquid medium, and 𝜃𝑜 is the static contact angle of silicone oil on the substrate in the liquid medium) indicates that complete macroscopic spreading will be observed. In cases where S and/or П < 0, the lubricant should be completely displaced by the addition of water on a hydrophilic surface, with potential dewetting on hydrophobic surfaces.13 Calculated values of A and S for our systems can be found in Table S1. For infused OTS-Si in water A = -1.44 10-21 J, with П > 0 indicating a stable silicone oil film, while in sucrose solution A = 0.46 10-21 J, with П < 0 indicating a metastable silicone oil film. These values and even the difference in sign are affected by errors in the values of refractive and dielectric constants used to calculate A. For both systems S ≈ 0 but slightly negative, which explains the dewetting of the silicone oil film into droplets observed. The Aizenberg group reported that the sliding angle of water droplets on a dewetted lubricant is indistinguishable from that on a stable lubricant film due to oleoplaning, within a certain range of velocities.13 In contrast to this finding, our results show that when the silicone oil thickness became thinner than 22 nm (Table 1), water droplets were no longer able to oleoplan, and rolled-off the OTS-Si at the same relatively high sliding angle as on an uninfused OTS-Si.


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Table 1. Water contact angle (CA), hysteresis (H), and sliding angle (SA) values in air on clean silicon wafers (Si) and OTS-coated silicon wafers (OTS-Si), infused with different silicone oil thickness, as determined by ellipsometry. All errors are ± 1 º unless stated otherwise. Sample

Static CA (°)

CAH (°)

SA (°)

Si

33

Pinning

> 90

Si excess oil

Wetting ridge Pinning

> 90

OTS-Si

109

15 ± 2

OTS-Si excess oil

Wetting ridge Wetting ridge < 1

OTS-Si 360 nm oil

106

3

2

OTS-Si 30 nm oil

106

3

3

OTS-Si 22 nm oil

107

5

7

OTS-Si 0 nm oil*

108

6

12 ± 2

8

*For this last step, the silicone oil thickness was decreased to zero by spraying the surface with a water stream. For advancing and receding contact angles, see Table S2.

To quantitatively determine the thickness of a stable silicon oil film remaining between dewetted oil droplets, ellipsometry was not suitable due to droplet spacing being micrometres in size. Instead, AFM force curves were used, which detect the presence of nano-thin liquid films by measuring the meniscus adhesive forces acting on the AFM tip upon approach to a liquid surface.1, 25, 42-44

Figure 2(a) shows approach and withdraw force curves performed on uninfused OTS-Si in

air. Both the jump-in distance (the distance after the cantilever bends in response of an attractive force) and jump-off distance (the distance where the force returns to zero) were around 5 - 7 nm due to a weak capillary attraction between the tip and the substrate, likely due to a water


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meniscus.45 Meniscus force measurements on infused OTS-Si in air are shown in Figures 3(a) and S3(b).46 A silicone oil film thickness of approx. 300 nm was applied on an OTS-Si via spin coating, before submersion in sucrose solution in the AFM liquid cell. The silicone oil dewetted into droplets of approx. 20-100 µm diameter, which could be easily seen with the X10 light microscope on the AFM. Force curves obtained on plain OTS-Si and on infused OTS-Si in the space between the dewetted droplets are shown in Figures 2(b) and 2(c). In the infused OTS-Si case longer-range adhesive forces were measured in the withdraw curves compared to the plain OTS-Si in sucrose solution, demonstrating the presence of a lubricant film. The jump-in distance of approx. 1.8 nm in the approach curve is indicative of the thickness of the film of silicone oil,46 with repeated measurements across multiple positions returning an average thickness of 1.8 ± 1.3 nm. This film thickness is similar to the thin films detected by Wang et al in similar systems.43 Measurements performed in water returned similar results (Figures S3(c) and (d)). Film thickness measurements by AFM are prone to errors of ≈ 1.5 nm due to potential thin film absorption onto the tip.25


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Figure 2. AFM approach and withdraw force curves on OTS-Si in air (a) non-infused, and in sucrose solution on OTS coated silicon (k = 2.9 N/m), (b) non-infused (k = 3.6 N/m), and (c) infused with 300 ± 2 nm silicone oil (k = 2.1 N/m). In (c) force curves were taken in the space between dewetted oil droplets. AFM cantilevers used had spring constants k = 2 – 4.5 N/m.

Hydrodynamic measurements were made with colloidal probe AFM to determine the effective slip length over this interface. Hydrodynamic force measurements were taken over 4000 nm at an


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approach rate of 40-80 μm/s (average compliance values taken from approach rates of 1-10 μm/s), with a ratio of viscosity of the silicone oil to sucrose solution ≈ 1:4. It has been shown that the effective slip length over a lubricant/liquid interface is influenced by the viscosity ratio of the two mediums, with a lower lubricant viscosity compared to the liquid resulting in more drag reduction.5 Each set of measurements was repeated 3 times with different cantilevers, over 3-4 positions on each sample. Measurements were taken on clean Si substrates, on OTS-Si substrates, and on OTSSi wafer infused with an initial thickness of approx. 300 nm silicone oil. To determine the effective slip length, the hydrodynamic forces Fh measured were fitted with the no-slip and the partial-slip boundary conditions, according to an established procedure.47 The effective slip length b used for fitting was chosen based on agreement with experimental data from distances 4 µm away from the surface, down to approx. 200 nm; at shorter distances shear rate effects may prevent a good agreement.48 Figure 3 shows hydrodynamic force measurements on different OTS –Si surfaces at an approach rate of 40 µm/s (Figures 3(a) and (b)), and a summary of the slip lengths across all experiments (Figure 3(c)). In all cases, the presence of finite interfacial slip could be surmised from the fact that the experimental hydrodynamic forces were lower than the no-slip theory. The average effective slip length on Si was found to be 7 ± 2 nm (Figure 3(c)), a value that agrees with previous results published by the authors on wettable surfaces.30 As expected based on the lower surface wettability, the slip length on OTS-Si was higher, 24 ± 3 nm (Figure 3(a)), consistent with literature values.30, 49 The effective slip length measured on the infused OTS-Si in between the dewetted oil droplets was found to be 29 ± 3 nm. Interestingly, the effective slip length values over both the uninfused and infused OTS-Si surfaces overlap, indicating that no further drag reduction was brought about by the infused silicone oil films. These results point to the fact that, within the


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timescale of our measurements (driving frequency 40-80 Hz), a 1.8 ± 1.3 nm thick layer of silicon oil is effectively immobilized or infused at each surface, this thickness being close to around 2Rg (Rg, the radius of gyration for the silicone oil used using the SAW model ≈ 0.89 nm50). A similar effect has been reported in surface force apparatus experiments at much lower shear rates (driving frequency 1-3 Hz).51-53 Our experiment does not allow to test whether the shear induced by the hydrodynamic measurement itself could further thin down the silicone oil film. Other potential explanations, such as shear thickening effects, can be excluded, based on the low average molecular weight of the silicone oil used (Mw ≈ 1250 g/mol), far below the entanglement length (34K), its rheological behavior is Newtonian.54-55 Our results are important in that they identify the limit of the slippery nature of SLIPS. Although structured surfaces lubricated with thick layers might induce large values of effective slip length, in the order of 100’s of nanometers,56 or as high as a few micrometers,5 at single locations where the solid surface features are exposed, their slippery nature fails. This work identifies the exact layer thickness at which this failure occurs on flat lubricated surfaces. The effective slip length measured agree with our macroscopic observation for droplet sliding angles on thin silicone oil films on OTS-Si: an infused silicon oil film of thickness below 3 nm can no longer impart slip due to its infusion in the underlying monolayer. This result is somewhat surprising as it was expected that any lubricant thickness would lead to a larger drag-reduction than that induced by a solid hydrophobic coating. For example, lubricant films of approx. 2 nm have been shown to be effective at reducing solid-solid friction.57-58


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Figure 3. (a) – (b) Hydrodynamic approach force curves using colloid probe AFM in sucrose solution at an approach rate of 40 µm/s on (a) OTS-Si, (b) OTS-Si infused with silicone oil. (c) Average values of effective slip length over multiple experiments. Colloid probes used had radius 8.5 – 9.5 µm, and spring constant k = 0.18 – 0.9 N/m.

Conclusions The stability of the lubricating film and the thickness of lubricating films was shown to be crucial to drag reduction in SLIPS. We have established that silicone oil films dewet on initially-lubricated


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OTS-Si substrates, upon immersion in sucrose solution, and that the minimum remaining thickness of lubricant (a 1.8 ± 1.3 nm-thin) does not provide additional drag reduction, beyond that provided by the hydrophobic coating. The average effective slip length of the 1.8 ± 1.3 nm thin silicone oil layer on an infused OTS-Si substrate was found to be 29 ± 3 nm, close to the 24 ± 2 nm on plain OTS-Si. This low effective slip length in the infused surface could be due to the immobilized nature of the silicone oil film due to its low thickness and or its infusion into the underlying OTS monolayer. This result sheds light on the likely mechanism of drag reduction in SLIPS. We know from our recent study that SLIPS are made up of areas with thick lubricant thickness and areas of nano-thin film lubricant.59 Our new results indicate that the areas were the lubricant is depleted, mostly the high points of the structured surfaces, are unlikely to provide significant drag reduction. A high incidence of these regions will therefore lead to an overall low potential for drag reduction in SLIPS. The slip results were compared to droplet sliding angle measurements on OTS-Si with different silicone oil thicknesses in air. Silicone oil films of thickness between 300 nm and approx. 30 ± 2 nm make OTS slippery, i.e. they reduce the water sliding angle of an OTS layer from 15° to 3°. Below a silicone oil thickness of 22 nm, the slippery properties are no longer observed, and the water sliding angle becomes similar to that on an uninfused OTS-Si. This study determined the limit of performance of slippery liquid-infused surfaces and established the threshold value of lubricant at which the slippery properties disappear. Nanoscale techniques such as colloid probe AFM will be important moving forward towards gaining a deeper, nanoscale understanding of SLIPS stability under shear, and drag reduction at the liquid-liquid interface.

ASSOCIATED CONTENT


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Supporting Information. Additional statistical details regarding hydrodynamic force experiments, Tapping Mode Atomic Force Micrographs of clean and OTS coated silicon wafers (Figure S1), optical micrographs of OTS coated silicon wafers infused with different thickness of silicon oil submerged in water (Figure S2), hamaker constants and spreading parameters for a silicone oil layer on different substrates in air water and sucrose solution (Table S1), advancing and receding contact angles for values presented in Table 1 (Table S2), pointed tip AFM approach and withdraw force curves on OTS silicon wafers in air and water with different silicone oil layer thicknesses (Figure S3), linearized hydrodynamic approach force curve using colloid probe AFM in sucrose solution compared to standard data representation at separation distance < 300 nm (Figure S4).

AUTHOR INFORMATION Corresponding Author *Associate Professor Chiara Neto School of Chemistry and the University of Sydney Nano Institute, The University of Sydney, New South Wales 2006, Australia. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources


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L.R.J.S. acknowledges the Surface Coatings Australia scholarship. The Authors acknowledge the Australian Research Council for funding (Linkage grant LP140100285). ACKNOWLEDGMENT The authors thank Prof Howard Stone for useful discussions. SEM was performed at the Australian Centre for Microscopy & Microanalysis.

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TOC Figure


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How Slippery are SLIPS? Measuring Effective Slip on Lubricated

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