Surface Functionalization for a Nontextured Liquid-Infused Surface

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Surface Functionalization for a Non-textured Liquid Infused Surface with Enhanced Lifetime Chi Zhang, Yanfeng Xia, Huan Zhang, and Nicole S. Zacharia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18021 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Applied Materials & Interfaces

Surface Functionalization for a Non-textured Liquid Infused Surface with Enhanced Lifetime Chi Zhang, Yanfeng Xia, Huan Zhang, Nicole S. Zacharia* Department of Polymer Engineering, University of Akron, Akron, Ohio, 44325, United States Keywords: slippery surface, self-cleaning, controlled wetting, liquid infused surface

Abstract: Liquid infused surfaces are a new class of self-cleaning surfaces having superior properties compared to similar surfaces. One challenge regarding these new surfaces is the eventually washing away or drainage of the lubricant, limiting usage. Presented here is a surface functionalization strategy to compatibilize the lubricant and surface, enhancing the lubricant's ability to remain on the surface even during washing. The strategy used here is the grafting of a layer of polydimethylsiloxane (PDMS) which stabilizes a layer of silicone oil. The effectiveness of this layer is studied as a function of PDMS molecular weight. The stable liquid layer can exist even in the absence of texture on the surface that is generally used to "lock" the lubricant in place. This strategy is shown to be effective on flat and textured surfaces. One advantage of a flat surface is that the composite liquid/solid surface can be studied using optical techniques such as ellipsometry that are difficult to employ in the presence of a rough solid surface. This method of surface compatibilization shows an enhanced lifetime when used on textured surfaces as well.

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This is a promising strategy for the enhanced longevity of liquid infused surfaces required for real world application. 1. Introduction Lubricated or liquid infused surfaces (LIS) have become an exciting emerging class of selfcleaning and “slippery” surfaces, created by infusing a textured surface with a liquid or lubricant. Although they have been shown to have quite promising properties, challenges remain regarding these surfaces. These challenges include truly understanding minimum texture requirements to achieve various performance metrics as well as how to extend the lifetimes of these surfaces.12 In this paradigm of self-cleaning composite liquid-solid surfaces, the water drops or whatever liquid of interest that is to be repelled float away from the surface on the lubricant layer. While the static contact angles on these surfaces are not usually over 150° (and so these surfaces are technically not superhydrophobic), these surfaces have been reported to be repellant to water and other liquids3 with very low roll off or sliding angles, to delay ice and frost formation456, and to be non-fouling with respect to bacteria amongst other applications78. Creation of slippery surfaces has been demonstrated using a wide variety of textured surfaces of different materials and geometries including polymers, metals, oxides, and silicon. Besides versatility of the substrate material, the requirements for geometry are also not very restrictive. Although some of the lowest sliding angles are reported for lubricated textures that start with hierarchical geometries such as lotus leaf mimic surfaces, slippery surfaces have been reported for a range of geometries including nanoscaled features alone or even microscaled features alone such as micron sized posts lithographically defined on silicon wafers.910 Compared to surfaces that mimic the lotus leaf, which have become something of a gold standard regarding superhydrophobic and self-cleaning surfaces, slippery surfaces are easier to fabricate specifically


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because of the more lax requirements for surface geometry, have some ability to self-heal as the lubricant can flow over scratches or other minor damage, and withstand more extreme environments like cold temperature and hydrostatic pressure. The challenge with these surfaces is that the lubricant, which repels liquid by flow along the surface also drain away from the film during this process.1112 Some suggested approaches to handle this include using a PDMS rubber layer as reservoir, distributing hydrophilic defects, or placing geometric surfaces features at specific length scales across the surface.131415 As mentioned, there has been some literature discussion as to what texture is necessary to create a stable liquid layer on a surface, but presented here is a demonstration that with suitable surface chemical functionalization self-cleaning slippery surfaces can be simply created on flat, non-textured surfaces. Thus far, in one study, Shiratori and coworkers16 show that using phenolic compounds pi-pi interactions can be utilized to fix a stable liquid layer on a flat surface and some mention was made of a stable fluorinated lubricant layer on the inside of a flat tube from the Aizenberg group’s work.10 We present here a study of slippery surfaces fabricated from silicone oil liquid layers stabilized on flat glass and silicon substrates by first grafting a layer of PDMS on those substrates. Although wetting has been examined on surfaces of PDMS brushes,14 lubricants have not previously been infused into brushes. This demonstrates that not only can a non-textured lubricant layer be stabilized, it can do so with a silicone chemistry that might be more well-suited to food contact surfaces or cosmetics or pharmaceutical packaging compared to other chemistries. We further demonstrate that this layer PDMS chains can be grafted onto a textured surface to improve the retention of lubricant in the texture as well. This PDMS-silicone oil surface has shown a promising future to reduce the drainage of the molecularly silicone oil lubricant layer, allowing for longer functional use of the surface. This surface can maintain its


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water repellency property under extreme conditions such as high temperature and exposure to steam. Ice formation is delayed, and the mobility of the condensed water droplets is promoted on this surface as compared to the analogous surface without the tethered PDMS chain layer. The ability to create a non-textured “slippery” coating also allows for optical characterization for which textured surfaces are not well suited. 2. Experimental Method 2.1. Surface Fabrication The design of this flat slippery surface was shown in Figure 1a. Silicon wafers and glass slides were both used as substrates. They were both cleaned by piranha solution (a mixture of 98% sulfuric acid and 30% hydrogen peroxide with a volume ratio of 7:3). The substrates were exposed to D4 (1,3,5,7-tetramethylcyclotetrasiloxane, Gelest Inc.) vapor in a sealed reactor for 2 days at 100 °C. Afterwards, the D4 covered surfaces were immersed into a solution of 20% weight percent vinyl-terminated PDMS (Mw 6,000, Gelest, Inc.) in hexanes (Fisher Chemical) with Karstedt’s catalyst (Pt catalyst, Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution, Sigma Aldrich). This reaction was carried out in a 20mL glass scintillation vial for 3 days at 50 °C. The chemical reaction is shown in Figure S3. The sample was finally rinsed with hexanes, acetone and DI water. All materials were used as received. The as-prepared samples were immersed into a silicone oil bath for one hour and was taken out and put vertically to remove the residue silicone oil. Further, a spin coater was used to coat silicone oil layer uniformly on the surface with 4000 rpm for 1 minute. A textured surface was also prepared and then functionalized in this manner before being infused with silicone oil. In order to prepare this textured

surface,

a

piece

of

silicon

wafer

was

immersed

into

0.1

mol/L

polydiallyldimethylammonium chloride (PDAC, Mw 400,000-500,000, 20% wt. in H2O, Sigma


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Aldrich) for 10 minutes and then into deionized water (DI water) baths for 1 minute, 3 times. The wafer was then immersed into 0.1 mol/L silica nanoparticle colloidal (14nm, Alfa Aesar) for 10 minutes and also DI water rinses for 1 minute, 3 times. This cycle of polycation deposition followed by negatively charged nanoparticle deposition was repeated for 10 times to fabricate a 10 bilayer of PDAC and SNP surfaces, which was labeled as (PDAC/SiO2)10. This (PDAC/ SiO2)10 film was exposed to D4 vapor and immersed in PDMS/hexanes. This surface was referred to (PDAC/ SiO2)10-D4-PDMS. Finally, silicone oil was infused into this texture surface in the same manner as the flat surface. 2.2. Characterization 2.1.1. Contact angle and sliding angle measurement Contact angle and sliding angle measurements were performed by a contact angle goniometer (VCA-Optima™ from AST products, Inc.). Milli-Q water (1018 Ω/cm) as well as organic liquids were used as probe liquid with volume of 2µL during contact angle measurement. For sliding angle measurements, a 5µL probe liquid droplet was first deposited on the surface. The substrate was tilted at a constant low rate. As soon as the droplet began to slide, the angle at which the substrate was tilted was recorded as the sliding angle of the probe liquid. If the probe water drop did not slide by the time a 15º tilt was achieved, the measurement was stopped and the sliding angle was recorded as 15 º, indicating a “non-slippery” surface. Each measurement was repeated 3-5 times at different areas of the surfaces. 2.2.2 Film thickness measurement Variable angle spectroscopic ellipsometer (VASE, M-2000 UV-visible-NIR-visible-NIR [250– 1680 nm] J. A. Woollam Co., Inc., Lincoln, NE, USA) was used to measure the thickness of films. The incident angle is fixed at 75°. A four-layer model, which is a 1.0 mm silicon substrate


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layer, a 1.0 nm fixed interface layer of Si-SiO2, a 1.0 thermal oxide SiO2 layer and a Cauchy layer with thickness, A and B fit parameters was employed to fit the ellipsometric data using Complete EASE software. Each measurement was at least repeated 3 times in order to get an average value of the film thickness. The 2D/3D mapping profiles were done by "ex-situ mapping" function of the ellipsometer. A typical mapping size was 0.8 cm by 0.8 cm and the data points were grid line fit, with 0.2 mm as the distance between each data point. 2.2.3. Surface robustness test As shown in Figure 1b, water drops were dripped onto the surface by a syringe pump with a constant feeding rate in a dropwise fashion. The diameter of the water drop is 1.5mm. Surfaces were cut into 3cm×1.5cm and divided into 3 areas. Sliding angles of all these 3 areas were measured by the contact angle goniometer. 2.2.4. Ice formation test The as-prepared samples are placed on a temperature-controlled aluminum cooling stage which is connected to a refrigerated circulator (Thermo ScientificTM A28 refrigerated bath, working temperature from -28°C to 200°C, Thermo Scientific Inc.) with the coolant (Bath fluid thermal C5, working temperature from -60°C to -110°C, Julabo Inc.). The temperature error of the circulator is ± 0.3 °C. A thermal couple (True RMS Millimeters, Fluke) is mounted on the surfaces to directly measured the temperatures. A small glass chamber is used for controlling ambient humidity. In other to adjust the humidity, nitrogen gas is mixed with water in a humidity controller. Humidity is precisely controlled by adjusting flow rate of nitrogen gas and measured by a hygro-thermometer (RH 101, EXTECH Instruments Inc. working range from 10% to 95%). The error or variability in the humidity measurement is ± 5%. An optical microscope (Olympus, Co.) is used to observe the ice formation process. Video was taken during this entire process.


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The time needed for complete coverage of each of samples on the substrates was measured to evaluate the surface anti-icing property. Each measurement was repeated 3 times to get the average “full frosting” time. The sizes of the droplets of different surfaces were measured and compared during ice formation. 3. Results and discussion

Figure 1. a) Fabrication of this flat robust liquid infused surface. b) Static water contact angle as well as sliding angle of flat liquid infused surface. Sample #1: Bare wafer-D4. #2: Silicon-waferD4-PDMS. 3: #Silicon-wafer-D4-PDMS-silicone oil. c) Sliding angles of different MW PDMS (ranging from 500 to 62,500) surfaces coated with silicone oil. d) Thickness of different MW PDMS surfaces (no oil). e) Thickness growth of PDMS (MW 6,000) surface. f) Water colored with methylene blue g) ketchup and h) mustard sauce slide on glass-D4-PDMS-silicone oil surface.


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The flat, stable LIS was fabricated as schematically depicted in Figure 1a and described in the experimental section. In brief, the surface is functionalized with a silane 1,3,5,7tetramethylcyclotetrasiloxane (D4), (orange) and then a layer of divinyl terminated PDMS (blue) with the help of hydrosilylation reaction (mechanism showed in Figure S3). Although both ends of the PDMS could attach to the surface, entropic considerations give one reason to believe on average only one end of the polymer is attached to the surface, especially at lower molecular weights, giving a layer of brushes.14 After spin coating silicone oil onto this PDMS functionalized surface, it can be visually confirmed that a stable layer of silicone oil remains. Staged coating strategies of creating this liquid layer were used; including soaking the PDMS grafted surface in silicone oil first, which has been used in other works to create an excess layer of oil.17 It was seen here that this results in thick lubricant layers as merely adding a drop on the surface and spin coating, but soaking was used to ensure full dissolution and swelling of the PDMS chains in the silicone oil. This lubricant layer was further eliminated by spin coating process. The final thickness of the lubricant is around 2 micrometers as shown in Figure 1e. Using different volumes of lubricant during spin coating also results in the same thickness of the liquid layer. A stable liquid layer does not exist when spin coating silicone oil directly onto a glass slide or piece of silicon wafer, nor is it the case for spin coating other types of oils (such as vegetable oil or olive oil) on either the bare or PDMS functionalized substrates. It is possible to achieve a stable layer of silicone oil on the silane only functionalized layer, but it is not a selfcleaning layer on the flat surface nor is it as robust as the silicone oil layer onto the PDMS brush surface. Furthermore, there does seem to be some critical amount of PDMS grafted on to the surface that is required for the stabilization of the liquid oil layer. Repeating the fabrication process with much more dilute PDMS solution (1 wt% versus 20 wt%) does not result in a layer


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of PDMS that is able to stabilize a silicone oil layer, showing that some critical density of PDMS on the surface is required. Spin coating Krytox, a fluorinated lubricant often used in creating liquid infused slippery surfaces, onto a layer of grafted PDMS brushes does not result in a low sliding angle surface. This shows that affinity between the functionalized surface and the oil is critical to create the stable, liquid layer on a non-textured surface. That is, the lubricant should be able to swell the layer of brushes. Furthermore, spin coating Krytox onto a fluorinated selfassembled monolayer (SAM) on a silicon wafer does not create a stable liquid layer. This can be verified by the fact that the sliding and static water contact angles do not change before and after spin coating the Krytox and are consistent with the fluorinated SAM rather than those consistent with liquid infused layers made with Krytox. Perhaps a higher molecular weight than that of a small molecule fluorinated silane is required to stabilize a liquid layer on a surface. The literature bears out the need for certain thermodynamic relationships between surface and liquid to create a stable slippery surface 18. The static contact angle and sliding angle of the non-textured stable liquid surface are shown in Figure 1b. The water contact angle is approximately 115° on the D4-PDMS-silicone oil surface, which is similar to that of water on other silicone oil infused LIS.1920 A silicon wafer functionalized only with the D4 silane is actually a water pinning surface (showed as 15 º). In this work, 15 º was considered to be a cut off for “non-sliding” surface, and those surfaces with that or higher sliding angles are shown in the plots at having a sliding angle of 15 º. For this reason, error bars are not shown on data for these particular samples as 15º is simply a signifier of a surface being non-sliding. The silicon wafer functionalized with the PDMS brushes only has a sliding angle for water of approximately 30°. However, spin coating the silicone oil onto the D4 functionalized surface on an underlying flat surface also results in a water pinning surface. Spin


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coating silicone oil onto a silicon wafer functionalized with PDMS of MW = 6000 g/mol results in a surface with a sliding angle for water of approximately 3°- 5°, indicating the successful fabrication of a stable liquid layer and furthermore a slippery surface. Subsequent spin coating of the surface retains this sliding angle. That is, putting the same sample back onto the spin coater and applying the same spin coating conditions as for the initial surface fabrication (2000 rpm, 30 sec) at least for 2 additional times does not change the water sliding angle within measurement error (~1.5°). When higher molecular weights of PDMS are used, sliding angles are higher, 15° or 20°. From previous studies by Hozumi et al, 14,21–25 the molecular weight of the PDMS grafted onto the surface in this manner influences the packing density as well as the chain mobility of the PDMS layer. For surfaces with just the PDMS layer and no lubricant, the sliding angle of MW 50,000 PDMS layer can be as high as 55° compared to sliding angle 15° for MW 6,000, sliding angle 40° for MW 20,000, showing that for all MW values the infusion of lubricant improved the sliding angle for water. As described above, multiple molecular weight (MW) values of PDMS were chosen to probe the influence of the surface functionalization on the overall properties of the stable liquid layer. Sliding angles of silicone oil infused into different molecular weight PDMS coated surface were measured, shown in Figure 1c. The first sample is the D4 functionalized surface with silicone oil spin coated onto it, which again is water pinning. The other samples represented in figure 1c are all also silicone oil coated with underlying PDMS brush layers with MW values of 500, 6,000, 27,500 and 62,500 g/mol. From these surfaces, the one with PDMS brushes of MW of 6,000 g/mol shows the lowest sliding angle and 500 MW gives a water pinning surface. A possible explanation is that MW of 6,000 g/mol represents a high enough MW value to swell with an appreciable amount of oil but the chains are not long enough to significantly increase the


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viscosity of the oil layer. As 500 g/mol gives a water pinning layer, it seems that a certain chain length is required to “tether” sufficient lubricant to the surface. Solubility of the brushes in the oil should also be decreasing as the molecular weight of the grafted layer increases, so decreased solvent quality could impact mobility on these layers. Figure 1d shows the thickness of these different MW PDMS chains. Other work has shown that this same system of the D4 silane with low MW PDMS grafted onto it results in a surface layer with high mobility, which may enhance flow of the silicone oil and therefore water drops in this case.14 Figure 1e describes the evolution of thickness throughout various stages of fabrication of the flat liquid layer, using PDMS MW of 6,000 g/mol. The thickness of the PDMS layer is approximately 6 nm, in agreement with the work of Houzumi et al.14 After spin coating the silicone oil, the total thickness increases to 2 µm. This flat silicone oil layer can be fabricated using glass slides or silicon wafers as the underlying substrate. One can see DI water, ketchup, and mustard sliding off the surface without staining, shown in Figures 1f to 1h, without streaking. This shows that the surface is selfcleaning with respect to various complex water based fluids such as various food products.


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Figure 2. a) Schematic showing how water was applied to the surfaces for measurement. Thickness mapping of flat wafer-D4-silicone oil surface b) before and c) after a dropwise 20 mL DI water washing. Thickness mapping of flat wafer-D4-PDMS-silicone oil d) before and e) after a dropwise 20 mL DI water washing. The arrows indicate the position where water hit the surface and water washing direction. f) lubricant volume change before and after 20mL DI water washing. g) Sliding angle change of different areas after 30mL and 40mL DI water wash. One of the most critical problems for different types of LIS is the drainage of the lubricant. To test the robustness of the stable lubricant layer on the flat surfaces, samples were washed with water drops (setup shown schematically in Figure 2a). Both D4 and D4-PDMS surfaces were coated with silicone oil for comparison. Figures 2b and 2d show the thickness mapping of these oil coated surfaces. The thickness of the oil layer is ~2µm in both cases from ellipsometric measurements, and as previously mentioned this layer is stable if the samples are put back on the spin coater and exposed to the same shear forces again. This refers to not only the sliding angle, but the thickness of the oil layer. Other works have shown that often newly fabricated LIS contain a thermodynamically excess amount of lubricant,26 but if this is the case at least the forces experienced by the sample during spin coating are not strong enough to remove any of this excess. If more oil is added to the surface in these subsequent spin coating steps (that is, the surface is fabricated first, then returned to the spin coater and another oil application step is performed), the overall thickness of the oil layer remains constant at 2 µm. This seems to be the maximum thickness that can be supported by the PDMS brushes. Figures 2c and 2e show thickness mappings of flat D4-silicone oil and flat D4-PDMS-silicone oil surface after having been washed in a dropwise manner with 20 mL of DI water, respectively. On the figures, the number 1 indicates where water first hits the surface. The impact of this water definitely


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displaces lubricant. It can be seen from 3D ellipsometric mapping data that the silicone oil is washed off to some degree in both cases, starting where the water impacts the surface. The water drops hit the surface on the x = 0 axis, and one can see from the mapping data that the oil in this area (area 1), for the PDMS brush surface in figure 2, is largely displaced due to the drop impact. However, in area 3, for the D4 functionalized surface, the surface thickness is close to 4 nm across the 0.5 x 2.5 cm rectangular area, which is essentially the same as the thickness of the silane. On the other hand, for the PDMS brush grafted surface the thickness changes over the length of the surface from as little as 4 nm to as much as ~80 nm. The remaining silicone oil on this PDMS brush surface however is thicker than what remains on the D4 surface and able to maintain a relatively low sliding as shown in Figure S1. Figure 2f shows the oil layer volume change after 20mL DI water washing. With the same size of washing water droplet, the remaining oil volume on PDMS surface is higher than D4 surface. This remaining layer of silicone oil is responsible for the surface slipperiness. Furthermore, we extended this water washing test to 40mL and we found after 40mL water wash, the sliding angle was approaching 15°, indicating a failure of the surface slipperiness. (shown in Figure 2g). D4-silicone oil surface is water pinning before and after washing. Water pinning surfaces in the context of these liquid infused surfaces have been explained with the idea that the water has such a high affinity for the underlying surface compared to the affinity of the oil for the surface that it can displace the lubricant, and the water drop sinks into the oil and is pinned by the surface. 27 The way in which water first impacts the surface is critical to the longevity of the surface. In another demonstration, we exposed our flat slippery surface to the flow of water from a faucet. In this case a high rate of turbulent flow was used, and holding the surface vertically under a running faucet for a full minute which results in 12 liters of water running across the surface does not


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dramatically change the slippery properties of D4-PDMS-silicone oil surface as shown in SI table 1. This can be seen in still images in Figure S2 as well as supplementary video 1. From these two different types of surface robustness tests in Figure 2 and S2 (dropwisely washing vs high flux washing), it shows the lubricant layer is under a severer drainage when undergoes a dropwisely washing, suggesting the force of the externally imposed flow can influence the life-times of the lubricant coatings.

Figure 3. Thickness mapping of a) D4-PDMS-silicone oil surface, b) 20mL DI water washed D4PDMS-silicone oil surface at the location of drop impact and c) Re-applied D4-PDMS-silicone oil surface. After washing the surface with water, silicone oil can again be added using the same spin coating procedure. Figure 3a shows an ellipsometric map of the coated surface and Figure 3b is the map showing the thickness at the location where the drop impact occurred after washing with 20 mL of DI water. Figure 3c then shows the ellipsometric mapping of the surface with reapplied lubricant coating, and it can be seen that this replenished oil layer is homogenous and its thickness is very similar to that of the oil layer on the original surface (~2µm). These oil layers are renewable after having been lost to water, a possible way for these surfaces to be used over longer times in practical application.


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Figure 4. a) Static contact angle and sliding angle of different types of surfaces as follows; #1: silicon wafer-silicone oil, #2: (PDAC/SiO2)10-silicone oil, #3: (PDAC/SiO2)10-D4-silicone oil, #4: (PDAC/SiO2)10-D4-PDMS-silicone oil. b) Surface thickness determined by ellipsometry. Sample #1: (PDAC/SiO2)10. #2: (PDAC/SiO2)10-D4 #3: (PDAC/SiO2)10-D4-PDMS #4: (PDAC/SiO2)10D4-PDMS-silicone oil. c) Side view time-lapse images showing water droplet sliding (PDAC/SiO2)10-D4-PDMS-Silicone oil surface. Sliding angle is ~3°. The grafting of PDMS to a flat surface has been shown to stabilize an oil layer for the creation of a self-cleaning surface. Furthermore, a similar functionalization with PDMS brushes can be used to enhance the properties of a textured liquid infused surface as well as helping to accommodate more lubricant. Textured surfaces were created using silica nanoparticles and polydimethyldiallyl ammonium chloride (PDAC) using the layer-by-layer (LbL) method, as previously reported,10 and these surfaces were then functionalized with the PDMS brushes of MW = 6,000 g/mol. These surfaces have a nanoscaled surface roughness as well as are approximately 70% void. Surface robustness was testing by the same washing method depicted


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in Figure 2a. The static contact angle and sliding angle of the surfaces are illustrated in Figure 4a. For silicon wafer and (PDAC/SiO2)10 coated silicone oil surfaces, both have a static water contact angle of approximately ~30° with a sliding angle larger than 15° (simply shown as 15° in Figure 1b). Comparing the data of sample #1 and #2, it indicates that during spin coating process, since rotate speed of the spin coater is 4000 rpm, the affinity between infused silicone oil and underlying surface is very weak. Although the silicone oil used here has a viscosity of 40 cSt, most of the silicone oil on this surface has already drained during spin coating process. Without any further surface modification, both silicone oil coated bare silicon wafer and (PDAC/SiO2)10 surface exhibit the hydrophilicity as well as high contact angle hysteresis. However, once applying D4 and grafting PDMS (MW: 6,000) onto the surface, the contact angle increases and the surface becomes fairly hydrophobic. The sliding angle decreases significantly. In Figure 4c, a 5 µL water droplet slides off unimpeded on a 3° incline. Growth of film thickness was determined by ellipsometry in Figure 4b. By depositing 10 bilayers of PDAC/SiO2 (sample #1), film thickness is determined to be ~34.25±0.29 nm. After applying cyclic D4 (sample #2) and PDMS (sample #3) with Mw 6,000 onto the surface, a continuous growth of film thickness was observed. The thickness of this PDMS is ~15 nm, which is larger than literature reports.14 Since the roughness of the underlying (PDAC/SiO2)10 cannot be negligible,10 a larger increase of thickness of PDMS tethered to D4 covered (PDAC/SiO2)10 surface is reasonable. As for the thickness of sample #4, (PDAC/SiO2)10-D4-PDMS surface was simply immersed into silicone oil for 1 hour to give the PDMS chains sufficient time to swell and reach an equilibrium state. At last, it was put vertically until no residual oil is seen to flow on the surface and then spin coated with a rotational speed of 4000 rpm in order to remove any excess oil. These data indicate that a low MW, liquid-like PDMS chains have a good mobility in silicone oil and According to Urata


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et. al28, alkyl chains will interact with low dielectric constant liquid, which is silicone oil in this case. The alkyl groups will extend into the silicone oil, resulting in the increase of thickness of PDMS. This PDMS chain perfusion and swelling in the lubricant reservoir helps to extend the longevity of this liquid coating under continuous shear forces.29 Therefore, we assume the high solubility of PDMS chains in the silicone oil will help the preservation of the infused lubricant during use of the sample (as shown in Figure S4).


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Figure 5. a) Images of methylene blue colored water droplets sliding on various substrates. b) sliding angle of water on (PDAC/SiO2)10-D4-PDMS-silicone oil coated on various substrates. c) Evolution of sliding angles on both (PDAC/SiO2)10-D4-silicone oil and (PDAC/SiO2)10-D4PDMS-silicone oil surfaces under an air flow over 11 days.


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One advantage of layer-by-layer (LbL) method is its ease in coating multiple types of substrates, which is also shown here. Figure 5a shows images which shows 20µL DI water sliding off from different substrates at ease within a very short time. More importantly, there was no dye (water dyed by methylene blue) trace left on the surfaces which indicated a great water repellency of these surfaces. The (PDAC/SiO2)10-D4-PDMS film was fabricated on silicon wafer, glass slides, polyethylene terephthalate (PET), aluminum foil, and polyethersulfone (PES) membrane. These surfaces all exhibited a sliding angle less than 5° (as shown in Figure 5b). However, the sliding angle of the surface deposited on filter paper (cellulose nitrite) substrate was high (>15°). PDMS chains coated silicone oil surface retains the easy-sliding property and liquid-repellency under air flow. Both (PDAC/SiO2)10-D4-silicone oil and (PDAC/SiO2)10-D4PDMS-silicone oil were placed in a standard laboratory chemical hood with a face velocity of 80 fpm for longevity measurements. A fume hood was chosen as a greater air flux would be experienced by the samples compared to ambient, speeding potential evaporation. Figure 5c shows the sliding angles changes. Due to the low vapor pressure of silicone oil, the oil layer is stable. The sliding angles on the two surfaces are essentially the same for the first 6 days. However, the difference in sliding angle becomes larger after 6 days. On the surface without PDMS chains (the (PDAC/SiO2)10-D4-silicone oil surface), aging results in a sliding angle larger than 10° after 8 days. However, the sliding angle of (PDAC/SiO2)10-D4-PDMS-silicone oil is still approximately 5° even after 11 days.


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Figure 6. Surfaces under extreme temperatures. a) Sliding angles of (PDAC/SiO2)10-D4-PDMSsilicone oil surfaces heated under 100 °C for ~48 hours. b) Sliding angles of (PDAC/SiO2)10-D4PDMS-silicone oil surface exposing to acidic steam for ~12 hours. Sliding angles of c) (PDAC/SiO2)10-D4-silicone oil and d) (PDAC/SiO2)10-D4-PDMS-silicone oil surfaces exposing to water steam for ~24 hours. Figure 6 shows that the (PDAC/SiO2)10-D4-PDMS-silicone oil surface can survive a wide range of temperatures. In Figure 6a, (PDAC/SiO2)10-D4-PDMS-silicone oil was heated on a 100 °C heating plate for ~48 hours. The sliding angle of the surface remained low (~5°) during this prolonged time period. According to Daniel et al.30 by infusing perfluorinated lubricant, Krytox 103, which is almost the same viscosity as silicone oil, into texture surface, they found that low


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sliding angle remained small at high temperatures range from 50°C to 150°C. Droplets sliding speed measurements at elevated temperatures were also conducted on our (PDAC/SiO2)10-D4PDMS-silicone oil surface (see Supporting Information Figure 2). The high temperature promotes the droplets mobility due to the decrease of infused lubricant viscosity. However, lubricant with high viscosity will hinder the movement of the droplets and leads to a higher sliding angle. Figure 6b is acidic steam test. (PDAC/SiO2)10-D4-PDMS-silicone oil surface is exposed to 0.5M HCl steam for 12 hours. The sliding angle of this surface also remained low. Therefore, a thermal-stable surface with liquid-repellency can be fabricated. With the advantages of withstanding autoclaving of this surface, the applications of anti-fouling surfaces used in sterilization process are also realized. Figure 6c and 6d are water steam tests. Both (PDAC/SiO2)10-D4-silicone oil and (PDAC/SiO2)10-D4-PDMS-silicone oil surfaces are exposed to steam. Without grafting PDMS the surface becomes essentially water pinning after exposure to steam for ~19 hours, which indicates that the silicone oil has drained. However, the sliding angle of this (PDAC/SiO2)10-D4-PDMS-silicone oil surface does not change very much after exposing to steam for ~24 hours.


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Figure 7.

a) Full frosting time of (PDAC/SiO2)10, (PDAC/SiO2)10-D4-PDMS, and

(PDAC/SiO2)10-D4-PDMS-silicone oil surfaces. Full frosting time of b) (PDAC/SiO2)10-D4silicone oil and c) (PDAC/SiO2)10-D4-PDMS-silicone oil surfaces after three icing-deicing cycles. Images showing the coalescence and mobility water drops on d) (PDAC/SiO2)10-D4PDMS and e) (PDAC/SiO2)10-D4-PDMS-silicone oil surfaces during ice formation. One can see that with the silicone oil the drops are able to grow larger on the surface before freezing, showing higher mobility at the surface as well as delayed ice formation. As the drops become large, they are swept away from the surface by gravity and nucleation begins again. According to Lin et. al31, there is a strong adhesion between water/ice and polar groups on a polar surface such as C=O or OH due the formation of hydrogen bonds and van der Waals’s forces. The surface layer of nonpolar silicone oil will not form such interactions with water, which may be a factor in delaying the formation of ice. Ice formation was tested on these


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surfaces under conditions of temperature of -5 ± 1 °C and relative humidity of 45 ± 5%. Full frosting time is defined as the time required for water to condense on the surface and a full layer of ice to be formed. Shown in Figure 7a, silicone oil infused surfaces can delay ice formation. Previous studies

4532[38]

have shown that by choosing an appropriate surface/lubricant system,

enhanced condensation and delayed ice formation can be realized on liquid infused surfaces. There would be several hydrophilic defects where frost growth is proceeding on (PDAC/SiO2)10D4-PDMS surface. Within those defective areas, water drops condensed and freeze. These sites are responsible for faster frosting. However, after being coated with silicone oil, the hydrophilic defects were buried under the surface and the thermal transport properties of the solid-liquid interface changed, the lubricant cloaked the water drop, all resulting in a delay in for drop condensation and ice formation. The criterion for cloaking4 is determined by the spreading coefficient, : = − − 4

(1)

where is the surface interfacial energy between water and air phase, is the surface interfacial energy between water and oil phase and is surface interfacial between oil and air phase. If > 0, it implies that the water droplet will be cloaked by the lubricant fluid. Whereas, < 0 implies that the lubricant will remain in the porous underlayer. was calculated using the Fowkes equation: / = + − 2

(2)

where and are the dispersion force contributions of the liquid surface tensions, which is 21.8 mN/m in this case. For nonpolar materials: = . In this experiment, =72.1

mN/m, =20.6 mN/m, = 20.6 + 72.1 − 2 × 20.6 × 21.8/ = 50.3 / . Thurs, = 72.1 − 50.3 − 20.6 = 1.2 />0.


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This implies that the water droplet should be cloaked by silicone oil in our surfaces. As the freezing temperature of silicone oil (-40 °C) is much lower than that of water and given its heat capacity, ice formation time of this silicone oil infused PDMS brushes surface is delayed dramatically (as shown in Figure 7a) at least in part to the cloaking of nucleated water drops. In this ice formation test, both (PDAC/SiO2)10-D4-silicone oil and (PDAC/SiO2)10-D4-PDMSsilicone oil surfaces were put in an environmental chamber and ice formation time was measured each three icing-deicing cycles. As shown in Figure 7b and 7c, this suggests that with the help of liquid-like PDMS, ice formation time of (PDAC/SiO2)10-D4-PDMS-silicone oil is a little longer than (PDAC/SiO2)10-D4-silicone oil, from which it can be inferred that the drainage of silicone oil layer is slowed to some extent during this icing-deicing cycles. Figure 7d and 7e show the ice formation process and water droplet mobility of (PDAC/SiO2)10-D4-PDMS and (PDAC/SiO2)10D4-PDMS-silicone oil surface. It can be seen that with the silicone oil layer, drops are able to grow larger before they freeze than without the lubricant. This shows both that there is surface mobility as well as that the water at the surface is not freezing. The droplets on (PDAC/SiO2)10D4-PDMS-silicone oil surface are highly mobile compared to the droplets on (PDAC/SiO2)10-D4PDMS surface. This mobility and coalescence of condensed water droplets creates a “sweeping” effect moving a layer of water away from the surface and gives opportunities for new drop to nucleate.3233

Through this process and moving away from the solid surface of water, ice

nucleation and formation on the surface is delayed. 4. Conclusion Presented here is a new approach for fabricating a robust flat self-cleaning liquid surface which does not rely on an underlying rough surface or macroscopic swollen rubber layer acting as a lubricant reservoir. This surface uses silicons, which are often used in food contact surfaces or


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medical devices in contrast to other reported methods for creating non-textured slippery surfaces. The strategy used here is the grafting of a mobile, low surface tension layer of PDMS brushes to the flat substrate. Lubricants with high chemical affinity to the PDMS layer, such as the silicone oil used here, are able to create a stable liquid layer when spin coated onto this PDMS layer. Adding the silicone oil lubricant to the layer of PDMS brushes is a novel approach, creating significantly improved self-cleaning properties. We showed that there is an optimal value of molecular weight of the PDMS grafted to the surface in order to create surfaces with low sliding angles for water drops. This kind of no-texture slippery surface can be characterized with optical techniques such as spectroscopic ellipsometry which cannot be used for textured surfaces. The same strategy can extend the lifetime of liquid/lubricant infused surfaces based on a porous underlying surface, shown here using a layer-by-layer film assembled with silica nanoparticles. We think this approach will bring new possibilities to the engineering of physically and chemically robust liquid-infused surfaces. The drainage of silicone oil and loss of properties of the surface is delayed by this layer of grafted low MW PDMS increasing the lifespan of the surface. This surface can maintain its water repellency under heating and exposing to steam, meaning that these surfaces could withstand sterilization processes. Ice formation is delayed and droplets mobility improved on the stable liquid layer compared to on the underlying solid surface due to the presence of the lubricant layer. The non-textured surface presented here is made entirely from silicones. Safety concerns have been raised with compounds such as perfluorooctanoic acid which has been widely used for water and oil repellent surfaces, leading to decisions to phase out the use of these molecules.3435 Similar concerns exist regarding the use of nanoparticles in consumer products.

3637

A fluorine and nanoparticle free surface such as the


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flat self-cleaning surface presented here may be able to be used in a range of applications such as food or cosmetics packaging. ASSOCIATED CONTENT Supporting information. Description of chemical modification of surface as well as more images and data regarding contact angles and sliding angles on these surfaces. AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed: [email protected] Author Contributions Funding Sources The authors would like to acknowledge NSF DMR 1425187 and the University of Akron’s NSF I-corps site CNS 1322061 for funding.

ACKNOWLEDGMENT The authors would like to thank Prof. Bryan D. Vogt for help with the ellipsometric mapping of the slippery surfaces and Dr. Alamgir Karim (both from Polymer Engineering at the University of Akron) for help with the icing experiments.

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Surface Functionalization for a Nontextured Liquid-Infused Surface

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