Multifunctional Textured Surfaces with Enhanced Friction and

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Multifunctional Textured Surfaces with Enhanced Friction and Hydrophobic Behaviors Produced by Fiber De-bonding and Pullout Reza Rizvi, Ali Anwer, Hani E. Naguib, Geoff Fernie, and Tilak Dutta ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11497 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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ACS Applied Materials & Interfaces

Multifunctional Textured Surfaces with Enhanced Friction and Hydrophobic Behaviors Produced by Fiber De-bonding and Pullout Reza Rizvi †, Ali Anwer, Hani Naguib*, Geoff Fernie and Tilak Dutta *Corresponding Author – Prof. H. Naguib ([email protected])

Dr. R. Rizvi, A. Anwer, Prof. H. Naguib Department of Mechanical and Industrial Engineering Department of Materials Science and Engineering University of Toronto 5 Kings College Cr., Toronto, M5S3G8, Canada

Prof. H. Naguib, Dr. G. Fernie, Dr. T. Dutta Institute of Biomaterials and Biomedical Engineering University of Toronto 164 College St., Toronto, M5S3G9, Canada

Dr. R. Rizvi, Prof. G. Fernie, Dr. T. Dutta Toronto Rehabilitation Institute

ACS Paragon Plus Environment

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University Health Network 550 University Ave., Toronto, M5G2A2, Canada Keywords: fiber composites, fiber pull out, surface texturing, ice friction, friction, slip resistance ABSTRACT Fiber de-bonding and pull-out are well understood processes that occur during damage and failure events in composite materials. In this study, we show how these mechanisms, under controlled conditions, can be used to produce multifunctional textured surfaces. A two-step process consisting of (1) achieving longitudinal fiber alignment followed by (2) cutting, rearranging and joining is used to produce the textured surfaces. This process employs common composite manufacturing techniques and uses no reactive chemicals or wet handling, making it suitable for scalability. This uniform textured surface is due to the fiber de-bonding and pull-out occurring during the cutting process. Using well-established fracture mechanics principles for composite materials, we demonstrate how different material parameters such as fiber geometry, fiber and matrix stiffness and strength, and interface behavior can be used to achieve multifunctional textured surfaces. The resulting textured surfaces show very high friction coefficients on wet ice (9X improvement), indicating their promising potential as materials for ice traction/tribology. Furthermore, the texturing enhances the surface’s hydrophobicity as indicated by an increase in the contact angle of water by 30%. The substantial improvements to surface tribology and hydrophobicity make fiber de-bonding and pull-out an effective, simple and scalable method of producing multifunctional textured surfaces.


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INTRODUCTION The hall-mark feature of a composite material is that its attributes is a combination of the attributes of its constituent phases. Often times these attributes (e.g. strength and ductility) are averaged between the constituent phases 1. In other instances, such as composite toughness 2, the combination of multiple phases produces a synergistic effect which exceeds the performance of either constituent phases. These obvious mechanical property benefits of composite materials, explains their high proliferation in our world today. With greater proliferation comes the motivation to find other value added applications of composites beyond the traditional mechanical/structural domain. Textured surfaces play an important role in both the natural and man-made domains. Common natural examples are the arrangements of fibrous septae on a gecko’s feet for reversible adhesion 3, the fractal-like papillae features on the lotus leaf for hydrophobicity and self-cleaning 4

, the ordered chitin fibrils in a butterfly’s wing giving rise to their structural coloration 5, and the

ridged denticles on the skins of sharks for reducing hydrodynamic drag and improving thrust 6. The discovery of these functional natural surfaces has spurred the development of their biomimicking alternatives such as dry adhesives 7, super-hydrophobic surfaces 4, and flexible lowdrag, high-thrust surfaces 6, to name a few. At the same time, several man-made challenges necessitate the use of synthetic textured surfaces such as light trapping structures on photovoltaic cells 8, shaped ceramic abrasives with long service life 9, and the infamous hook and loop structure of the Velcro® reversible fasteners 10. Synthetic approaches commonly employ subtractive processes to impart a surface texture, such as imprint or photo-lithography 11, which utilizes a mask or a mold to define features, or such as laser ablation/texturing 12, where material is removed by a raster scan of high powered lasers. A major shortcoming of these subtractive


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processes, is that it is difficult to introduce compositional heterogeneity across the surface. Usually, the textured features are compositionally the same as the substrate they are made from. If the features are required to be different than the substrate, then additional pre- and postprocessing steps such as deposition and etching are required which add to the complexity and the high costs of these processes. Here we introduce a technique of surface texturing based on fiber debonding and pullout in fiber reinforced composite materials. This technique utilizes traditional composites processing techniques and maintains compositional heterogeneity across the prepared surface. We start by demonstrating how the mechanics of damage to a fiber composite in a controlled manner can be used to fabricate functional textured surfaces consisting of pulled-out fibers and their sheathings. With this understanding, we demonstrate control of the surface microstructure through careful selection of fiber or matrix materials. We then demonstrate a few applications of textured composite materials made with this technique in surface wetting for enhanced hydrophobicity, and in tribology, significantly building on our previous understanding 13 of ice-tribology of composites materials. These textured surfaces can have feature sizes and hence functionalities similar to those surfaces produced using more complex and expensive manufacturing methods such as photolithography 11 or laser ablation/texturing 12. Furthermore, these textured surfaces, similar to the composites they are made from, are heterogeneous in nature, consisting of two or more phases, which can provide either individual functionalities or a synergistic functionality. The first step in a two-step process to produce these textured composite surfaces consists of mixing the fibrous and polymeric phases, in this case, 4 vol.% fibrous phase in a StyreneButadiene-Styrene (SBS) elastomer, together followed by flow based longitudinal alignment (Figure 1) of the fibrous phase through processing techniques such as extrusion and compression


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molding or injection molding (used in this study). In the second step, a sharp tool (e.g. a stainless steel blade secured on an anvil cutter) (Figure 1) is used to transversally cut the composite such that the aligned fibers are oriented normal to the cut surface. The resulting cut surface consists of thousands of microscopic fibrous protrusions (Figure 1), which are caused by fiber de-bonding and pull-out during the cutting procedure. These protrusions are present regardless of the type of fibrous phase present; fibers that have been assessed thus far are Alumina (Al2O3), Basalt (an igneous mineral, B), Carbon (CF), Glass (GF), Poly(p-phenylene-2,6-benzobisoxoazole) - liquid crystal polymer (PBO-LCP), Silicon Carbide (SiC) and Stainless Steel (SS). These materials represent polymer, metal and ceramic fibers – yet when they are embedded in a soft polymer matrix and transversally cut, they all display prominent surface texturing (Figure 2, higher and lower magnifications available in Figure S1 – Supporting Information) through fiber de-bonding and pull-out. Fiber de-bonding and pull-out are commonly observed phenomena during composite damage and failure. In fact, these phenomena are used as a toughening mechanisms to impede crack growth and improve the fracture toughness of brittle materials in ceramic matrix composites (CMCs) 14. Several investigators have studied fiber de-bonding and fiber pull-out and their origin and mechanisms are well understood. The work of Kelly 15 describes the energy expended as work of de-bonding during the process of composite fracture. Building on this, Wells and Beaumont 16 modelled the de-bonding and pull-out phenomena and predicted the pullout lengths based on Weibullian statistics of fiber strengths.


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Figure 1. Schematic illustrating the production process of surface texturing of fiber composites using longitudinal alignment followed by transversal cutting. The fiber de-bonding and pull-out phenomena during the cutting process results in textured surfaces.

Figure 2. Surface texturing of SBS elastomer composites materials with uniformly distributed fibrous protrusions are possible with a multitude of materials as the fiber phase.


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Theory The cutting process of longitudinally aligned fiber composites is similar in mechanics (Figure 3) to mode-1 tensile tests of notched fiber composite specimens widely used to study composite fracture mechanics 17. In a compliant material, such as a rubber or elastomer, a propagating tear is created in front of the cutting tool (Figure 3a), similar to a crack front in a notched tensile test. As the matrix tears open, all local tensile stresses are carried by the intact fibers. The comprehensive work of Wells and Beaumont 16, showed that the fiber de-bonding and pull-out failure mechanism occur because of (1) a critical de-bonding stress, σ d , being reached at the interface between the fiber and the matrix, followed by (2) additional length ( x ) dependent interfacial friction stress, f (x ) , being transferred to the fiber, after the de-bonding event. These two contributions to the total axial fiber stress, σ ( x ) , are additive - i.e. σ ( x ) = σ d + f ( x ) . Outwater and Murphy’s 18 energy based interfacial de-bonding model states that the axial stress,

σ d , required to cause interfacial de-bonding between the fiber and the matrix is:

σ d = 2 E f G2 C d f

(Equation 1)

where d f is the fiber diameter, E f is the fiber modulus, and G 2 c is the critical strain energy release rate for mode-2 type interfacial fracture between the fiber and the matrix. The frictional contribution, f (x ) , to the fiber stress arises due to some degree of contraction of the matrix onto the fiber during processing, which needs to be overcome if fiber pull-out is to occur. A complete expression 16, detailing the axial stress distribution along the fiber length, with the de-bonded crack front being the origin of the x co-ordinate (Figure 3b), is given as

σ (x) = σ p − (σ p − σ d )exp(− βx )

(Equation 2)


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where σ p = ε 0 E f ν f is the maximum possible stress due to friction, and β =

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µν f Em is a E f d f (1 + ν m )

characteristic elastic co-efficient of the interface. Here, ε 0 , is the misfit strain characterizing the matrix contraction on the fiber upon processing, E and ν , are the Young’s modulus and the Poisson’s ratio, of the fiber and matrix phase (denoted by subscripts f and m, respectively) and µ , is the coefficient of friction between the fiber and the matrix phase. A key concept of Equation 2 is that it accounts for the Poisson’s contraction of the fiber during the pull-out process, with σ p , characterizing the maximum stress due to friction, which occurs when the fiber’s Poisson’s contraction is equal to the matrix’s residual contraction strain. Assuming the stressed fiber to have a uniform failure strength, σ f , then fiber failure in Equation 2, would occur when the fiber

(

)

stress equals the fiber’s failure strength - i.e. σ f = σ p − σ p − σ d exp(− βld 2) . Note that this occurs on the crack plane ( x = l d 2 ), where the fiber stress is at a maximum (Figure 3b), suggesting that there shouldn’t be any fiber pullout. However, most microscopic fibrous materials do not have a uniform strength along their axis – often times the presence of defects at such small scales becomes the strength limiting condition [19,20]. This is particularly true for brittle materials such as glasses or ceramics, but also applies to even ductile materials such as polymers and metals which upon spatial constriction (e.g. micro-fibers and nano-whiskers) undergo significant strength increase at the expense of ductility [21–23]. When cut, composites containing such microscopic fibers will fail wherever the total fiber stress exceeds the local defect dependent strength, as depicted in Figure 3c. Statistically speaking, the likelihood of a fiber defect lying at a point in plane with the crack front is very low; instead, failure is most likely to happen on either side of the plane. As a result, one side of the cut surface will contain a


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protruding fiber while the other side will contain its opposite sheath, along with the remainder of the fiber, if any.

Figure 3. The cutting process (a) responsible for the surface texturing by fiber de-bonding and pull-out. Schematics illustrates the mechanisms by which (b) a fiber of uniform strength fractures, and (c) a fiber of varying strength fractures, when cut. Upon cutting, the protrusions pull-out length, l p ,distributions (d) reflect the characteristics of the fibers that were used to fabricate the composite. Solid lines depict the log-normal fit to the length distribution data.

The above expression for fiber failure can now be rearranged and solved for the debonded length, l d , which is a function of the de-bond stress, fiber and matrix properties and interfacial friction.

ld =

2

β

ln

σ p −σ d σ p −σ f

(Equation 3)


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The de-bonded length is an upper limit indicator 16 of the fiber pull-out length, l p , since only the de-bonded portion of the fiber can fracture and be pulled-out. Therefore, inferences can be made about the pull-out lengths by examining the de-bonded length (Equation 3) and which parameters influence it. The degree of texturing can be improved by increasing the de-bonding and hence pull-out lengths, which according to Equation 3 can be done in several ways. For instance, increasing fiber diameter ( d f ) decreases de-bonding stress ( σ d ) and decreases the characteristic elastic co-efficient ( β ) – this will result in greater de-bonding (and hence pull-out) lengths. This is supported by the SEM observations of Figure 2 and quantified in Figure 3d, where thinner diameter fibers such as alumina (Al2O3) fibers with ~ 5µm diameters have significantly less pullout lengths (20µm) than the thicker 50µm Steel (pullout length: 170µm) and 7µm Carbon (35µm), 13µm Glass (37µm) and 13µm Basalt (46µm) fibers. Further evidence is provided by surface roughness measurements (Figure S2), which indicates that the average surface roughness increases as the diameter of the fibrous phase is increased. The angles (Figure S3) that the pulled-out fibers make with the horizontal cut surface were characterized based on analysis of the SEM observations (Figure 2). Stiffer materials such as Basalt and Glass had higher preferential orientation angles of 70° and 68°, respectively. Whereas, the flexible PBO liquid crystal polymer fibers had essentially no preferential orientations indicated by a low average angle of orientation of 52° and a large standard deviation of 24°. Equation 3 also predicts that fibers with higher stiffness ( E f ) and higher failure strengths ( σ f ) reinforcing a more compliant matrix (lower E m ) will result in greater pull out lengths. Furthermore, a poorer wetting between the fibers, manifested as a lower critical strain energy release rate ( G 2 c ) of their interface, will result in lower stresses to cause de-bonding ( σ d ) and


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hence higher de-bonding and pull-out lengths according to Equation 3. To validate this hypothesis, carbon and glass fiber reinforced Thermoplastic Polyurethane (TPU) samples were surface textured using an identical process as the SBS surfaces. The TPU used is a stiffer elastomer than SBS (85 vs. 55 Shore A durometer value) with greater tear strength and abrasion resistance. Furthermore, its chemical structure, consisting of diols and isocyanates, is more polar than SBS, hence, it should favor better surface wetting with inorganic fibrous phases. The morphology comparison between TPU and SBS, shown in Figure 4, confirms the hypothesis, where the surfaces textured using SBS as the matrix display significantly greater pull-out lengths (Figure 4a) and surface roughness values (Figure 4b) than the TPU samples. Furthermore, SBS exhibits poorer wetting of the fibers as the protrusions are void of any adhered polymer along the length of the fiber. In comparison, better TPU wetting on the protruded fibers is easily distinguished by the conically adhered polymer layer surrounding the base of the exposed fibers.

Figure 4. Scanning electron micrographs (a) after cutting and re-arranging is done in SBS and TPU elastomer containing 4 vol.% carbon and glass fibers. The surface morphology along with surface roughness profiles (b) indicate that texturing is more prominent in the more compliant and less wetting SBS elastomer than the stiffer and better wetting TPU elastomer.


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Ice Friction An interesting and practical application for these textured surfaces is in achieving high traction when sliding on ice in colder climates. Under these conditions, typical soft materials such as polymers, rubbers and elastomers have very low friction coefficients (µ 0.35) in the first run, but values in subsequent runs are sharply reduced, due to breakage of longer, unstable fibers. The Alumina fibers, despite being brittle, have a much smaller diameter and hence smaller pull-out lengths, resulting in significantly lower COF values (0.246±0.009) than other brittle materials – Basalt: 0.329±0.020, Glass: 0.289±0.011 and SiC: 0.290±0.013. Contrast this with ductile and deformable fibers like Steel (0.251±0.008) and PBOLCP (0.172±0.012), which had significantly smaller COF than the brittle fibers (Figure 5f). Based on our analysis, Carbon fibers, possess the right combination of stiffness and toughness and along with their large pull-out lengths, provide the optimum COF on wet ice.


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Figure 5. Experimental setup (a) for measuring the friction of surface textured composites on wet-ice substrates. Friction slide test profiles for a (b) pure SBS elastomer, and (c) surface textured SBS-4vol% carbon fiber composite tested at a nominal load of 600N and a slide speed of 300 mm/s. The textured composite (4 vol% Carbon) shows significantly higher static coefficients of friction (first peak) characterized by the (d) indentation and shearing of the ice contact. After repeated use (N=9) on ice, the protruding fibers (e) are still intact and the composite still maintains a high friction coefficient (f) regardless of the fiber types tested.

Hydrophobicity Apart from ice tribology, the effectiveness of texturing surfaces by fiber pull-out and debonding for enhancing hydrophobicity can also be demonstrated (Figure 6). We’ve measured the contact angles for the composite surfaces with and without surface texturing. The contact angles of water on smooth SBS and TPU substrate are both similar at 83.6° and 83.8°, respectively, making them each slightly hydrophilic (Figure 6a and Figure S8). Addition of a small amount of fibrous phase to these materials does not alter the surface wetting characteristics because of the presence of a microscopic polymer skin layer; this is generally a consequence of processing that is done in the melt or solution state. After texturing, the contact angle for SBS and TPU


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composites containing 16 vol% carbon fiber (CF) increases to 110.1° and 102.7°, respectively. This represents a 31.7% and 22.6% increase over the contact angles of the SBS and TPU, respectively.

Figure 6. Typical water droplet shapes along with the measured contact angles (a) for SBS and TPU elastomers and their textured composite surfaces containing 16 vol% carbon fiber. Plot (b) showing the variation of contact angle with carbon fiber content for SBS and TPU textured composites – the inset shows the SEM surface morphology at a fiber content of 16 vol%. Plot (c) comparing the contact angle of textured SBS composite surfaces containing 4 vol% of various fibers.

A steady, non-linear increase in the contact angle is observed (Figure 6b), as the volume fraction of carbon fiber increases in the textured composites containing either SBS or TPU. However, at higher fiber contents (16 vol% CF), there is a 7.4° difference in the contact angles between SBS and TPU composites. The higher contact angle of SBS composites is attributed to the marked difference in surface morphology (Figure 6b, inset) between the two materials. At these compositions, the SBS composite still retains the fibrous protrusions that are visible at


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lower fiber contents (Figure 2 and Figure S1). In contrast, the TPU composite morphology has significantly lower protrusion lengths at high fiber contents. This morphological observation indicates that there is a fine balance between the stress-field interactions of adjacent fibers – controlled by the composition dependence of inter-fiber distance during the cutting process, and the interfacial strength between the fiber and the matrix phase. The type of fibers used in the textured composite also plays a significant role in their surface wetting behavior. The average contact angle of textured composites containing 4 vol% of different microscopic fibers is shown in Figure 6c. Fibers that result in the largest protrusion lengths, such as PBO, Basalt, Glass and Carbon (Figure 3d), provide some of the largest improvements to contact length. One exception to this are the steel fibers, which because of their larger size (Figure S1) are not as densely distributed on the surface to cause a significant influence to the wetting behavior. The largest improvement in contact angles was observed for the PBO liquid crystal fibers (111°), which are relatively softer and more flexible than the other fibers. After texturing, their ductility provides them with a distinct surface morphology which consists of highly deformed fibers with low angular orientations (Figures S1 and S3). Two main contributing factors, surface roughness and compositional heterogeneity, explain the contact angle improvements when composite materials are surface textured by fiber de-bonding and pull-out. The first contribution, based on the well-known Wenzel

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theory of

surface wetting, is from the added surface roughness with increasing fiber volume fraction (Figure S4). This added surface roughness, brought about by the protruding fibers, amplifies the true droplet contact area, when compared to the much smoother SBS and TPU surfaces. Higher contact area due to increased surface roughness at the droplet-solid interface amplifies the net energy balance between the solid-gas and solid-liquid interface energies such that hydrophilic


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surfaces (θπ/2) become more hydrophobic indicated by an increase in contact angle. The second, and more significant, contribution comes from the compositional heterogeneity present at the droplet-solid surface interface. The well-known Cassie’s 28 condition of surface wetting for heterogenous systems is generally applied for textured surfaces containing the solid surface and air pockets as the two phases present underneath the droplet interface. In the present scenario, there are two possible heterogeneous surfaces configurations that might exist (schematized in Figure S9). The first configuration consists of the droplet resting on a heterogenous surface containing both the fibrous phase and the polymeric matrix. In this case, Cassie’s condition predicts the apparent contact angle to be proportional to the contribution of the area fractions of each constituent phase and their own equilibrium contact angle. According to this configuration, if the fibrous phase is more hydrophilic than the polymer matrix, then the apparent contact angle should decrease, while if the fibrous phase is more hydrophobic, then the apparent contact angle should increase. The second, more complex but more probable configuration is based on the droplet resting on a tri-phase heterogenous surface consisting of the fibrous phase, the matrix phase and any trapped air pockets. Air pockets beneath the droplet can get trapped when the inter-fiber spacing distance is low, especially at high fiber contents. Air pockets can also be trapped in the remaining sheaths in the matrix phase from the fiber debonding and pull-out process during fabrication. Any entrapped air, whether between the fibers or in the fiber sheaths, will make a significant contribution to the increase in contact angle according to Cassie’s model, since air has a contact angle of 180°. These air pockets in the triphase surface configuration would explain why even hydrophilic fibers such as Al2O3 and Glass


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exhibit an increase in contact angle when the Cassie model actually predicts a decrease in their respective two phase configuration without any air pockets. CONCLUSION In this study we developed novel surface textured composites with high friction and improved hydrophobicity. These composites were prepared in bulk using melt extrusion, injection molding. This was followed by shear assisted fiber-debonding by means of cutting the composite to expose the textured surface consisting of embedded fiber phase. The frictional properties were obtained using SBS as the matrix phase and Alumina, Basalt, Carbon, Glass, Steel, SiC and PBO as the fiber phase and compared with pure SBS as the baseline. We discovered 4 vol% CF-SBS composite to possess the highest COF (0.334±0.013) due to their optimum stiffness, toughness and protrusion lengths from the fiber debonding process. This represents a 930% increase in the COF as compared to it pure counterpart. The hydrophobic characteristics of these surface textured composites were evaluated using SBS and TPU as the matrix phases and varying vol% of carbon fiber from 0 to 16 vol% as the fiber phase. Compared with neat matrices, increasing vol% of carbon fiber improved the contact angle asymptotically to around 22.7% with TPU matrix and 31.3% with SBS matrix. This increase in contact angle was a result of both induced surface roughness and compositional heterogeneity by the carbon fibers causing wenzel and Cassie baxter configuration of the water droplet. We have successfully demonstrated the applicability of these easy-to-produce functional surfaces for enhanced icefriction and hydrophobicity applications – substantial improvements in ice COF and contact angle were obtained over non-textured polymeric surfaces. These are just two applications of functional composite textured surfaces by fiber de-bonding and pull-out with many more yet to be explored.


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Experimental Section Materials: The compliant matrix materials consisted of thermoplastic Styrene-Butadiene-Styrene (SBS) elastomer (D2122 DO-N, Kraton Performance Polymers, Houston TX) and Thermoplastic Polyurethane (TPU) elastomer (Desmopan 385E, Bayer Materials Science, Germany) was used as the compliant matrix material. The SBS had a density of 0.93g/cc and TPU had a density of 1.2g/cc. Several materials representing polymers, metals and ceramics were evaluated as the fibrous phase in the textured composites. Polycrystalline Alumina (Al2O3) fibers (MAFTEC ALS, Mitsubishi Plastics, Japan) with a density of 3.5g/cc and an average diameter of 5µm and length of 1mm, chopped Basalt fibers (MB-CF-13, Mafic SA, Ireland) with a density of 2.67g/cc and an average diameter of 13µm and length of 3mm, Polyacrylonitrile (PAN) precursor based chopped Carbon fibers (Panex 35, Zoltek, St. Louis MO) with a density of 1.81g/cc and an average diameter of 7.2µm and length of 3mm, chopped Glass fibers (Chopvantage 3075, PPG Fiber Glass, Cheswick PA), with a density of 2.51g/cc and an average diameter of 13µm and length of 3.2mm, chopped Poly(p-phenylene-2,6-benzobisoxoazole) liquid crystal polymer fibers (Zylon AS, Toyobo Co. Ltd., Japan) with a density of 1.54g/cc and an average diameter of 12µm and length of 3mm, polycrystalline Silicon Carbide (SiC) fibers (Sylramic SiC fiber, COI Ceramics, Magna UT) with a density of 2.95g/cc and an average diameter of 10µm and length of 5mm and short Stainless Steel fibers (SO-INOX, Green Steel Group, Italy) based on AISI 434 wire steel with a density of 7.7 g/cc and an average diameter of 70µm and length of 1.5mm were used as the fibrous phase.

Fabrication of Textured Composites: Composites containing various fiber types were prepared with SBS or TPU elastomer using commonly employed polymer composite processing


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techniques of twin-screw compounding and compression molding. The twin-screw compounding was done in continuous mode using a DSM Xplore 15 (DSM, Netherlands) lab-scale mixer at a temperature of 200°C, a screw speed of 20rpm. The extrudate was pelletized and compounded again, in order to ensure homogenous mixing of the two phases. Injection molding of the unidirectionally aligned composites was carried out on a benchtop DSM Xplore injection molding unit (DSM, Netherlands) at a mold temperature of 50°C, injection pressure of 115 psi, and fill time of 10s to create final molded dimensions of 60mm by 13mm by 2mm. Next the surface texturing was done in a three step process of cutting, re-orienting and refusing the molded composites. In the first step, cutting was done by transversally shearing the molded samples using a sharpened stainless steel scalpel with the aid of a template with 3mm spacing. This process of shearing the molded composites is what produces the surface textured morphology through fiber de-bonding and pull-out (schematized in Figure 3). Next the cut fiber strips are re-oriented orthogonally (90° rotation) so that the cut face is positioned laterally and the molded faces are arranged longitudinally. These cut strips are then placed in clamped rig (Figure S10) and held together with minimal clamping pressure for handling. The last process in the assembly is of re-fusing these strips together to make a solid material for practical applications. The re-fusion process is based on thermal diffusion of the polymer across the clamped molded faces of the strip and the temperature of the process is based on thermal properties and rheology of the composite material. Through experimentation, the optimum fusing temperature for SBS and TPU was found to be 145 and 160 °C, respectively, for a total time of 10min. The final dimensions of the sample depend on the starting dimensions of the molded piece as well as the cutting interval (3mm) – typically, 10 cut strips were joined together to make


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samples of dimensions 20 x 13 x 3 mm3. Two of these samples can be further applied on an adhesive backing strip to yield larger (20 x 26 x 3 mm3) samples for ice friction characterization.

Characterization of Textured Composites: The root mean square average surface roughness of the samples was measured using a Mitutoyo SJ-210 linear surface roughness probe (Mitutoyo, Japan). The scan distance was 4mm while the scan speed was 0.5mm/s. A Gaussian filter was applied to determine the mean-line with a cut-off wavelength of 0.8mm. Surfaces were evaluated using the arithmetic mean of the absolute values relative to the filtered mean line. The morphology of the surfaces were viewed at a 30° tilt using a JEOL JSM6600 scanning electron microscope (Jeol Corp., Japan) operated at 20kV, with the surfaces made conductive using a thin sputter coated layer of platinum. The average protrusion lengths, angles and densities were manually measured and analyzed on the open source ImageJ (National Institutes of Health) image analysis program 29, accounting for 30° tilt correction of both the protrusion angles and the 2D plane of view. The coefficient of friction of the compliant fiber composites were measured using a customized SATRA slip resistance testing machine STM 603 (SATRA, UK) based on ASTM F2913-2011 30. The machine is capable of applying a vertical force normal to the two contacting surfaces and then dragging the surfaces against each other at a pre-set strain rate, while monitoring the horizontal force required to drag the surfaces. The coefficient of friction (COF) is defined as the ratio between the horizontal friction (FF) and the normal force (FN) and is monitored continuously. For the purpose of this study, one of the surface was the fiber composite mounted on a custom rig and the other surface was 5mm thick wet ice maintained between -4 and -1°C. The relative humidity was recorded as being between 48 and 66%, although frost


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buildup effects were negligible since the ice surface was smoothed and wetted with a damp cloth 10s prior to testing. A normal force of 600N (~1MPa) was applied on the contact between the sample and ice with a dwell time of 0.2s. Then the ice tray was slid while under pressure of the sample at a rate (velocity) of testing of 300mm/s. The first peak COF value observed was defined as the static coefficient of friction. A total of 10 runs on freshly smoothed ice surfaces were performed. For most fibrous materials, the first run value was observed to be higher, while the rest of the 9 runs were slight lower and very similar (Figure S7); this is attributed to some initial fiber re-arrangement and breakage in the sample. For the purposes of analysis, the last 9 runs were averaged and reported as the materials COF and friction force values. The contact angle of the fiber composite was measured using a standard plano-convexo magnifying lens with a +20 diopter power and a 10.1 Mega Pixel Panasonic camera. The sample was mounted on an adjustable platform to avoid parallax errors. Adhering to ASTM D7490 31, 6 drops of 20µL distilled water was applied at 6 different locations on the sample surface using a standard manual micro pipette. Contact angle measurements were done using ImageJ, and the contact angle plugin

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. The plugin procedure relies on defining two base line points and three

edge points, through which the software estimates the droplet shape as an ellipse of best fit. The left and right contact angles are averaged and reported as per the standard.

Supporting Information: Different magnification SEM images, linear surface roughness scan profiles, fiber pullout angle distribution, varying vol% Carbon fiber linear roughness scan profile for TPU and SBS composites, first friction cycle run traces, repeated COF runs cycles, water droplet shape estimation, schematic illustration of Cassie heterogeneous surface models, sample


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holding rig, blade tip SEM and slit SEM image showing protruding fibers and backscatter, EDX images showing fiber wetting. Author Information Reza Rizvi †, Ali Anwer, Hani Naguib*, Geoff Fernie and Tilak Dutta *Corresponding-Author: Prof. H. Naguib Department of Materials Science and Engineering University of Toronto 184 College St., Toronto, M5S3E4, Canada

Prof. H. Naguib Corresponding-Author, Dr. G. Fernie, Dr. T. Dutta Institute of Biomaterials and Biomedical Engineering University of Toronto 164 College St., Toronto, M5S3G9, Canada

Dr. R. Rizvi, A. Anwer, Prof. H. Naguib Department of Mechanical and Industrial Engineering University of Toronto 5 Kings College Cr., Toronto, M5S3G8, Canada E-mail: [email protected]

Dr. R. Rizvi, Prof. G. Fernie, Dr. T. Dutta Toronto Rehabilitation Institute


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University Health Network 550 University Ave., Toronto, M5G2A2, Canada

Present Addresses †Dr. Reza Rizvi Department of Mechanical, Industrial and Manufacturing Engineering University of Toledo 2801 W. Bancroft St. MS312 Toledo, OH 43606-3390 (419) 530 8237 [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) Acknowledgements We would like to acknowledge Funding from Technology Evaluation in the Elderly Network Centers of Excellence (TVN-NCE), Natural Sciences and Engineering Research Council (NSERC) of Canada, Ontario Research Fund, Governments of Ontario and Canada.

References


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software)

Function textured surfaces using fibrous elastomer composites are fabricated using a simple method involving fiber de-bonding and pull-out. The effect of fiber material, shape and content on the surface morphology is elucidated. The texturing makes these surfaces functional for friction and surface wetting applications, with improvements in coefficient of friction and water contact angle of 900% and 31%, respectively, demonstrated.


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Multifunctional Textured Surfaces with Enhanced Friction and

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