Creation of superhydrophobic wood surfaces by plasma etching and thin-film deposition

    Creation of superhydrophobic wood surfaces by plasma etching and thin-film deposition Linkun Xie, Zhenguan Tang, Lu Jiang, Victor Breedveld, Dennis W. Hess PII: DOI: Reference:

S0257-8972(15)30290-5 doi: 10.1016/j.surfcoat.2015.09.052 SCT 20605

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

23 July 2015 25 September 2015 28 September 2015

Please cite this article as: Linkun Xie, Zhenguan Tang, Lu Jiang, Victor Breedveld, Dennis W. Hess, Creation of superhydrophobic wood surfaces by plasma etching and thinfilm deposition, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.09.052

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ACCEPTED MANUSCRIPT Creation of superhydrophobic wood surfaces by plasma etching and thin-film deposition Linkun Xie a, b, Zhenguan Tang b, Lu Jiang b, Victor Breedveld b, *, Dennis W. Hess b, * a

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Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, People’s Republic of China b School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States

ABSTRACT: Superhydrophobic wood has been created using a combination of O2 plasma

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etching and plasma deposition of thin films to achieve the necessary combination of surface roughness and chemistry. Inherently hydrophobic fluorocarbon films (from pentafluoroethane

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(PFE) precursor) and hydrophilic diamond-like carbon (DLC) coatings (from acetylene

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precursor) were both used to create highly water repellent substrates. The effect of O2 plasma

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etching on surface roughness was investigated using Scanning Electron Microscopy (SEM) and Laser Scanning Confocal Microscope (LSCM) profilometry. The wetting behavior of the

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resulting wood was determined by static water contact angle and droplet sliding angle measurements. Wood samples subjected to O2 plasma etching prior to fluorocarbon

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deposition exhibited “roll-off” superhydrophobicity with low sliding angles; the sample in this study with the most extreme wetting properties has the highest water contact angle and lowest sliding angle reported to date for modified wood substrates (WCA 161.2°±1.5° and sliding angle ~ 15°), without affecting visual appearance of the wood. Due to our ability to control roughness, etched samples that were coated with hydrophilic DLC films displayed superhydrophobic behavior (WCA), although the surface was “sticky” in that water droplets did not slide or dislodge from vertically-held substrates. Keywords: Wood; Water repellency; Plasma etching; Plasma deposition; Fluorocarbon film; Diamond-like carbon (DLC) film; Superhydrophobic surface 1

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Corresponding author. Tel.: +1-404-894-5134 (V. B.); +1-404-894-5922 (D. W. H.). E-mail address: victor. [email protected] (V. B.), [email protected] (D. W. H.).

1. Introduction

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Wood is a three-dimensional polymeric composite with cellulose, hemicelluloses and lignin

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as main components [1,2]. In addition, wood contains small amount of extractives, including lipids, phenolic compounds, terpenoids, fatty acids, resin acids, steryl esters, sterol, and waxes [3,4]. In modern society, wood is used as a building, engineering and decorative material

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because it is economical, renewable and aesthetically pleasing [1]. Furthermore, wood has a

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high strength-to-weight ratio and requires less energy during manufacturing than steel or cement-based materials. The greatest disadvantage of wood in these applications is its

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hygroscopicity [5]; hydroxyl groups on the porous surface readily form hydrogen bonds with

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sorbed water molecules[6,7]. As a result of water uptake, wood suffers from dimensional

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instability [8] and accelerated biological (fungi, bacteria, insects) and photochemical degradation during the natural weathering process that occurs both outdoors and indoors [9].

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Hydrophobic surface protective coatings and sealants that block pores can save the wood products from degradation by fungi, bacteria and insects [10]. However, the search for long-lasting, economic wood treatment alternatives to paints and varnishes continues. Over the years, various methods have been reported to modify wood hydrophobicity, including acetylation [11], metal oxide deposition [12-14], modification with chlorosilanes [15,16], and silicone polymer grafting [8]. The majority of these methods require the use of wet chemistry, which in many instances causes environmental concerns due to the use of solvents and resulting waste streams, and economical challenges resulting from the need for additional processing steps and large consumption of chemicals due to sorption into the porous wood substrate. Moreover, most of these wet chemistry methods employed to modify 2

ACCEPTED MANUSCRIPT wood substrates change its distinctive physical appearance and/or mechanical strength. These concerns have inspired the search for alternative methods that modify only the outer surface

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layer of the wood. Vapor phase approaches such as those that use glow discharges or plasmas

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for the functionalization of wood surfaces are of much interest [17].

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Plasma techniques are dry, clean processes with minimal environmental concerns [9]; the treatment affects only the outermost layers of the wood surface [18]. Therefore, plasma

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modification does not change bulk wood properties [9] and chemical consumption and waste generation are limited. Recently published literature has focused on two classes of materials

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for plasma-assisted deposition: siloxanes and fluorine-containing polymers. For example, the

of

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plasma-generated

polydimethylsiloxane/additive

film

[19].

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deposition

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weathering resistance of southern yellow pine wood surfaces has been improved by

Hexamethyldisiloxane (HMDSO) has also been shown to be a viable film precursor to create

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hydrophobicity on various wood surfaces [2,9,17,20-22]. Fluorine-containing monomers such as CF4 [23], and SF6 [9] have been reported to modify the hydrophobicity of spruce, chestnut

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poplar, and pine wood surfaces. In addition, pine was plasma-treated using ethylene, acetylene, 1-butene and vinyl acetate as precursors to render the wood surface hydrophobic [5]. Highly hydrophobic maple wood surfaces were prepared by atmospheric treatments

with

ethylene,

methane,

chlorotrifluoroethylene

and

hexafluoropropylene precursors [24]. Although the modified wood surfaces in all of these plasma-based studies exhibited increased hydrophobicity, the static water contact angles (WCA) reported were all less than 145° [18,24], well below the generally accepted threshold of 150° for superhydrophobicity [25-27]. Moreover, water droplet adhesion, which can be characterized via droplet sliding angle or contact angle hysteresis, was high in prior studies, 3

ACCEPTED MANUSCRIPT thereby preventing easy removal of droplets and self-cleaning effects that are often associated with superhydrophobicity. Our previous studies on the modification of cellulose substrates

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[28-30], independently controlled physical roughness and surface chemistry to achieve

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water droplet wetting and adhesion on cellulose surfaces.

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superhydrophobic behavior, thereby demonstrating that these properties are critical to tuning

Wood surfaces have inherent roughness as a result of the micro- and nano-structures of

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different wood cell types (tracheids, fibres, parenchyma, vessels) [31], but lack the nanoscale roughness that is required for “roll-off” superhydrophobicity [28,32,33]. In this paper, we

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demonstrate that selective etching of wood with an O2 plasma can be used to tune the

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nanoscale roughness, so that subsequent deposition of a low surface energy film generates

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superhydrophobicity.

Selective plasma etching is based on the premise that amorphous regions of polymeric

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substrates will be more susceptible to reactive etching than are crystalline regions [28,34]. In prior work on paper and cotton substrates, our group and other researchers have shown that

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this approach is applicable to cellulose, one of the main components of wood [28-30,35-37]. In addition, lignin (aromatic compound) is more resistant to plasma etching than are cellulose and hemicelluloses (aliphatic compounds) [38]. The inherent chemical heterogeneity of wood has been used to take advantage of differences in etching rates among the wood components, thereby generating surface roughness on wood upon plasma exposure [38]. In this study, we present the results of the effect of O2 etching and subsequent plasma-assisted deposition of thin films on the wetting behavior of wood surfaces. In particular, we demonstrate the importance of controlling the wood substrate roughness to achieve superhydrophobicity. We underscore this by showing that even an inherently 4

ACCEPTED MANUSCRIPT hydrophilic coating material (diamond-like carbon, DLC) can impart superhydrophobicity to wood when the surface roughness is tuned appropriately. Fluoropolymer coatings were used

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to achieve the highest water contact angles and lowest sliding angles reported to date for

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wood substrates.

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2. Experimental 2.1. Materials

Golden chinkapin (Castanopsis chrysophylla) sliced veneer wood specimens were obtained

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from the Department of Natural Resource Ecology and Management at Iowa State University.

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The specimen were cut into thin slices of 50 mm×15 mm×0.8 mm along the radial direction of the log, and surface plasma treatments were performed on these radial sections. Before

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treatment, all samples were sanded with 120 grit sandpaper and cleaned with a nitrogen gas

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jet to eliminate particulate residue from the wood surfaces; finally the wood was oven-dried at

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103 ℃ for 4 h. The sanding process assisted the elimination of surface contamination and created uniform, reproducible roughness on the wood surfaces prior to further roughening by

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the plasma treatment. Silicon wafers (100-oriented), which were used as control substrates to enable deposited film characterization, were cut into sizes of 15 mm × 15 mm, cleaned with acetone, methanol, isopropanol washes (BDH, ACS grade, 99%) and dried using nitrogen gas. Pentafluoroethane (PFE) monomer gas (N4 grade, 99.99%) was kindly donated by Dr. Ashwini Sinha (Praxair). Argon carrier gas (Ultra High Purity, 99.99%), Acetylene (C2H2) (Ultra Pure Carrier, 100%), nitrogen (Ultra High Purity, 99.999%) and oxygen (Ultra Pure Carrier, 99.996%) were purchased from Airgas Inc. (Radnor, PA). 2.2. Plasma processing 2.2.1. Plasma etching in O2 and plasma deposition of pentafluoroethane (PFE) films Plasma etching and deposition were performed in a 6-inch diameter parallel-plate rf 5

ACCEPTED MANUSCRIPT (13.56MHz) discharge reactor (Kurt J. Lesker Co., Jefferson Hills, PA). Wood and silicon wafer samples were placed on the bottom grounded electrode and heated to 110 ℃ for all

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experiments using Omegalux CIR 2015 cartridge heaters (Omega Engineering Inc., Stamford,

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CT) and monitored by a Syskon RKC temperature controller (RKC Instrument Inc., South

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Bend, IN). More detailed information about this reactor can be found elsewhere [28]. Before any etching or deposition treatment, the reactor system was evacuated to a base pressure of ~

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1.0×10-2 Torr. Etching with an O2 plasma was carried out at a working pressure of 0.5 Torr and oxygen flow rate of 75 standard cubic centimeters per minute (SCCM). Fluorocarbon

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(PFE) film deposition was conducted at a working pressure of 1.0 Torr with PFE precursor

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flowing at 20 SCCM and argon carrier gas flowing at 75 SCCM. The applied rf power during

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etching and deposition was 120 W. After deposition, the reactor system was back-filled to atmospheric pressure with nitrogen gas; samples were then removed from the reactor for

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surface characterization.

2.2.2. Plasma etching in O2 and deposition of DLC films

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A 2.5-inch parallel-plate rf (13.56MHz) discharge reactor [29] was used to etch wood surfaces and subsequently deposit DLC films onto wood; this (different) reactor was used to prevent undesirable cross-contamination by fluorine species during DLC studies. The samples were processed as described above for PFE films. The bottom grounded electrode was again heated to 110 ℃ using Omegalux CSH-101120 cartridge heaters (Omega Engineering Inc., Stamford, CT) and monitored by a platen temperature controller (Tek-Vac Industries Inc., Brentwood, NY). Etching with an O2 plasma was performed at a working pressure of 0.5 Torr and an oxygen flow rate of 20 SCCM. DLC film deposition was also conducted at a working pressure of 0.5 Torr from a mixture of acetylene (C2H2) and argon at flow rates of 10 SCCM 6

ACCEPTED MANUSCRIPT and 30 SCCM, respectively. The applied rf power during etching and deposition was 120 W. 2.3. Characterization

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2.3.1. SEM and LSCM imaging

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Scanning Electron Microscopy (SEM) of unmodified and plasma-modified wood surfaces

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was performed with a Hitachi SU8230 cold field-emission SEM (Hitachi High-Technologies Co., Japan) at an accelerating voltage no higher than 5.0 keV. All samples were sputter-coated

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with Ag/Pd to mitigate charging effects during imaging.

The wood surface roughness, quantified through various roughness parameters such as

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average surface roughness (Sa), root mean square roughness (Sq) and peak-to-valley

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roughness (Sz), was measured for unetched and etched samples by LEXT OLS4100 Laser

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Scanning Confocal Microscope (LSCM) profilometry with a laser wavelength of 405nm (Olympus Co., Japan), using a 20x objective and a scan area of 643 μm×646 μm.

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2.3.2. Deposition thickness measurements Deposition thickness variation with deposition time on silicon wafers for PFE and DLC

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films was determined with an M-2000V ellipsometer (J. A. Woollam Co., Lincoln, NE). Three locations were measured on each sample and the average value was used to represent the deposition thickness. From these data, deposition rates for PFE and DLC films, which were constant over the deposition times investigated, were 191 ± 9 nm/min and 50 ± 3 nm/min, respectively. 2.3.3. Contact angle measurements Water contact angles were measured with a Rame-Hart CA goniometer (Model 290, Succasunna, NJ) using the sessile drop method and 4 μL deionized water drops. Sliding angles were determined by tilting the sample stage at a rate of one degree per second until the 7

ACCEPTED MANUSCRIPT drops started moving and rolled off the sample surface. Sliding angles were measured both along and perpendicular to the grain directions of the wood samples. Due to inherent

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variability and inhomogeneity in wood samples, the reported contact angles and sliding angles

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represent the average of at least eight measurements at different locations on each sample.

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2.3.4. XPS measurements

The elemental composition and chemical bonding information of the deposited PFE and

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DLC films on the wood surfaces were determined with a Thermo K-Alpha XPS (Thermo Fisher Scientific, West Palm Beach, FL) equipped with a monochromatic Al Kα X-ray source

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(hv = 1486.6 eV) and operating at a vacuum below 10-7 Pa. Curve fitting

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Avantage 5.934 software.

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(Gaussian-Lorentzian) of the high resolution C1s spectra was performed with Thermo

3. Results and discussion

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3.1. Effect of O2 plasma etching

Roughness enhancement via plasma etching is based on selective etching rather than

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homogeneous etching; the latter would result in reduced surface roughness [39]. Wood, as mentioned previously, is a biopolymeric composite of mostly cellulose, hemicelluloses and lignin. The SEM micrographs showing the evolution of surface morphology with etch time are presented in Fig. 1. The unetched wood surface shows much smoother features than the etched samples. After O2 plasma etching, the wood displays a roughened surface with nanometer-scale characteristics (Fig. 1b, c and d) that are not observed on the unetched sample (inset in Fig. 1d highlights the nanometer-scale structures). We ascribe the features on the roughened surface to both lignin and crystalline portions of the cellulose component of wood, which are exposed after selective etching of the surrounding amorphous cellulose by 8

ACCEPTED MANUSCRIPT the O2 plasma [28,38]. In order to investigate the wood surface roughness more quantitatively, the same samples were characterized with a laser scanning confocal microscope (Fig. 2).

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From the topography of the 3D confocal microscopy images, it can be clearly seen that the

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wood surface roughness increases with longer etch time. In addition, roughness parameters

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average roughness (Sa), root mean square roughness (Sq) and peak to valley roughness (Sz) increase with prolonged etch time (Table I). These results indicate that etch time in an oxygen

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plasma provides excellent control over the degree of roughness of wood substrates, which is critical to establish wetting and adhesion properties.

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3.2. Water contact angle and sliding angle

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Static water contact angle (WCA) is normally used as a criterion for the characterization of

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surface hydrophobicity, but this parameter alone is inadequate to fully describe wetting and adhesion of water droplets on hydrophobic surfaces [34,40]. Additional measures for adhesion,

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such as droplet sliding angle (critical tilt angle beyond which a droplet of known volume slides along a substrate) or contact angle hysteresis (difference between advancing and

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receding contact angles) are often reported [41]. A surface with a high water contact angle does not necessarily exhibit a low sliding angle [32]; even superhydrophobic substrates can exhibit droplet adhesion strengths ranging from “roll-off” to “sticky” [28]. In this paper we invoke static water contact angle and droplet sliding angle as critical parameters for evaluating the properties of hydrophobic wood surfaces. As discussed in the introduction to this paper, untreated wood is a porous, hydrophilic material owing to the abundant hydroxyl groups of the polysaccharides and lignin constituents. It is therefore not possible to measure a static contact angle for untreated wood samples, because the water droplet spreads out along the grain and is quickly absorbed into the wood. 9

ACCEPTED MANUSCRIPT Hence, we denote the contact angle of the untreated wood surface as “0°”, indicating its naturally occurring, highly hydrophilic behavior. After deposition of hydrophobic

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fluorocarbon (PFE) films, the wood surfaces exhibited stable (super)hydrophobic properties

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that could be quantified as a function of plasma etch time (i.e. roughness) and deposition time

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(i.e. film thickness); the results of those experiments are presented in Fig. 3. As a reference, the water contact angle (θ) of PFE-deposited on a flat silicon wafer is 103.4°±0.9° (Fig. 4a).

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To highlight the role of surface roughness as a critical factor for imparting superhydrophobic properties, Fig. 3a shows the contact angle and sliding angle as a function of etch time for a

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constant PFE layer thickness of 382 nm (2 min deposition). From this graph, it is obvious that

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the contact angles increase substantially from 140.3°±2.2° for the unetched sample to

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157.0°±1.2° after 5 min of etching, and then remain essentially constant for etch times up to ~30 min. However, significant changes are observed in the sliding angles over this range of

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etch times, with the sliding angles decreasing gradually with increased etch time. Wood with greater nanoscale roughness provides the ability to trap more and larger air pockets between

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the substrate and water droplet, leading to lower adhesion and lower sliding angles. To further understand the effect of surface roughness on hydrophobicity, water contact angles were determined as a function of PFE film thickness for wood surfaces without prior O2 plasma etching (Fig. 3b), after 10 min etching (Fig. 3c) and after 30 min etching (Fig. 3d). These results (Fig. 3b) demonstrate again that the roughness of intrinsic micro- and nano-structures on wood surfaces is insufficient to achieve superhydrophobicity (WCA > 150°); for all unetched samples, water drops remain “pinned” to the wood surface even when the substrate is tilted to 90°. As a result, the sliding angle is undefined for all samples in Fig. 3b. In contrast, wood surfaces subjected to O2 plasma etching for either 10 min or 30 min display higher 10

ACCEPTED MANUSCRIPT contact angles (all superhydrophobic) and lower sliding angles independent of PFE film thickness in the thickness range investigated (Fig. 3c and Fig. 3d). The qualitative trends in

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Figs 3b-d provide further evidence for the effect of roughness on wetting behavior. Thin

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plasma-deposited layers are known to accentuate existing roughness on surfaces due to the

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growth mechanism of such layers [42,43], while thicker layers smooth out the substrate topography (akin to snow layers on a pebble path). Therefore, the unetched sample (Fig. 3b),

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which has minimal nanoscale roughness, displays a gradual decrease in WCA as a function of PFE layer thickness, while the sample with the greatest nanoscale roughness (30 min etching,

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Fig. 3d) shows an initial jump in WCA (and decrease in sliding angle) due to the roughness

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enhancement, followed by a gradually decreasing WCA due to surface smoothing effects. The

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intermediate sample (10 min etching, Fig. 3c), with lower nanoscale roughness, even shows an increasing sliding angle after the PFE layer thickness exceeds 287 nm due to the

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disappearance of the nanoscale roughness that is needed to maintain low sliding angles. It should be noted that the inherent anisotropy of wood surfaces is maintained after etching

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and film deposition, as indicated by subtle, but significant, differences in the droplet sliding angles along and perpendicular to the grain directions; the sliding angles along the grain direction are always smaller than those across the grain. SEM images of wood surfaces etched for 30 min and then coated with different PFE thicknesses are shown in Fig. 5. With an increase in deposition time, the granular size resulting from PFE deposition increases (Fig. 5b) while the surface morphology changes to a more dense structure (Fig. 5c). It should be noted that the sample with 30 min etching and 191 nm layer thickness exhibits the most extreme wetting behavior reported to date for hydrophobically modified wood substrates without noticeably affecting the visual appearance 11

ACCEPTED MANUSCRIPT of the wood: water contact angle 161.2° ± 1.5° and sliding angles of 12.9°± 2.8°(along grain) and 17.5°±5.4° (across grain).

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To highlight the dominant role of roughness on wetting properties, superhydrophobic wood

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surfaces (WCA > 150°) were prepared by depositing hydrophilic diamond-like carbon (DLC)

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films. The water contact angle of DLC deposited on a flat silicon wafer is as low as 63.6°±0.8° (Fig. 4b); by definition, DLC is therefore regarded as hydrophilic. However, rough

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wood surfaces coated with DLC are hydrophobic. Contact angles for a 150 nm DLC film deposited on wood surfaces are plotted against the etch time in Fig. 6a. This graph reveals that

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the contact angle increases with increased etch time up to ~ 20 min and then remains constant

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at ~ 150°. It is obvious that the 20 min etched wood has a substantially higher contact angle

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(152.5°±2.8°) than unetched wood (118°±2.8°). The correlations between water contact angles, plasma deposition thickness and etch time are further demonstrated in Fig. 6b. This

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graph shows similar effects to those observed for PFE (Fig. 5): gradually decreasing WCA with increasing deposition thickness due to surface roughness reduction, but the effects are

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less pronounced than those for PFE because of the lower DLC thicknesses. SEM images of wood surfaces etched for 30 min and subsequently coated with DLC layers of different thickness are shown in Fig. 7. These micrographs indeed reveal that the packing of DLC structures on the wood surface becomes more dense and somewhat less structured on the nanoscale as deposition time increases. In spite of their high water contact angles (>150°), the superhydrophobic DLC-deposited substrates do not exhibit low adhesion. Rather non-wetting water droplets remain strongly attached to the wood surface, even if the substrate is positioned vertically. It is well-known that surface energy and surface roughness are the two factors that influence the wettability of 12

ACCEPTED MANUSCRIPT a solid surface. As shown in Fig. 4b, the DLC film is hydrophilic, while a wood surface created by O2 plasma etching and plasma deposition of a DLC film is “sticky”

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etched wood renders the substrate “roll off” superhydrophobic.

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superhydrophobic. In comparison, deposition of much more hydrophobic PFE film onto

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3.3. XPS analyses

Table II shows the surface atomic concentration for untreated, PFE-deposited and DLC-deposited wood samples. Clearly the oxygen concentration on the wood surface is

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reduced by the deposition of fluorocarbon or hydrocarbon films. Analysis showed that ~ 48%

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fluorine atoms were present on the PFE-deposited samples and the percentage of carbon atoms on DLC-deposited surfaces was close to 94%.

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Selected high resolution C1s spectra of untreated, PFE-deposited (30 min etching, 191 nm

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PFE layer thickness), and DLC-deposited (30 min etching, 100 nm DLC layer thickness)

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samples are shown in Fig. 8. Fig. 8a shows the deconvoluted XPS spectrum of untreated wood. Typical signals for lignocellulosic materials are evident in this figure: C-C or C-H at

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284.8 eV, carbon bonded to one oxygen at 286.5 eV, O–C–O and C=O in polysaccharides and keto or aldehyde groups at 287.6 eV [23,44]. Since the oxygen content of the PFE-deposited wood surface is so low (see Table II), peak deconvolution of the C1s spectrum was carried out assuming carbon to be bonded to only fluorine or CFx species. The C1s spectrum was deconvoluted into five peaks (Fig. 8b) corresponding to CF3 (294.3 eV), CF2 (292.1 eV), CF (289.4 eV), C-CFX (287.4 eV), and C-C or C-H (285.8 eV) groups on the basis of literature reports [45-47]. The XPS C1s spectra indicated that the surface of the PFE film (Fig. 8b) had -CF3, -CF2, -CF, and -CH groups present. Furthermore, film composition does not change significantly as a function of increased deposition time (Table II). The relative abundance of -C-CFx (25.8%) and other fluorocarbon species suggests that this film is highly cross-linked 13

ACCEPTED MANUSCRIPT [45,47]. A deconvoluted high resolution C1s spectrum of a 100 nm DLC film displayed only peaks

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at 284.6 eV and 285.4 eV, which are attributed to sp2 carbon (C=C) and sp3 bulk carbon (C-C)

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(Fig. 8c) [48,49]. Because of the low oxygen content formed on the wood surface from air

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contamination (Table II), (C-O) bonds were not detected by the spectral fit (Fig. 8c). This chemical characterization of the treated wood surfaces confirms that no unexpected

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contamination due to the treatments performed is detectable. 3. Conclusions

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We have demonstrated the ability to fabricate superhydrophobic wood surfaces with

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tunable wettability and water droplet adhesion by depositing either a thin hydrophobic PFE or

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a hydrophilic DLC film in combination with O2 plasma etching to create nano- and microscale roughness. The wettability was studied by water contact angle (WCA) and droplet

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sliding angle measurements. The WCA of the wood surface etched for 30 min and after coating with a ~191 nm thick PFE film was as high as 161.2° ± 1.5°; this sample displayed

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low sliding angles of 12.9°± 2.8° (along grain) and 17.5°±5.4° (across grain). High WCA values were also obtained by deposition of inherently hydrophilic DLC films on etched wood substrates. For the wood surface etched for 30 min and then coated with a ~100 nm DLC film, the WCA was 153.7°± 2.7°, but water droplets strongly adhered to this surface even when the substrate was held vertically. These observations can be explained based on the combination of the observed topography of wood surfaces after etching and film deposition, and the hydrophobic property of the deposited film. Acknowledgments This work was financially supported by the National Natural Science Foundation of China 14

ACCEPTED MANUSCRIPT (grant no. 31260159) and Yunnan Provincial Application Basic Research project (No. 2012FB166) and by the Renewable Bioproducts Institute at the Georgia Institute of

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Technology. Appreciation is extended to Professor Monlin Kuo (Iowa State University) for

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providing the wood samples and to Dr. Ashwini Sinha (Praxair) for generously donating the

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pentafluoroethane (PFE) precursor.

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[21] O. Levasseur, L. Stafford, N. Gherardi, N. Naudé, E. Beche, J. Esvan, P. Blanchet, B. Riedl, A. Sarkissian, Surf. Coat. Technol. 234 (2013) 42- 47.

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[22] R. Mahlberg, H. E. M. Niemi, F. Denes, R. M. Rowell, Int. J. Adhes. Adhes. 18 (1998) 283-297.

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[23] B. Poaty, B. Riedl, P. Blanchet, V. Blanchard, L. Stafford, Wood Sci. Technol. 47 (2013) 411– 422.

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[24] G. Toriz, M. G. Gutiérrez, V. González-Alvarez, A. Wendel, P. Gatenholm, A. D. Martínez-Gómez, J. Adhes. Sci. Technol. 22 (2008) 2059-2078. [25] P. Roach, N. J. Shirtcliffe, M. I. Newton, Soft

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[26] Y. L. Zhang, H. Xia, E. Kim, H. B. Sun, Soft Matter 8 (2012) 11217–11231.

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[27] F. Xia, L. Jiang, Adv. Mater. 20 (2008) 2842 –2858.

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[28] B. Balu, V. Breedveld, D. W. Hess, Langmuir 24 (2008) 4785 – 4790. [29] L. Li, S. Roethel, V. Breedveld, D. W. Hess, Cellulose 20 (2013) 3219 –3226. [30] L. Li, V. Breedveld, D. W. Hess, ACS Appl. Mater. Interfaces 5 (2013) 5381−5386.

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[31] S. Mayo, R. Evans, F. Chen, R. Lagerstrom, J. Phys.: Conf. Ser., 186 (2009) 12105. [32] M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir 16 (2000) 5754 – 5760. [33] B. He, N. A. Patankar, J. Lee, Langmuir 19 (2003) 4999 –5003. [34] J. P. Youngblood, T. J. McCarthy, Macromolecules 32 (1999) 6800-6806. [35] M. N. Mirvakili, S. G. Hatzikiriakos, P. Englezos, ACS Appl. Mater. Interfaces 5 (2013) 9057−9066. [36] D. Caschera, B. Cortese, A. Mezzi, M. Brucale, G. M. Ingo, G. Gigli, G. Padeletti, Langmuir 29 (2013) 2775−2783. [37] D. Caschera, A. Mezzi, L. Cerri, T. de Caro, C. Riccucci, G. M. Ingo, G. Padeletti, M. Biasiucci, G. Gigli, B. Cortese, Cellulose 21 (2014) 741–756. 17

ACCEPTED MANUSCRIPT [38] A. Jamali, P. D. Evans, Wood Sci. Technol. 45 (2011) 169 –182. [39] R. Jafari, S. Asadollahi, M. Farzaneh, Plasma Chem. Plasma Process 33 (2013) 177-200.

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[49] M. H. Ahmed, J. A. Byrne, J. A. D. McLaughlin, A. Elhissi, W. Ahmed, Appl. Surf. Sci. 273 (2013) 507-514.

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ACCEPTED MANUSCRIPT Table I: Average roughness (Sa), root mean square roughness (Sq), and peak-to-valley roughness (Sz) measurements obtained with laser scanning confocal microscopy.

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Sq (μm ± μm) 3.52 ± 0.31 4.08 ± 0.84 8.25 ± 1.19 9.47 ± 1.44

Sz (μm±μm) 80.46 ± 3.77 93.78 ± 13.88 112.24 ± 18.75 132.13 ± 5.88

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Sa (μm ± μm) 2.48 ± 0.19 2.72 ± 0.70 5.62 ± 0.72 6.26 ± 1.03

Sample Unetched Etching 10 min Etching 20 min Etching 30 min

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ACCEPTED MANUSCRIPT Table II: The XPS atomic concentrations of untreated, PFE-deposited and DLC-deposited wood substrates (wood was etched for 30 min before performing deposition treatment).

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Untreated 1 min PFE deposition 3 min PFE deposition 2 min DLC deposition 4 min DLC deposition

Stoichiometry ratio O/C F/C 0.40 0.03 0.97 0.03 0.97 0.06 0.07

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Atomic concentration (%) C O F 71.5 28.6 50.0 1.6 48.4 50.0 1.3 48.7 94.0 6.1 93.7 6.3

Wood sample

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ACCEPTED MANUSCRIPT List of figure captions Fig. 1. SEM images of wood surfaces before and after etching with an O2 plasma in the

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2.5-inch parallel-plate reactor. (a) unetched, (b) etching 10 min, (c) etching 20 min, (d)

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etching 30 min.

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Fig. 2. Laser scanning confocal microscope images of wood surfaces before and after etching with O2 plasma in the 2.5-inch parallel-plate reactor. (a) unetched, (b) etching 10 min, (c)

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etching 20 min, (d) etching 30 min.

Fig. 3. Water contact angles (left axis) and sliding angles (right axis) for wood surfaces with

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PFE films: (a) as function of etch time at fixed 382 nm (2 min deposition) PFE layer thickness;

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(b), (c) and (d) as function of PFE layer thickness for different etch times of 0, 10, 30 min,

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respectively. Black line represents static water contact angle. Blue line represents sliding angle along grain and red dotted line represents sliding angle across grain.

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Fig. 4. Water contact angles on a silicon wafer after deposition of: (a) PFE for 2 min (thickness 382 nm; θ = 103.4°±0.9°), and (b) DLC for 2 min (thickness 100 nm; θ =

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63.6°±0.8°).

Fig.5. SEM images of wood surfaces etched for 30 min followed by PFE deposition for (a) 191 nm, (b) 382 nm and (c) 573 nm. Fig. 6. Water contact angles for wood surfaces deposited with DLC: (a) as function of etch time at fixed 150 nm (3 min deposition) DLC layer thickness, (b) as function of DLC layer thickness for different etch times of 0, 10, 20, 30 min. Fig. 7. SEM images of wood surfaces etched for 30 min, followed by DLC deposition for (a) 50 nm, (b) 100 nm, (c) 150 nm and (d) 200 nm. Fig. 8. C1s XPS spectra of wood (a) untreated, (b) 30 min O2-etched and coated with 191 nm 21

ACCEPTED MANUSCRIPT PFE thicknesses and (c) 30 min O2-etched and coated with 100 nm DLC thicknesses. Black line represents raw XPS spectra. Red line represents fitted curve and dotted line represents

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ACCEPTED MANUSCRIPT Highlights • Superhydrophobic wood surfaces were created by plasma etching and plasma film

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deposition.

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• Wood roughness control by plasma etching allowed control of water droplet repellency and

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adhesion.

• Etched wood samples coated with a hydrophobic (fluorocarbon) film exhibited “roll-off”

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superhydrophobicity with low droplet sliding angles.

• Etched wood samples coated with a hydrophilic (amorphous carbon) film showed

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superhydrophobic properties, but were “sticky”.

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