Smooth and Transparent Films Showing Paradoxical Surface Properties

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Smooth and Transparent Films Showing Paradoxical Surface Properties: The Lower the Static Contact Angle, the Better the Water Sliding Performance Sohei Kaneko, Chihiro Urata, Tomoya Sato, Roland Hönes, and Atsushi Hozumi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00206 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Smooth and Transparent Films Showing Paradoxical Surface

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Properties:

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The Lower the Static Contact Angle, the Better the Water Sliding

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Performance

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Sohei Kaneko,1 Chihiro Urata,2 Tomoya Sato,2 Roland Hönes2

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and Atsushi Hozumi2,*

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1Nippon

Paint Surf Chemicals. Co., Ltd., 4–1–15 Minami-Shinagawa,

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Shinagawa, Tokyo 140–8675, Japan

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2National

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2266–98 Anagahora, Shimo–Shidami, Moriyama, Nagoya 463–8560, Japan

Institute of Advanced Industrial Science and Technology (AIST),

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*Email: [email protected]

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KEYWORDS: static contact angle ・ dynamic contact angle ・ substrate tilt

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angle・hydrophilicity・sliding property・liquid-like・drainage performance

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ABSTRACT

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Smooth and transparent hydrophilic films showing excellent water

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sliding properties were prepared by using a sol-gel solution of 2-[methoxy

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(ethyleneoxy)10propyl]trimethoxysilane and tetraethoxysilane. The resulting

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hybrid films were statically hydrophilic (static water contact angles (CAs)

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were in the range of 30°–45°), but water droplets (50 L) could move

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smoothly on an inclined surface (minimum sliding angle was 6°) without

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pinning or tailing because of low CA hysteresis (5° ± 1°).

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Thanks to this hybrid film formation on aluminum (Al) substrate,

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drainage performance during condensation and frosting/defrosting markedly

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improved, compared to hydrophilic, bare Al or hydrophobic monolayer-

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covered Al substrates.

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INTRODUCTION

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Surface wetting/dewetting properties, such as (super)hydrophilicity and

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(super)hydrophobicity, are important issues in surface science/chemistry and

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industrial applications, and have been generally estimated by the magnitude

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of static contact angles (CAs, θS). In the scientific community, we recognize

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that hydrophobic surfaces usually exhibit θS values of water above 90°, while

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hydrophilic surfaces display θS values of water below 90°.1 In particular,

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surfaces characterized by θS values of water exceeding 150° or nearly 0° are

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referred to as superhydrophobic or superhydrophilic surfaces, respectively.1

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It is commonly believed that water droplets spread out and stick to

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(super)hydrophilic surfaces because of ionic or hydrogen bonding between

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the solid surface and the water droplet.2,3 In an effort to reduce these

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interactions, (super)hydrophobic treatments using low surface energy

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materials have been applied to various substrates.4-7 Among the various

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(super)hydrophobic treatments reported so far, in order to improve liquid (in

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this study, water) sliding or removing properties, minimizing CA hysteresis

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(θ  difference between advancing (θA) and receding (θR) CAs or θcos :

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difference between cosθR and cosθA) is key.8,9

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F = m g (sin θT) = k w γLV (cos θR – cos θA) = k w γLV Δθcos

(1)

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m: the mass of the water droplet, g: the gravitational constant, θT: sliding/tilt

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angle, k : a constant that depends on droplet shape, w: the width of the water

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droplet, γLV: the liquid (water)-vapor surface tension

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According to equation (1), when water drops exhibit only weak lateral

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adhesion on an inclined surface (low CA hysteresis), a relatively low θT is 3 ACS Paragon Plus Environment

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sufficient for the gravitational force (F = m g (sin θT)) to overcome the

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adhesion force, even if the static CA is low.8,9 This indicates that when CA

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hysteresis is negligible, small activation energies lead to droplet movement.

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Thus, both water and oil droplets easily slide off of the surface at very low

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θT values without pinning, tailing, or substantial drop shape deformation;

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independent of the magnitude of static CAs.

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To realize such low CA hysteresis surfaces, “liquid-like” surfaces proposed

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by McCarthy’s and our groups10-21 are advantageous. Because of the high

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mobility of functional surface-tethered (mainly alkyl) groups, “liquid-like”

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surfaces tend to exhibit superior dynamic dewettability, on which various

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probe (polar and nonpolar, with high and low surface tension) liquids are

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able to move smoothly without pinning or tailing.10-21 For example,

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smooth/flat surfaces covered with specific-featured monolayers,10-12 low-

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molecular-weight polydimethylsiloxane (PDMS) films/brushes,13-19 silicone

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films,20 and organic/inorganic hybrid films21 exhibited “liquid-like” nature,

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showing negligible CA hysteresis and excellent liquid droplet mobility.

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These surfaces are typically hydrophobic (θS values of water generally

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exceeding 100°) and oleophilic (θS values of oils are generally below 40°) in

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a static situation in all cases,10-21 because their outermost surfaces are mainly

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covered with alkyl groups. In addition, these studies mainly used highly

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smooth and chemically reactive Si wafers.10,13-19

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In this study, our goal lies in designing smooth and transparent hydrophilic

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surfaces showing excellent water sliding properties. This may lead to the

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development of novel coatings with enhanced drainage performance, for

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example, for the fins in heat exchangers of air-conditioners (generally made

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of aluminum (Al) and Al alloys).22 In order to improve the overall heat

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exchange performance, it is necessary to prevent air flow blockage, as


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condensed water droplets will form water-bridges between the fins causing

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serious air pressure drops and water splashes.23 In an effort to avoid this,

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hydrophobic/superhydrophobic treatments on metal substrates have been

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demonstrated

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improve water drainage performance, defrosting efficiency, and frosting

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resistance, they are not suitable for improving heat exchange efficiency

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because condensed water droplets tend to form water-bridges between the

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narrow gaps of Al fins (about 1–2 mm).23 Superhydrophobic treatments28,29-

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extensively.24-34

Although

hydrophobic

treatments24-28

using nanostructures were reported to be more effective in both frosting

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prevention and defrosting, compared to hydrophobic and hydrophilic

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treatments. However, the formation of textured surfaces is rather

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complicated. Most of them generally display poor mechanical/optical

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properties, and are easily contaminated by oily substances because of their

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large surface area, in comparison to flat surfaces.12,35

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Alternatively, (super)hydrophilic treatments using plasma or hydrophilic

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polymer films have been also demonstrated.28,36-40 However, water tends to

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spread and considerable amounts of water pin to conventional

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(super)hydrophilic surfaces because of strong surface wettability and

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adhesive forces, indicating poor water sliding properties.28,41,42 If the retained

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water is not completely evaporated from the surface, it will form an ice layer

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in the next frosting period.28 Thus, the development of simple and effective

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hydrophilization methods that can provide both hydrophilicity and excellent

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water sliding properties (good drainage performance) is in high

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demand. Until now, to the best of the authors’ knowledge, investigations into

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such functional surfaces are lacking, because such surfaces are considered to

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be paradoxical per se.


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To fabricate these unusual hydrophilic surfaces showing excellent water

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sliding properties, we used a simple sol-gel solution containing hydrophilic

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pegylated organosilane (PEG-Si) and tetraethoxysilane (TEOS) as

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precursors for film deposition (we hereafter refer to this film as PEG-hybrid

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films). Papra et al. reported the formation of PEG monolayers on Si wafers

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using 2-[methoxypoly(ethyleneoxy)propyl]trimethoxysilane through a

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liquid phase process.43 The resulting monolayer-covered Si surfaces were

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hydrophilic with small dynamic CAs (θA/θR of ~38°/~34°) and low CA

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hysteresis (3–7°), but unfortunately, water sliding properties (T values) were

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not reported.43 In addition, because the thickness of the PEG-Si monolayers

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were only 1.1–1.7 nm, surface wettability was still considered to be greatly

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influenced by the substrate surface roughness and morphology, which

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restricts the applicability of monolayer based approaches to highly smooth

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Si wafers and comparable substrates. In contrast, our sol-gel PEG-hybrid

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films promise to be transparent, smooth/flat, and sufficiently thick (about

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200 nm), so that static/dynamic wetting properties of our samples will be

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independent of the surface roughness of the substrates employed. Moreover,

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water solubility, excluded volume effect, and the high mobility of PEG44, 45

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may endow flexibility to PEG chains at the surface, providing “liquid-like”

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surface properties.10-21 In spite of our transparent hybrid film surfaces

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displaying hydrophilic nature, water droplets (50 L) slid easily and cleanly

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down the surface upon slight tilting (θT < 10°). Our approach is also a facile

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one-pot sol-gel method, expected to be widely reproducible to prepare PEG-

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hybrid films over relatively large areas on various substrates with excellent

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adhesion.

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RESULTS AND DISCUSSIONS　 6 ACS Paragon Plus Environment

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Our PEG-hybrid films were prepared using a conventional co-hydrolysis

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and co-condensation method reported previously.21, 46, 47 In this study, we

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initially chose two different types of PEG-Si containing either methoxy

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(CH3O–) or hydroxy (HO–) 　 terminated PEG chains (Figure 1a, we

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hereafter refer to these two PEG-Si as PEG-M and PEG-OH, respectively).

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All precursor solutions were highly transparent. By spin coating these

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solutions, flat/smooth and highly transparent films were obtained. For

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example, glass slides and Al substrates coated with PEG-hybrid films

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retained their transparency and metallic luster, respectively (Figure 1b, in

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this case, PEG-M was used). As shown in Figure 2, atomic force microscope

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(AFM) images revealed a highly smooth and defect-free surface at a short

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length scale (3 × 3 μm2). The average root-mean-square roughness (Rrms) of

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these sample surfaces was estimated to be 0.2–0.3 nm from AFM images.

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The average thickness of PEG-hybrid films was in the range of 202–217 nm,

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and not noticeably dependent on precursor molecules, precursor

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compositions, or preparation conditions used. No peeling or cracking of the

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films was observed in a Scotch® tape peeling test and even after submersion

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in water for a few days.

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As a control experiment, PEG-M monolayers were also prepared on Si

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wafers through a liquid phase process, following literature procedures.43 We

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did not prepare PEG-OH monolayers in this study because the PEG-OH

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molecule is unstable and does not give well-ordered monolayers due to the

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spontaneous inter/intramolecular crosslinking between the terminal HO–

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groups of the PEG units and alkoxysilyl (in our present case, ethoxysilyl

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((C2H5O)3Si-) groups. The average thickness of the PEG-M monolayer was

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about 2.7 nm, as determined by ellipsometry. The Rrms of our monolayer

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samples was estimated to be 0.2 nm from the acquired AFM images (see the


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Supporting Information, Figure S1), which is in good agreement with

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reported values.29 The θS/θA/θR values of water for PEG-M monolayer-

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covered Si wafer were 42° ± 2°/45° ± 3°/37° ± 1°. The substrate θT values

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for 50 μL water drops on this surface were in the range of 5° ± 2°.

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Changes in θS values of water for our sample surfaces (coated on the glass

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slides) as a function of the molar ratio of PEG-Si and TEOS ([PEG-

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Si]/[TEOS]) were plotted together with that of the PEG-M monolayer-

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covered Si wafer (Figure 3). The TEOS-only film surface ([PEG-Si]/[TEOS]

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= 0) initially showed higher static water CA (θS = 51°) than the PEG-hybrid

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film surfaces. However, the θS value of water for the TEOS-only film surface

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was unstable because of the gradual hydrolysis of surface-exposed C2H5O–

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groups. In this study, we selected an acid-catalyzed sol-gel reaction to obtain

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stable, homogeneous, and transparent precursor solutions. It is well-known

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that some ethoxysilyl (C2H5O-Si) groups in TEOS remain unreacted in acid-

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catalyzed sol-gel reactions.48 Thus, some C2H5O-Si groups are expected to

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still exist within oligomers of hydrolyzed and condensed TEOS species in

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the precursor solution. Because of the low surface energy of C2H5O-Si

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groups, oligomers containing C2H5O-Si groups may preferentially migrate

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toward the outer surface during coating and drying steps, forming a surface

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partially covered with C2H5O-Si groups (see the Supporting Information,

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Figure S3). Indeed, the S value of the initial TEOS-only film was about 50°

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(see the Supporting Information, Figure S2), which is higher than that of Si-

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OH group-terminated surfaces (< 10°), indicating the presence of C2H5O-Si

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groups. Some of the surface C2H5O-Si groups are expected to be gradually

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hydrolyzed by immersion in water for 12 h (Figure S2), forming more

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hydrophilic Si-OH groups (Figure S3). This result suggests that the TEOS-

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only film must be initially covered partially with hydrolysable C2H5O-Si 8 ACS Paragon Plus Environment

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groups. As can be seen in Figure 3, when the [PEG-Si]/[TEOS] was in the

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range of 0.15–0.25, the θS values of water for both PEG-M and PEG-OH

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hybrid film surfaces reached their minimum values of 39° ± 1° and 32° ± 1°,

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respectively, and increased slightly going to [PEG-Si]/[TEOS] of 0.8. These

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hybrid films showed reasonable hydrolytic stability compared to the TEOS-

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only film. In a similar fashion to TEOS-only film, it is expected that

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unreacted C2H5O-Si species exist in both the precursor solutions and the

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resulting PEG-hybrid films. However, for example, even after immersion of

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our PEG-M hybrid films ([PEG-Si]/[TEOS]=0.07 and 0.15, as TEOS-rich

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samples) in water for 12 h, surface wettability barely changed, while the

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changes in S value for TEOS-only film was quite large (see the Supporting

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Information, Figure S2). Therefore, we believe that the terminal methoxy

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groups on PEG chains, unlike C2H5O-Si groups, preferentially dominate the

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outermost surface, while the residual alkoxy groups are most likely buried

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within the PEG chains, leading to appropriate stability of our PEG-M hybrid

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films in water. The resulting θS values of water for PEG-OH hybrid film

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surfaces were smaller than those on the PEG-M hybrid film surfaces because

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the terminal functional groups on the former surfaces are composed of higher

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surface energy functional groups, such as –OC2H4–OH, than those on the

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latter surfaces (–OC2H4–OCH3).

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　　As obvious from the picture of a water droplet placed on the PEG-M

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hybrid film surface ([PEG-Si]/[TEOS] = 0.15, Figure 3), our surface can be

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considered to be statically hydrophilic because θS value of water were small

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(about 40°). The critical surface tension of the PEG-M hybrid film ([PEG-

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Si]/[TEOS] = 0.15), as determined by a Zisman plot,49 was about 30.9 mN/m.

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Water (72.8 mN/m), diiodomethane (50.8 mN/m), ethylene glycol (47.9

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mN/m), and propylene glycol (35.4 mN/m) were used as probe liquids. 9 ACS Paragon Plus Environment

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However, in contrast to this static situation, the resulting film surfaces

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exhibited unusual water sliding behavior. As can be seen in Figure 4, CA

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hysteresis and θT values (50 μL water50) of PEG-M hybrid film surfaces were

4

small. They markedly decreased from 13° ± 2° ([PEG-Si]/[TEOS] = 0.8) to

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6° ± 2° ([PEG-Si]/[TEOS] = 0.15). Although the ease of movement of water

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droplet (θT values) strongly depended on the volumes on the surface of PEG-

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M hybrid film ([PEG-Si]/[TEOS] = 0.15, see the Supporting Information,

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Figure S4), overall, water droplets could slide off of our sample surfaces

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smoothly without pinning or tailing, in spite of the statically hydrophilic

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nature (see the Supporting Information, Movie S1). There were no

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significant differences in surface properties between the PEG-M monolayers

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and the PEG-M hybrid films. On the contrary, water sliding property of

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PEG-OH hybrid film surface (θT = 23˚) was inferior to that of PEG-M hybrid

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film and monolayer surfaces. This was probably due to the partial

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crosslinking of the terminal HO– groups on the PEG chains with C2H5OSi–

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groups, reducing the flexibility and mobility of the PEG chains (see the

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Supporting Information, Figure S5) and/or hydrogen bonding between the

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HO– groups and the water droplet. Thus, our PEG-M hybrid films were

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found to be capable of removing water effectively from the surfaces,

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indicating excellent drainage performance.

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By taking advantage of this unusual water sliding property of our PEG-

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hybrid films, drainage performance of our samples ([PEG-Si]/[TEOS] =

23

0.15) during frosting/defrosting experiments was investigated. Here, we

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chose perfluorinated monolayers51 prepared from perfluoroalkylsilane

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(FAS13, CF3(CF2)5CH2CH2Si(OCH3)3) as a representative hydrophobic

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surface and UV-ozone cleaned bare Al substrate (finely polished) as a non-


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sliding hydrophilic surface. The average thickness of the FAS13 monolayer

2

was estimated by ellipsometry to be about 1.0 nm.

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The static/dynamic wetting properties of our three samples are summarized

4

in Table 1. We first demonstrated water condensation and compared their

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water sliding properties through a narrow gap (1.0 mm, typical pitch of Al

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fins in heat exchangers of home-use air-conditioners.23) between the sample

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surface and the cover glass (Figure 6). The samples were maintained at 1 °C

8

to induce water condensation on their surfaces.

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As shown in Figure 5, the minimum volume of water which could slide off

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of the vertical PEG-M hybrid film- and FAS13 monolayer-formed Al

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substrates was found to be about 3 L and 8 L, respectively. They

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correspond to water droplet heights of about 0.59 mm and 1.70 mm,

13

respectively. As expected, condensed water droplets on the PEG-M hybrid

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film surface could flow down smoothly through the narrow gap without

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pinning (Figure 6b and c, see the Supporting Information, Movie S2 left)

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because of the sufficient lower CA hysteresis (5°) and height of the water

17

droplets (less than 1 mm). A continuous trail of water was clearly observed

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(Figure 6c). On the contrary, the FAS13 monolayer-covered Al substrate did

19

not exhibit good water sliding behavior, in spite of having higher water

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repellency as shown in Table 1. In this case, condensed water droplets

21

gradually accumulated to become larger droplets and then slid off of the

22

surface, but got trapped in the gap (Figure 6d-f, see the Supporting

23

Information, Movie S2 right). Based on these results, we can conclude that

24

the hydrophobic surface was not effective, but the hydrophilic surface with

25

low CA hysteresis was superior to achieve the best water sliding properties

26

through the narrow fin gap.


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Next, drainage performance under frosting/defrosting conditions on UV-

2

ozone cleaned bare Al (finely polished), PEG-M hybrid film- and FAS13

3

monolayer-covered Al substrates were compared. Figure 7 shows snapshots

4

of these three samples from the full movie (see the Supporting Information,

5

Movie S3) before testing (at 25 °C, panels a-c), during a period of frosting

6

(at –10 °C, panels d-f), after a period of defrosting (at 1 °C, panels g-i), and

7

during a period of drying (at 25 °C, panels j-l). As shown in Figure 7 and

8

Movie S3, water condensed and frosted on all surfaces during the frosting

9

step. However, the shapes of the frost formed on the FAS13 monolayer-

10

covered Al substrate (panel f) were quite different from those of the bare and

11

PEG-M hybrid film-covered Al substrates, because of the large differences

12

in absolute CAs.28 FAS13 monolayer-covered Al substrate, because of its

13

large CA hysteresis, had water pinned to its surface during the defrosting

14

step (panel i). To fully remove water, the substrate had to be heated at 25 °C

15

for at least 60 min. In contrast, on both bare and PEG-M hybrid film-coated

16

Al substrates, the frost layer was densely packed and fully covered the

17

surface (panels d and e, respectively). In addition, water droplets spread on

18

and were firmly pinned to the bare Al substrate during the defrosting step

19

(panel g). As a result, condensed water droplets were difficult to completely

20

eliminate during the drying step (panel j). Water droplets could only be

21

removed through evaporation by heating the substrate at 25 °C for at least

22

90 min, which was slightly longer than that required for the FAS13

23

monolayer-covered Al substrate. In contrast, as can be seen in panels h and

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k, our PEG-M hybrid film surface demonstrated excellent drainage

25

performance. Frost smoothly slid off of the surface once it melted (at around

26

1 °C); it did not remain on the surface and left a clean surface behind (panel

27

k, see the Supporting Information, Movie S3). These results clearly indicate


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that our hydrophilic PEG-M hybrid film surfaces, unlike hydrophobic

2

treatments, possess excellent drainage performance (in particular, through

3

the narrow gap) not only during condensation, but also during frosting and

4

defrosting.

5 6 7

CONCLUSIONS We have successfully demonstrated a simple, reproducible, and scalable

8

method to prepare smooth and transparent hydrophilic films showing

9

unusual water sliding properties via a sol-gel solution containing hydrophilic

10

polyethyleneglycol-substituted organosilane (PEG-Si) and tetraethoxysilane

11

(TEOS). Although the resulting PEG-hybrid film surfaces were hydrophilic

12

with low static water contact angles (in the range of 31–44°), water droplets

13

(50 L) could move smoothly on our surfaces without pinning or tailing

14

(minimum sliding angle was 6°). The dynamic wetting properties of our

15

PEG-hybrid films were dependent on both the identity of terminal (CH3O–

16

or HO–) groups on PEG chains, and the PEG-Si/TEOS ratios. The final

17

water sliding properties of the hybrid films prepared with CH3O-terminated

18

PEG-Si were superior to films prepared with OH-terminated PEG-Si. This

19

difference may arise from hydrogen bonding between the HO– groups on

20

PEG chains and the water droplet, as well as from partial crosslinking with

21

PEG-Si alkoxysilyl (C2H5O–Si) groups. These interactions hinder water

22

droplet contact line motion, resulting in larger θT values. Unfortunately, the

23

detailed mechanism of this unusual wetting behavior has not been clearly

24

identified, but we believe that both high mobility of PEG chains and TEOS

25

acting as a molecular spacer21 probably lead to low packing densities of the

26

PEG chains. This may help induce “liquid-like” properties to our sample

27

surfaces, resulting in low activation energy barriers for contact line


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Page 14 of 31

1

displacement, and consequently, in excellent water sliding behavior and

2

drainage performance after defrosting. There were no marked differences in

3

surface wetting properties between the PEG monolayers and our PEG-hybrid

4

films. However, thanks to the addition of TEOS, sufficiently thick,

5

continuous, and smooth hybrid films could be achieved even at room

6

temperature without specific pre-treatments. Thus, our technique is

7

applicable not only to highly smooth Si wafers and glass slides, but also to

8

other practical substrates with excellent adhesion, since TEOS acts as a

9

binder. We expect our hybrid films showing excellent water sliding

10

properties will be readily applicable not only to fins in heat exchangers of

11

air-conditioners and energy-efficient heat exchangers using supercritical

12

CO2, but also for practical anti-(bio)fouling and -corrosion coatings.

13 14

EXPERIMENTAL　SECTION

15

Materials

16

Ethanol (> 99.0%) was purchased from Amakasu Chemical Industries Co.,

17

Ltd., Japan. Toluene (> 99.0%), n-hexane (> 96.0%), 0.1M HCl were

18

purchased from FUJIFILM Wako Pure Chemicals Co., Ltd., Japan.

19

Deionized water with a resistivity of 18.2 MΩ·cm-1 (Milli-Q) was used for

20

all rinsing processes and water contact angle (CA) measurements.

21

[Hydroxy(ethyleneoxy)8-12propyl]triethoxysilane (PEG-OH, HO-(C2H4O)8-

22

12-Si(OC2H5)3,

23

tetrahydrooctyl)trimethoxysilane (FAS13, CF3(CF2)5CH2CH2Si(OCH3)3)

24

were

25

(ethyleneoxy)10propyl]trimethoxysilane,

26

Si(OCH3)3, Dynasylan 4148) was purchased from Evonik, Germany. All

27

chemicals were used as received without further purification. n-type Si (100)

50%

purchased

in from

ethanol) Gelest

and Inc.,

(tridecafluoro-1,1,2,2,USA.

(PEG-M,


2-[Methoxy

CH3O-(C2H4O)10-

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1

wafers were purchased from Shin-Etsu Handotai, Co., Ltd., Japan. Pre-

2

cleaned glass slides (S7213) were purchased from Matsunami Glass Ind. Ltd.,

3

Japan. The 100-nm-thick Al layer-deposited Si wafers were gifts from Prof.

4

Thomas J. McCarthy of Univ. of Massachusetts, Amherst. The Al layer was

5

deposited over a 30-nm-thick Ti binding layer. Al plates (A1100, purity >

6

99％) were purchased from, Nippon Testpanel Co., Ltd., Japan. They were

7

cut into 50 × 50 mm2, and then finely polished until mirror like by TDC

8

Corporation, Japan. These Al substrates were used for drainage performance

9

test.

10 11

Preparation of sol-gel hybrid films　

12

Precursor solutions were prepared by mixing PEG-Si (PEG-M or PEG-OH)

13

and TEOS in an ethanol/hydrochloric acid solution for 24 h at room

14

temperature (25 ± 2 °C, 45± 5 % relative humidity (RH)) to induce

15

hydrolysis and partial condensation. Typical molar ratios of precursor

16

solutions were 0–0.8 : 1 : 22–70 : 8–175 : 2×10-4–0.018 = PEG-

17

Si/TEOS/ethanol/water/HCl. They were spin-cast onto glass slides (24 ×

18

72 mm2), UV/ozone-cleaned Si (20 × 30 mm2), or finely polished and

19

UV/ozone-cleaned Al substrates (50 × 50 mm2) at room temperature (50 ±

20

5% RH). For example, in the case of Si wafers, 0.2 mL of precursor solution

21

was first placed on the cleaned substrate and allowed to completely spread

22

across the surface. The substrate was then spun at 1000 rpm for 5 s and 2000

23

rpm for 10 s at room temperature in ambient air. All samples were then dried

24

at 80 °C for 3 h. To remove impurities existing on the sample surfaces, the

25

dried sampled were washed with Milli-Q water for 1 min and then blown dry

26

with N2 gas.

27


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1

Preparation of PEG-M monolayers

2

As a control experiment, PEG-M (Figure 1a) monomeric layers were

3

prepared on Si substrates (20 × 40 mm2) through a liquid phase process,

4

according to a previously reported procedure.43 UV/ozone-cleaned Si

5

substrates were immersed into a solution of PEG-M (0.01 mmol/L) in

6

toluene with a catalytic amount of conc. HCl (about 0.8 mL/L) at room

7

temperature for 72 h. Afterwards, all samples were rinsed with toluene,

8

ethanol (twice), Milli-Q water (twice), in that order, to remove excessive

9

physisorbed PEG-M derived species, and then dried with N2 gas. Atomic

10

concentrations corresponding to C, O, and Si of the PEG-M-monolayer-

11

covered Si surface were determined by XPS to be 33.4, 51.6, and 15.0 at%,

12

respectively.

13 14

Preparation of perfluoroalkylsilane monolayers

15

Perfluoroalkylsilane (FAS13) monomeric layers were prepared by a

16

chemical vapor deposition (CVD) method, according to our previous

17

report.51 Finely polished Al substrates (50 × 50 mm2) or Al-deposited Si

18

substrates (10 × 10 mm2) were first rinsed with n-hexane and Milli-Q water

19

to remove contaminants. Next, they were photochemically cleaned by using

20

UV/ozone treatment so as to be completely hydrophilic with a water θS value

21

of 5° or less. After this cleaning, each of the substrates was immediately

22

placed into a Teflon® container (60 cm3), together with a small glass vial

23

containing 0.2 cm3 of FAS13, inside a glove box filled with dry N2 gas

24

(relative humidity of less than 5%). The container was sealed with a cap and

25

then heated for 24 h at 120 °C in an oven. After this period, excessively

26

physisorbed FAS13 molecules were removed from the substrate surfaces by

27

sonication in n-hexane for 30 min, rinsed with Milli-Q water, and then blown


Page 16 of 31

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1

dry with N2 gas. Atomic concentrations corresponding to C, O, F, Si and Al

2

of the FAS13-monolayer-covered Al substrates were determined by XPS to

3

be 15.5, 45.2, 20.9, 2.24, and 16.1 at%, respectively.

4 5

Drainage tests

6

Two different types of drainage tests were demonstrated using a Peltier

7

cooler (20 × 20 cm2), which was vertically placed as shown in Figure 6.

8 9

Test A: First, narrow slits (1.0 mm gap) were prepared on the top of the

10

sample surfaces. In this trial, PEG-M hybrid film- ([PEG-Si]/[TEOS] =0.15)

11

and FAS13 monolayer-covered Al (finely polished) substrates were

12

employed. They were fixed to the Peltier cooler and then cooled to 1 °C

13

within 1 min, while the surrounding temperature was kept at 25 °C with a

14

relative humidity of 100 % (to accelerate the test). These conditions were

15

maintained for 2 h.

16 17

Test B: Three samples (UV/ozone-cleaned bare, PEG-M hybrid film- ([PEG-

18

Si]/[TEOS] =0.15), and FAS13 monolayer-covered finely polished Al

19

substrates) were fixed to the Peltier cooler. During the cooling step, the

20

surface temperature reached –10 °C within a few min. The surface

21

temperature was then maintained at –10 °C for 30 min to induce frosting.

22

After the frosting step, the surface temperature was increased to 25 °C at a

23

rate of 0.5 °C/min. Finally, the temperature was maintained at 25 °C for 2 h.

24 25

Characterization

26

Water contact angles (CAs) were measured using a DropMaster DM-501

27

(Kyowa Interface Science Co., Ltd., Japan) and the software (FAMAS,


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1

version 3.7.2), equipped with an electric syringe pump (AD300). For the

2

static water CA (θS) measurements, 1–10 L water droplets were used. In

3

the case of dynamic water CA measurements, 10 L water droplets were first

4

placed on the sample surface (the needle of the dosing syringe was immersed

5

into the droplet). Next, water was gradually added until the three-phase

6

contact line of the water droplet had noticeably moved all, while capturing

7

droplet images. The advancing CA (θA) was determined by analyzing the

8

droplet shape, via instrument software, when the three-phase contact line

9

began to move. The receding CA (θR) was determined analogously by

10

removing water from the droplet. Static/dynamic water CAs were gathered

11

at three different points on each sample. The θS/θA/θR values reported here

12

are the averages of the three measured values. Sliding/tilt angles (θT) were

13

measured using pre-deposited 50 L water droplets.50 The sample substrates

14

were automatically tilted until the droplet started to slide off. The surface

15

morphologies of the samples were observed by atomic force microscopy

16

(AFM, VN-8000, Keyence, Japan) in tapping mode using standard

17

cantilevers (OP-75041). Film thicknesses of our PEG-hybrid films were

18

measured using the spectroscopic mode of a confocal microscope

19

(OPTELICS® HYBRID, Lasertec Corporation, Japan). The error in

20

thickness determination for an identical sample was about ±5 nm. The

21

thicknesses of our monomeric layers were also determined using

22

spectroscopic ellipsometry (M-2000, J. A. Woollam Co., United States).

23

Measurements were conducted at incidence angles of 50, 60, 70 and 80,

24

and wavelengths λ = 250–1700 nm. Film thicknesses were calculated using

25

optical constants and modelling available in the device software. The error

26

in thickness determination for an identical sample was about ± 0.25 nm.

27

Surface chemical compositions of our samples were studied using X-ray 18 ACS Paragon Plus Environment

Page 18 of 31

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1

photoelectron spectroscopy (XPS). Spectra were acquired using a Physical

2

Electronics Quantum 2000 spectrometer with a 200 m spot size and

3

monochromatic Al K radiation (1486.68 eV). The X-ray source was

4

operated at 50 W and 15 kV with the analyzer's constant pass energy at 29.35

5

eV. The pressure in the analysis chamber was about 6 × 10–6 Pa during

6

measurements. Core-level signals were obtained at photoelectron take-off

7

angle of 90°. The binding energy (BE) scales were referenced to 284.6 eV

8

as determined by the locations of the maximum peaks on the C1s spectra

9

corresponding to hydrocarbon (CHx).

10 11

ASSOCIATED CONTENT

12

Supporting Information

13

The Supporting Information is available free of charge on the ACS

14

Publications website at DOI:

15

AFM images; static water contact angles and contact angle hysteresis data

16

after immersion in water; schematic illustrations of sample surfaces; sliding

17

angle data acquired at different water droplet volumes (DOCX)

18

Movie S1 (QT)

19

Movie S2 (QT)

20

Movie S3 (QT)

21 22

AUTHOR INFORMATION

23

Corresponding Author

24

*E-mail: [email protected]

25 26

ORCID/Research ID

27

Atsushi Hozumi: 0000-0003-4375-3785/L-9357-2018 19 ACS Paragon Plus Environment

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Page 20 of 31

1

NOTES

2

The authors declare no competing financial interest.

3 4 5

ACKNOWLEDGMENTS The

authors

would

like

to

express

their

gratitude

to

6

Dr. Hiroshi Kakiuchida of AIST, and Dr. Makoto Yagihashi of Nagoya

7

Municipal Industrial Research Institute, for their technical assistance.

8 9 10 11

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Figure 1. Schematic illustration of this study. (a) Molecular structures of two different types of PEG-silanes (PEG-Si) used in this study. (b) Coating process and appearances of samples. Inset images show i) PEG-M and ii) PEG-OH hybrid films formed on glass slides (24 × 72 mm2), and ⅲ) PEG-M hybrid film formed on an aluminum (Al) substrate (50 × 50 mm2).

Figure 2. Typical AFM images of (a) PEG-M and (b) PEG-OH hybrid films formed on Si wafers (20 × 30 mm2). The scanned area was 3 μm x 3 μm.

Static water contact angle (θS , ° )

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● ● ● ●

TEOS-only film PEG-M hybrid film PEG-OH hybrid film PEG-M monolayer

[PEG]/[TEOS]


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Figure 3. Changes in static water contact angles (CAs) of four different types of sample surfaces as a function of molar ratio of PEG-Si and TEOS ([PEG-Si]/[TEOS]). Inset image is a water drop profile (3 μL) on PEG-M hybrid film surface ([PEGSi]/[TEOS]=0.15). (b)

● ● ● ●

TEOS-only film PEG-M hybrid film PEG-OH hybrid film PEG-M monolayer

[PEG]/[TEOS]

(c)

Sliding angle ( θT, ° )

(a)

Contact angle hysteresis (˚)

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Dynamic water contact angle (˚)

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[PEG]/[TEOS]

[PEG]/[TEOS]

Figure 4. Changes in (a) dynamic water CAs (θA: solid circles, θR: open circles), (b) CA hysteresis (Δθ: solid circles), and (c) sliding angles (θT, 50 μL water) of four different types of sample surfaces as a function of [PEG-Si]/[TEOS] ratio. Table 1. Static/dynamic wetting properties of Al substrates with and without hydrophilic or hydrophobic coating for drainage tests.


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Figure 5. Correlation of volume of water droplet with height of water droplet placed on PEG-M hybrid film ([PEG-Si]/[TEOS]=0.15) and FAS13 monolayer formed on Al substrates. Red arrows indicate the minimum volume of water required for the droplet to slide off of the vertically inclined surfaces.

Figure 6. Sliding behaviors of condensed water droplets on PEG-M hybrid ([PEGSi]/[TEOS]=0.15) and FAS13 monolayer formed on finely polished Al (50 × 50 mm2) substrates at 1 °C. The samples were firmly attached to an upright Peltier cooler (tilt angle of 90°). (a) Appearances of test set up just after cooling, (b) 20 min, (c) 22 min, (d) 60 min, (e) 62 min, and (f) after 120 min.


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Figure 7. Frosting/defrosting behaviors of UV-ozone cleaned bare (a, d, g, j), PEG-M hybrid ([PEG-Si]/[TEOS]=0.15, b, e, h, k), and FAS13 monolayer formed on finely polished Al (50 × 50 mm2) substrates (c, f, i, l)). The samples were firmly attached to an upright Peltier cooler (tilt angle of 90°). (a, b, c) Before test, (d, e, f) frosting step, (g, h, i) defrosting step, and (j, k, l) drying step.


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Smooth and Transparent Films Showing Paradoxical Surface Properties

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