Highly durable superhydrophobic coatings with gradient density by movable spraymethodMizuki Tenjimbayashi and Seimei Shiratori Citation: Journal of Applied Physics 116, 114310 (2014); doi: 10.1063/1.4895777 View online: http://dx.doi.org/10.1063/1.4895777 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spray-coating process in preparing PTFE-PPS composite super-hydrophobic coating AIP Advances 4, 031327 (2014); 10.1063/1.4868377 Superhydrophobicity on transparent fluorinated ethylene propylene films with nano-protrusion morphology by Ar+O2 plasma etching: Study of the degradation in hydrophobicity after exposure to the environment J. Appl. Phys. 114, 164307 (2013); 10.1063/1.4826897 Mechanically durable superhydrophobic surfaces prepared by abrading J. Appl. Phys. 114, 124902 (2013); 10.1063/1.4822028 Experimental study of skin friction drag reduction on superhydrophobic flat plates in high Reynolds numberboundary layer flow Phys. Fluids 25, 025103 (2013); 10.1063/1.4791602 Mechanisms for hydrophilic/hydrophobic wetting transitions on cellulose cotton fibers coated using Al2O3 atomiclayer deposition J. Vac. Sci. Technol. A 30, 01A163 (2012); 10.1116/1.3671942

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Highly durable superhydrophobic coatings with gradient densityby movable spray method

Mizuki Tenjimbayashia),b) and Seimei Shiratoria),c)

School of Integrated Design Engineering Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama,Kanagawa 223-8522, Japan

(Received 4 August 2014; accepted 4 September 2014; published online 18 September 2014)

Superhydrophobic surface is expected to be applied in anti-fouling, anti-icing, and anti-bacterial.

However, practical use is interrupted by low mechanical strength, time-consuming process, and

limited coating substrate. Here highly durable superhydrophobic coatings were prepared by simple

and novel spraying method, which sprays with changing the “spray distance between substrate

and spray” (SD), named “movable spray method.” We prepared the solution that changes wettabil-

ity and durability with spraying distance by mixing SiO2 nanoparticles and ethyl alpha cyanoacry-

late polymer (EAC). Then, we evaluated the chemical components and surface morphologies of

each spraying distance coatings (0� 50 cm) by XPS, SEM, and laser scanning microscope. It

revealed that surface roughness and SiO2/EAC ratio increased as the SD increases. Thus, durable

superhydrophobic coatings were designed by spraying with increasing SD gradually. Glow

discharge-optical emission spectrometry analysis revealed that designed coatings showed the

gradual increase of SiO2/EAC ratio. As a result, coatings prepared on glass, wood, or alumi-

num substrates maintained their superhydrophobicity up to the abrasion at 40 kPa. This mova-

ble spray method is simple coating by the wet process and prepares robust hydrophobic

coating on complex shape and large area substrates. The gradient functional surface was found

to have mechanical durability and superhydrophobicity, and wide area applications will be

expected. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4895777]

I. INTRODUCTION

The emerging field of biomimetics to mimic biology or

nature is getting a lot attention to develop nanomaterials, nano-

devices, and processes which provide desirable properties.

Liquid repellent surface which mimics the leg of water slider,

butterfly wings, and lotus leaves is called superhydrophobic-

ity.1–3 Much research has been devoted to superhydrophobic-

ity, in which apparent contact angles with water of >150� and

low sliding angles can be obtained. Superhydrophobicity is

applied in anti-fouling, anti-icing, anti-bacterial, anti-fogging,

and flow enhancement applications.4–12 The laboratory fabrica-

tion of superhydrophobic surfaces was inspired by the mor-

phology of lotus leaves1–3 and requires low surface energy and

roughness to achieve enhanced water repellency.13 The rela-

tionship between surface energy and water repellency is given

by Young’s equation,14 which concerns surface energy and liq-

uid surface energy. A lower surface energy results in greater

water repellency. Fluorinated hydrocarbons (cSV ¼ 6 mN/m)

are often used to prepare low surface energy materials.

However, fluorine components are high cost, easily react with

other material, and the environmental exposure of fluorine

components reportedly impedes nerve growth in children.15

Therefore, it is desirable to fabricate superhydrophobic surfa-

ces without using fluorine.16 Achieving this, tends to be more

difficult than when using fluorine components, and reports of

high water repellency in the absence of fluorinated materials

are very few. The relationship between roughness and water

repellency is given by the Wenzel and Cassie and Baxter mod-

els.17 Wenzel’s model involves the droplet completely contact-

ing with a rough surface, and shows that the roughness

enhances wettability. Cassie and Baxter’s model involves the

droplet contacting with a mixed material surface, in which air

is trapped by the roughness. Distinguishing between the two

models involves determining whether the surface is sufficiently

rough to trap air between the liquid and surface. Cassie and

Baxter’s model conditions result in higher water repellency,

because of the small contact area between liquid and surface.18

Therefore, high roughness, enough to trap air is required for

high water repellency. However, such superhydrophobic surfa-

ces have low mechanical strength, and easily lose their water

repellency because of the surface rough structure. Various

approaches for preparing mechanically durable superhydro-

phobic coatings have been reported.19–22 Wang et al. reported

that a polytetrafluoroethylene/polyvinylidene fluoride compos-

ite passed the 200 times abrasion test with a gloved hand at

10 kPa.20 Zou et al. prepared epoxy coatings on cotton fabrics,

which passed the 1000 times abrasion test with sand paper at

200 Pa.23 However, most reports require overly strong sub-

strates or more time. Mechanical durability on various sub-

strates and a facile coating process are required for industrial

applications. Maintaining the nanostructure is important for

ensuring a robust film. Forming an intermediate layer is also

essential for increasing the adhesion of films to various sub-

strates.24 Thus, two layers are required on substrate, which is

a)Electronic addresses: soccer.tentuku.in13@gmail.com and

shiratori@appi.keio.ac.jpb)M.T. conceived, designed and carried out the experiments, analyzed the

data, and wrote the paper.c)S.S. gave scientific advice and commented on the manuscript.

0021-8979/2014/116(11)/114310/7/$30.00 VC 2014 AIP Publishing LLC116, 114310-1

JOURNAL OF APPLIED PHYSICS 116, 114310 (2014)

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hydrophobic nanoparticles as upper layer, and an adhesive

resin polymer as lower layer. These two layers ideally require

the structure that adhesive resin polymer layer gradually shifts

to hydrophobic nanoparticles layer (that is the ratio of poly-

mer/nanoparticles gradually changes). To the best of our

knowledge, no reports showed the fabrication method of gradi-

ent structure and discussed the relationship of surface compo-

nent, morphology, and wettability with 3 phases (i.e.,

nanoparticle, resin, and air) in authors’ surveys. Recently, our

laboratory reported superhydrophobic coatings with hierarchi-

cal structure by spraying hydrophobic nanoparticle dispersion

to evaporation of solvent by self-assembly.25 Here, mixing

resin to the nanoparticle dispersion and spraying control may

be able to fabricate gradient density with robust hydrophobic

coating. Thus, in this article, Ethyl-alpha-cyanoacrylate (EAC)

is chosen as adhesive resin, which instantly polymerizes in the

air when spraying. And by changing the spraying distance

between spray and substrate (SD), we evaluated the adhesion

and deposition process, coated surface component, morphol-

ogy, and wettability relationship of hydrophobic nanoparticles

and EAC. In addition, we constructed the “movable spray

method” to fabricate the robust hydrophobic coating with gra-

dient density (hydrophobic nanoparticles and EAC) by contin-

uous increase of SD. This movable spray method is simple

coating by the wet process; prepares robust hydrophobic coat-

ing on complex shape and large area substrates. Recently,

Layer-by-Layer coating method gets much attention in indus-

try because of the same property.26 Compared with this

method, our coating method can save the time much shorter,

and it can be applied for wide applications (such as auto mo-

bile, bone adhesive,27 and finger print kits28), because EAC is

versatile adhesive with durability29,30 and our fabrication was

biocompatible material and involved eco-friendly materials.31

II. EXPERIMENTAL SECTION

A. Materials

EAC (purity: 99.5%) was purchased from Kobunshi

Shouji Co. Ltd., Japan. SiO2 nanoparticles (average particle

diameter: �12 nm) modified with hexamethyldisilazane

were purchased from Aerosil RX200, Evonik Industries,

Germany. These materials were selected because of their

biocompatibility. Glass (Length 76�Width 26�Thickness

1 mm, Matsunami Glass Industries, Japan); wood (Agathis,

Length 600�Width 100�Thickness 5 mm, Simachu Co.,

Ltd., Japan); and aluminum (HAO0313, Length 300�Width

100�Thickness 0.3 mm, Hikari Co., Ltd., Japan) were used

as substrates.

B. Adjustment of spraying solution

The spraying mixture was obtained by ultrasonically

dispersing 3.7 wt. % of SiO2 nanoparticles in acetone for

10 min. 11.1 wt. % of EAC was added and the dispersion

was stirred for 3 h.

C. Spraying conditions

Spraying conditions were determined using referring

conditions.32 The spraying pressure of a sprayer (Airtex,

Japan, nozzle diameter: 0.6 mm) was 0.7 MPa, and the

amount of sprayed mixture was 5 ml. The distance between

the SD was varied. Surfaces were coated by spraying at dis-

tances of up to 50 cm. A sample where the spray mixture

was casted on the substrate was also prepared for comparison

and defined as 0 cm of SD.

D. Characterization

X-ray photoelectron spectroscopy (XPS, JPS-9010TR,

JEOL, Japan) was used to investigate the chemical composi-

tion of the surface. Laser was used for this analysis. Surface

structures were observed using a Color 3D laser scanning

microscope (VK-9710, Keyence, Japan) and field-emission

scanning electron microscopy (SEM, S4700, Hitachi, Japan).

Surface wettability was determined by measuring the contact

angle with water, using a device created from LabView.

Measurements were made using the sessile drop method

with �10 ll droplets of deionized water (Aquarius GS-

500.CPW, Advantec, Japan, 18.2 MX). Surface mechanical

durability was observed using an abrasion device (Tribogear

Type 18L, Shinto Scientific Co., Ltd., Japan). Cotton fabrics

were used as the abrasive material. Each test consisted of 10

rounds at variable pressure. Chemical components were

measured by glow discharge optical emission spectroscopy

(GD-OES, GDProfiler2, HORIBA Scientific Co., Ltd.,

France).

E. Movable spray

When movable spraying method was used, the spray dis-

tance was gradually changed to allow the gradient density to

be prepared. We characterized surfaces prepared at each

spray distance, and then assessed the suitability of the prepa-

ration method for preparing gradient density.

III. RESULTS AND DISCUSSION

A. Surface analysis with spray distance

The spray distance affected the resulting surface coat-

ings composition and morphology. Fig. 1(a) shows the XPS

analysis of coatings prepared at each spray distance. Each

sample showed 4 characteristic peaks. Si-2p peak was

observed in 97 eV (In theory 99 eV) of binding energy and

Si-2s peak was observed in 157 eV (In theory 151 eV) from

SiO2 nanoparticles. C-1s peak was observed in 288 eV (In

theory 285 eV) from trimethylsilyl groups modifying SiO2

nanoparticles. N-1s peak was observed in 402 eV (In theory

398 eV) from EAC. Magnified images of XPS analysis are

shown in Figures 1(b) and 1(c). It is noteworthy that Si and

N peaks were observed, since the surface consisted of SiO2

nanoparticles and EAC which contains –C�N functionality.

There were no changes in binding energies between different

distances, and therefore no differences in chemical binding.

The Si and N XPS peak intensities increased with increasing

spray distance, because of the increase in surface roughness

and XPS scanning area. The Si/N surface ratio was calcu-

lated and is shown in Figure 1(b). The Si/N ratio increased

from 0.77 to 1.62 with increasing spray distance from 0 cm

to 30 cm, reflecting an increase in surface nanoparticle

114310-2 M. Tenjimbayashi and S. Shiratori J. Appl. Phys. 116, 114310 (2014)

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content. In the spray distance from 30 cm to 50 cm, the Si/N

ratio was almost constant. That is probably because all sol-

vency of spraying mixture evaporated when arriving sub-

strate (when spray distance> 30 cm) and mixture reaction

ended, resulting in almost same content mixture piles when

spraying distance is more than 30 cm. The morphologies of

particles in coatings prepared at increasing spray distances

are shown by the SEM images in Figs. 2(a)–2(c) and

2(a0)–2(c0). Root mean square roughness (Rrms) values

increased from 1.097 lm to 20.256 lm with increasing spray

distance, as shown by the laser scanning microscope images

in Figs. 2(d)–2(f). The change in spraying mixture is shown

schematically in Fig. 2(g), and is used to explain the

observed morphologies and chemical changes. The mixture

consisted of acetone, EAC monomer, and SiO2 nanopar-

ticles. EAC initially dissolved in acetone, and both acetone

and part of EAC evaporated to a greater degree with increas-

ing spray distance. We carried out the EAC evaporation test,

in which EAC dissolved in acetone heated 24 h at 50 �C and

then, compared the weight of EAC, resulting less than 1% of

EAC evaporate with acetone. Hence, the mixture aggregated

by surface tension caused by acetone evaporation, and

almost all of EAC enter into the mixture with acetone.

Acetone evaporation promoted a decrease in surface EAC

content and an increase in surface nanoparticle content. EAC

polymerized and solidified upon complete acetone

FIG. 1. (a) XPS analysis of coatings

prepared at various spray distances. Si

and N peaks were analyzed. (b) and (c)

Magnified images of XPS analysis. (d)

Spray distance versus surface Si/N ra-

tio, as analyzed by XPS. Higher Si/N

ratios indicate higher surface nanopar-

ticle occupancy.

FIG. 2. SEM images of the top views

of coatings prepared at spray distances

of (a) and (a0) 10; (b) and (b0) 20; and

(c) and (c0) 30 cm. Laser microscope

images of the surface structure of coat-

ings prepared at spray distances of (d)

10, (e) 20, and (f) 30 cm. (g)

Schematic of the spraying process. The

sprayed mixture changed with increas-

ing spray distance as indicated by

arrows. Evaporation of acetone cause

the aggregation of EAC and polymer-

ization of EAC is accelerated.

114310-3 M. Tenjimbayashi and S. Shiratori J. Appl. Phys. 116, 114310 (2014)

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evaporation. Acetone evaporated to a lesser degree at a spray

distance of 10 cm, as shown in Figs. 2(a) and 2(d). Thus,

most of the sprayed solution consisted of acetone. Little resin

or nanoparticles were present, and a very flat liquid surface

formed on the substrate. Most acetone evaporated at a spray

distance is 30 cm, as shown in Figs. 2(c) and 2(f). Thus,

solid-to-solid adhesion occurred, because EAC polymerized

during spraying and acetone evaporation. A rough surface

was formed, and the ratio of EAC monomer may be

decreased. Therefore, the surface roughness and nanoparticle

surface content increased, and the adhesion area decreased,

with increasing spraying distance. Fig. 3(a) shows the wett-

ability and durability of coatings prepared at each spray dis-

tance. Water contact angle increased and durability

decreased with increasing spray distance. Durability was

quantified by the decreasing rate of contact angle after abra-

sion test. Surface modeling and simulation were used to

interpret the surface wettability and durability results. The

Cassie-Baxter and Wenzel models are applicable to struc-

tures consisting of two materials (i.e., air and another mate-

rial).17,33 The current system consisted of three materials

(i.e., EAC, nanoparticles, and air). Therefore, we used the

Cassie-Baxter and Wenzel models and adjusted our surface.

To make the model for our system, the Cassie-Baxter equa-

tion was derived for mixed substrates containing air, EAC,

and SiO2 nanoparticles. The Cassie-Baxter equation is

cos hCB ¼ f1 cos h1 þ f2 cos h2 ðf1 þ f2 ¼ 1Þ; (1)

where hCB is the apparent contact angle, f is the surface ratio

of the materials on the surface indicated in subscript, and his the flat contact angle of the relevant material. Next, we

discuss the conditions when material 2 consists of materials

3 and 4, as shown in Fig. 3(a).

The flat contact angle of material 2 is

cos h2 ¼ f3 cos h3 þ f4 cos h4 ðf3 þ f4 ¼ 1Þ: (2)

Substituting Eq. (2) for Eq. (1) yields

cos hCB ¼ f1 cos h1 þ f2f3 cos h3 þ f2f4 cos h4

ðf1 þ f2 ¼ 1; f3 þ f4 ¼ 1Þ: (3)

Rearranging Eq. (3) yields

cos hCB ¼ f1 cos h1 þ f2 cos h2 þ f3 cos h3 ðf1 þ f2 þ f3 ¼ 1Þ:(4)

The flat contact angle of EAC, SiO2 nanoparticles, and air

was 52� (as measured by us), 99�,and 180�, respectively.34

Substituting these values into Eq. (4) yields

cos hCB ¼ fEAC cos 52� þ fparticle cos 99� þ fair cos 180�

ffi 0:616� 1:616fair � 0:772fparticle

ðfair þ fparticle � 1; fEAC ¼ 1� fair � fparticleÞ:(5)

The Wenzel equation is shown in Fig. 3(b).

The flat contact angle of the EAC and SiO2 nanoparticle

mixture obtained by substituting Eq. (1) is

cos hMixture ¼ fEAC cos 52� þ fparticle cos 99�

ðfEAC þ fparticle ¼ 1Þ: (6)

The Wenzel equation is shown in Eq. (7)

cos hW ¼ r cos hf lat ðr 1Þ; (7)

where r is the surface roughness factor. Substituting Eq. (6)

for Eq. (7) yields

cos hW ¼ rðfEAC cos 52� þ fparticle cos 99�Þffi rð0:7721fEAC � 0:1564Þðr > 1; fEAC þ fparticle ¼ 1Þ: (8)

Equations (5) and (8) were used to simulate the relationship

of contact angle and surface structure, as shown in Figs. 4(b)

and 4(c). The contact angle of region I was �52�, which our

experiments indicated was comparable with the flat contact

angle of polymerized EAC. Fig. 1(b) shows that the ratio of

Si/N was 0.77, indicating that SiO2 nanoparticle is embedded

under EAC polymer layer and that thickness is very thin

(probably 5–10 nm), because the escape depth of MGKalaser is about 5–10 nm. The contact angle in region II was

higher than that in region I. Between regions I and II, an

increase in r resulted in a decrease in contact angle, as shown

in Fig. 4(b), since the surface flat contact angle is hydrophilic

(water contact angle< 90�). Therefore, nanoparticles existed

on the surface, and the simulation in Fig. 4(b) indicated a

nanoparticle surface occupancy of �60%–80% in region II.

The structure was modeled as shown in Fig. 4(d). The con-

tact angle at region III was much higher than that at region

II, perhaps because of the transfer between the Cassie-BaxterFIG. 3. (a) Cassie-Baxter model for three-component mixtures. (b) Wenzel

model for three-component mixtures.

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and Wenzel states. Hence, we conducted transfer experi-

ments to check if they were consistent with Cassie-Baxter or

Wenzel behavior.35 Regions II and III were in Wenzel and

Cassie-Baxter states, respectively, as shown in Fig. 5. The

reason for the transfer from Wenzel to Cassie-Baxter state

was the significant increase in fair. Region III contained

75%–90% air, while region II contained 0%–40%, if it can

be applied to the Cassie-Baxter model, as shown in Fig. 4(c).

The reason for the significant increase in fair was the increase

in surface particles rate and forms hierarchical structure as

shown in Fig. 2(b). Numerous SiO2 nanoparticles were

exposed on the surface because they had aggregated upon

the shrinking of the sprayed droplets, and formed surface

nano/micro structures.25 Figs. 2(b) and 3(e) show that EAC

and SiO2 nanoparticles were inter-dispersed. Adhesion

between SiO2 nanoparticles decreased in region IV because

of the decreased EAC monomer content shown in Figs. 2(c)

and 3(f).

B. Designing gradient density structure by movablespray method

Variable distance spraying was used to prepare the dura-

ble water repellent structure. Changing the spraying distance

FIG. 4. (a) Contact angle versus spray distance. The contact angle before abrasion indicates wettability, and a decreasing contact angle after abrasion indicates

durability. (b) and (c) Graphs produced from Eqs. (1) and (2), respectively. Regions I–IV indicates contact angle areas shown in (a). (d)–(f) Models of struc-

tures for regions II–IV.

FIG. 5. Cassie-Wenzel transition observations. Transition from Cassie and

Wenzel states was observed at spray distances of 10 and 20 cm. The water

droplet remained on, and was repelled from the substrate in the former and

latter spray distances, respectively. In the latter, the droplet remained

attached to the syringe, despite being pressed against the substrate.

114310-5 M. Tenjimbayashi and S. Shiratori J. Appl. Phys. 116, 114310 (2014)

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has changed the morphology of the coating on the substrate.

The spray distance was gradually changed from 10 to 50 cm.

We named this spraying method as “Movable spraying meth-

od.” Closer spraying formed the adhesion layer, where EAC

adhesion part is comparably occupied, and more distant

spraying formed the water repellent layer, where adhesive

EAC layer decreases and SiO2 nanoparticles pile up on the

surface. After that, the piling layer that consisted of only

SiO2 nanoparticles and had little adhesion strength was

removed and SiO2 nanoparticles EAC bonded were

remained, as shown in Fig. 6. Fig. 7(a) shows GD-OES anal-

ysis. The amount of EAC is in proportion to N peak and the

amount of SiO2 nanoparticles is in proportion to Si peak.

Etching time (ET) is in proportion to the distance from sur-

face to substrate (Surface depth). N peak is observed when

ET¼ 24.8 s. In the region of ET¼ 0� 24.8 s, N intensity

gradually increases with etching time. Small Si peak was

observed when ET¼ 2.8 s, and gradually decreased as the

increase of ET (>2.8 s). The surface structure is hierarchical

structure with SiO2 nanoparticles as shown in Fig. 2(c) and

gradually the density of SiO2 nanoparticle increases in the

region of ET¼ 0� 2.8 s. And in the region of

ET¼ 2.8� 24.8 s, the density of EAC increases and that of

SiO2 nanoparticle decreases. This surface is the heterogene-

ously mixed structure of nanoparticle and resin; hence, we

cannot estimate the etching rate (¼Etching thickness/

Etched time (m/s)). However, in the region of ET¼�30.0 s,

it is observed that Al peak becomes constant, and both Si

and N peak gradually decrease with ET. The decrease of Si

and N peak indicates that both EAC and SiO2 nanoparticles

in the interface of substrate and coating are etched. And the

ET that Si and N peaks become no intense and Al constant

the time of all coating is etched and Al is exposed on the sur-

face. Therefore, the region of ET¼ 0� 24.8 s is the

trustworthy area to check the coating structure tendency,

and, in this area, ET can be replaced with surface depth of

coating. Fig. 7(b) shows the relationship with integrated sur-

face depth and components peak of the coating. The outer-

most surface is filled with SiO2 nanoparticles and little EAC.

And gradually the density of SiO2 nanoparticles decreases

and that of EAC increases while entering into the substrate.

From this data, the gradient density structured thin film with

SiO2 nanoparticles and EAC was confirmed to be fabricated.

Cross-sectional SEM imaging (Fig. 8) and abrasion testing

on aluminum, wood, and glass substrates (Fig. 9) were used

to characterize the variable distance sprayed coatings. Fig. 8

shows surfaces filled with nanoparticles, showing that the

surface is consisted of nanoparticles forming 10 lm rough-

ness. And the middle layer is filled with nanoparticle/EAC

mixture. The adhesion layer contained nanoparticles, and it

is the difference with the adhesion layer without SiO2 nano-

particles. Adding SiO2 nanoparticles in acetone/EAC

enhanced thermal stability and prevented EAC evaporation36

and intermolecular forces37 resulting in the enhancement of

the mechanical durability. Fig. 9 shows that coatings retained

their superhydrophobicity after abrasion testing at 40 kPa.

(Abrasion performances are shown in supplementary mate-

rial.38) Coatings consisting of only nanoparticles lost their

superhydrophobicity upon testing at 10 kPa. There were no

differences in performance between the three substrates.

FIG. 6. Preparation of the high durability superhydrophobic surface. The surface was prepared by movable spraying.

FIG. 7. It shows GD-OES analysis of coating by movable spray method on

the Al substrate. Etching time is in proportion to the distance from surface to

substrate. (a) Intensity versus Etching time. (b) Normalized NPs and EAC

intensity versus Surface depth.

FIG. 8. SEM images of (a) the tilting direction, where the dashed line indi-

cates the border between the top and side of cross section of coatings: (b)

top view, and (c) side view.

114310-6 M. Tenjimbayashi and S. Shiratori J. Appl. Phys. 116, 114310 (2014)

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While the substrate surface roughness may have differed,

spraying at short distance effectively rendered them all flat.

IV. CONCLUSIONS

The morphology and chemical composition of a mixture

of EAC, SiO2 nanoparticles, and acetone changed with

increasing spray distance and were analyzed by arranging

Cassie-Baxter and Wenzel models. These models are appli-

cable to the analysis of structures consisting of two mixed

materials. A method for preparing gradient density layered

structures was established using one-step spraying. This fac-

ile process was biocompatible and involved eco-friendly

materials. Coatings prepared on aluminum, wood, and glass

substrates retained their superhydrophobicity after abrasion

at 40 kPa, while coatings with constant spray distance lost

their superhydrophobicity after abrasion at 10 kPa. In this pa-

per, we reported the gradient functional surface with me-

chanical durability and superhydrophobicity (i.e., contact

angle of SiO2/EAC ratio, Rrms values). Movable spraying

method is probably applicable to fabricate gradient func-

tional surface when mixing 2 different functional materials

which changes morphology or composition with spray

distance.

ACKNOWLEDGMENTS

We are grateful to Dr. Kouji Fujimoto whose comments

and suggestions were greatly valuable throughout our study,

and Dr. Kyu-Hong Kyung who gave us the advice of writing

this article. We are indebted to Dr. Yoshio Hotta and Mr.

Kengo manabe, whose relevant comments were an enormous

help.

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movies about durability of coated glass, which kept superhydrophobicity

after wearing with cotton fabrics many times, and about colored water

slips on the coated glass with tilting about 6� after 50 kPa abrasion with

cotton fabrics.

FIG. 9. Abrasion pressure versus contact angle for aluminum-, wood-, and

glass-coated substrates. Abrasion was conducted for 10 rounds with cellu-

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