1280 New J. Chem., 2012, 36, 1280–1284 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

Cite this: New J. Chem., 2012, 36, 1280–1284

A facile route to mechanically durable responsive surfaces with reversible

wettability switchingw

Xiaotao Zhu,ab

Zhaozhu Zhang,*aKun Wang,

aJin Yang,

abXianghui Xu,

a

Xuehu Men*aand Xiaoyan Zhou

ab

Received (in Montpellier, France) 9th January 2012, Accepted 2nd March 2012

DOI: 10.1039/c2nj00014h

Development of responsive surfaces with switchable hydrophobicity is hindered by their limited

mechanical stability. Herein, to address this problem, we fabricated a mechanically durable

responsive surface of CNTs/polyethylene composite coating by a facile route. Superhydrophobic

CNTs/polyethylene composite coating was fabricated by a hot-pressing process, followed by Ag

deposition and surface fluoration. Air-plasma treatment and surface fluoration were used to tune

the surface chemical composition and thereby to alter the surface wettability between

superhydrophobicity and superhydrophilicity. The mechanical stability of the resulting responsive

surface was evaluated by an abrasion test, and the results showed that the surface still exhibited

its reversible wettability switching even after repeated abrasion. Furthermore, the function of

reversible wettability switching of the responsive surface can be regenerated when failure of this

function occurred.

1. Introduction

Superhydrophobic surfaces have attracted immense commercial

and academic interest during the past several decades,1–3 and

extensive artificial superhydrophobic surfaces have already been

fabricated in different ways.4,5 Recently, various research groups

have also tried to develop responsive surfaces that can effectively

switch their wetting properties in response to environmental

stimulus, due to their wide applicability in various fields including

the development of self-cleaning surfaces, antifogging films,

microfluidics, tunable optical lenses, and so forth.6–10 These

include surfaces that alter their wettability in response to

changes in temperature,11,12 electric potential,13,14 mechanical

deformation,15 pH value,16,17 and others.18–21 These external

stimuli can effectively trigger chemical or topographic changes

at the surface and thereby alter the value of the contact

angle (y) for water droplets from greater than 1501 to almost 01.

Although great progress has been achieved in studying the

preparation and properties of responsive surfaces, for practical

applications, there exist still many problems to be solved. One

main challenge that must be overcome is improving the

mechanical stability of the responsive surfaces. The rough

surface texture of the responsive surface is easily damaged or

destroyed by mechanical contact and cannot be repaired auto-

matically, which makes the surface lose its reversible wettability

switching easily during normal use. Thus, for the development of

responsive surfaces, materials and techniques need to be explored

to provide mechanically robust surfaces. Despite the importance

of mechanical stability in engineering responsive surfaces, this

issue has received relatively little attention. For example, for the

responsive surfaces obtained so far, only an initial characterization

of the wetting properties is performed, the stability of the wett-

ability switching under mechanical stress has not been evaluated.

In this study, we fabricated a mechanically durable responsive

surface by a facile route. The alternation of air-plasma treatment

and surface fluorination enable the resulting surface to switch

its wetting characteristics from superhydrophobicity to super-

hydrophilicity easily and reproducibly. Moreover, for the first

time, the mechanical stability of the resulting responsive surface is

characterized, and the results show that reversible wettability

switching of the surface is maintained even after repeated abrasion,

demonstrating its stability under mechanical stress. Moreover,

reversible wettability switching of the surface can be regenerated

for repeated use when loss of wettability switching occurred after a

long-term use. This study provides a new avenue to extending the

lifespan of responsive surfaces with reversible wettability switching.

2. Experimental

Materials

Pristine multiwalled carbon nanotubes (CNTs) were

purchased from Chengdu Organic Chemicals Co. Ltd., China

a State Key Laboratory of Solid Lubrication, Lanzhou Institute ofChemical Physics, Chinese Academy of Sciences, Tianshui Road 18th,Lanzhou 730000, PR China. E-mail: zzzhang@licp.cas.cn,xhmen@licp.cas.cn; Fax: +86-931-4968098

bGraduate School, Chinese Academy of Sciences, Beijing 100039,PR China

w Electronic supplementary information (ESI) available: Experimentaldetails, XPS, XRD, and FTIR data. See DOI: 10.1039/c2nj00014h

NJC Dynamic Article Links

www.rsc.org/njc PAPER

Publ

ishe

d on

19

Mar

ch 2

012.

Dow

nloa

ded

by U

nive

rsity

of

Pitts

burg

h on

28/

10/2

014

16:1

5:08

. View Article Online / Journal Homepage / Table of Contents for this issue

This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 1280–1284 1281

(purity 499.9%, with a diameter and length of 30–50 nm and

about 30 mm, respectively), and used as received. Ultra-

high molecular weight polyethylene (UHMWPE, molecular

weight = 3 � 106 g mol�1) was purchased from Chenguang

Research Institute of Chemical Industry. Ammonia solution

(25–27 wt%), silver nitrate, and glucose were supplied by

Sinopharm Chemical Reagent Company. 1H,1H,2H,2H-

perfluorodecanethiol was purchased from Sigma–Aldrich.

Preparation of the superhydrophobic CNTs/

UHMWPE composite

0.5 g of UHMWPE was added into a mold and pressed with a

pressure of 1.5 MPa at room temperature for 10 s to form an

incompact disc. Subsequently, the powder of pristine multi-

walled CNTs (0.1 g) was distributed on the surface of the disc

and pressed under a pressure of 4 MPa. The mold was then

heated at 180 1C for 40 min. After cooling, the sample was

demolded and ultrasonically cleaned with ethanol three times

to remove the CNTs that are not firmly embedded inside the

UHMWPE substrate. The CNTs/UHMWPE composite was

immersed in 0.5 M [Ag(NH3)2]+ solution (pH = 10.8) for

45 min, and then transferred into a 0.5 M glucose solution for

30 min, followed by rinsing with water. Subsequently, the

sample was immersed in an ethanol solution of 1H,1H,2H,2H-

perfluorodecanethiol (1 mM) for 30 s, and then placed in an

oven to dry for 30 min at 100 1C.

Characterization

Field-emission scanning electron microscopy (JEOL JSM-6701F

FESEM) was used to observe the morphology of sample

surfaces. X-Ray photoelectron spectroscopy (XPS) data were

acquired using the VGESCALAB210 X-ray photoelectron

spectrometer, using Mg Ka line (hn = 1253.6 eV) as an

excitation source. Contact angles (CAs) and sliding angles

(SAs) were measured by the sessile drop method using a

KRUSS DSA 100 (KRUSS) apparatus. The average contact

angle (CA) and sliding angle (SA) values were determined by

measuring the same sample at five different positions.

3. Results and discussion

Fig. 1 shows the surface morphology of the CNTs/UHMWPE

composite surface before and after Ag deposition. It can be

observed that the original composite surface is not flat, and

CNTs are exposed outside the UHMWPE surface, showing a

hair-like morphology (see Fig. 1a–c). The embedding depth of

CNTs in UHMWPE was about 200 mm (see ESIw). Some

protrusions and pores with different sizes are distributed on

the surface so that a large fraction of air can be trapped in

them, which is essential for formation of a solid–liquid–air

composite interface.22 After Ag deposition, the CNTs surface,

as shown in Fig. 1d and e, is covered with an Ag layer

composed of particles with diameter about 500 nm. Interestingly,

some Ag particles form coralloid aggregates on the composite

surface, which further enhance the surface roughness (Fig. 1f),

and this rough surface morphology could provide the needed

textures to enable the achievement of superhydrophobicity. The

surface texture of the resulting Ag deposited CNTs/UHMWPE

composite (henceforth denoted as Ag-composite) did not exhibit

observable changes after placing in an ultrasonic bath containing

ethanol for 30 min, indicating the strong attachment between

Ag particles and composite surface, and this enhanced attach-

ment was beneficial to improving the mechanical stability of

the Ag-composite.

After dip-coating with perfluorodecanethiol, the surface

details of the Ag-composite were preserved, but the surface

fluorination process tuned the surface of Ag-composite to

superhydrophobic. Small droplets of water appeared to float

on the superhydrophobic surface, and the bright, reflective

surface visible underneath the water droplets (Fig. 2a) was a

signature of trapped air and the establishment of a composite

solid–liquid–air interface.22,23 The formation of this composite

state enhanced super-repellence by promoting a high CA

(1561) as well as a low SA (31), as shown in Fig. 2b. Furthermore,

when a water droplet vertically hits the superhydrophobic surface,

it can bounce away from the surface. With continuous bouncing

on this superhydrophobic surface, the water droplet moved from

left to right rapidly (for less than 3 s). This bouncing and

Fig. 1 FESEM images of the CNTs/UHMWPE composite surface (a) before and (d) after Ag deposition. (b, c) and (e, f) are the magnified images

of (a) and (d), respectively.

Publ

ishe

d on

19

Mar

ch 2

012.

Dow

nloa

ded

by U

nive

rsity

of

Pitts

burg

h on

28/

10/2

014

16:1

5:08

. View Article Online

1282 New J. Chem., 2012, 36, 1280–1284 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

rolling process is clearly shown in Fig. 3, and this result

indicates that the coated Ag-composite possesses robust super-

hydrophobicity under dynamic wetting.

The surface wettability of the coated Ag-composite surface

can be systematically tuned by air-plasma treatment. It was

found that the CA of water decreased from 1561 to 1391 after

air-plasma treatment (at the power of 10 W) for 30 s and then

decreased gradually as the processing time proceeded (Fig. 4a).

After air-plasma treatment for 90 s, the non-wetted surface

transits to a fully wetted one, and water droplets completely

wet the surface, leading to near-zero CA. The wettability

variation is attributed to the change of surface chemical

composition, as there was no observable change in the surface

structure after plasma treatment. It is well-known that oxidative

(oxygen and air) plasma treatment is an effective tool for

improving wettability by creating –COOH, –CO, and other

relevant functional groups.24–26 XPS analysis demonstrated

Fig. 2 Digital photographs of water droplets on the superhydrophobic

composite surface (a) and the profile of a spherical water droplet

exhibiting a CA of 1561 and a SA of 31 (b).Fig. 3 Snapshots of a water droplet of 10 mL vertically hitting the

superhydrophobic surface.

Fig. 4 (a) The influence of plasma processing time on the hydrophobicity of the surface; (b) reversible wettability switching with water can

be achieved reproducibly and rapidly by the alternation of plasma treatment and surface fluorination; (c) XPS spectra of (c) F 1 s, (d) C 1 s, and

(e) O 1 s of the surfaces before and after air-plasma treatment. The intensity is decreasing for C 1 s and F 1 s peaks but increasing for the O 1 s peak

as the air-plasma treatment proceeds.

Publ

ishe

d on

19

Mar

ch 2

012.

Dow

nloa

ded

by U

nive

rsity

of

Pitts

burg

h on

28/

10/2

014

16:1

5:08

. View Article Online

This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 1280–1284 1283

that after air-plasma treatment, the relative amount of oxygen

increased, and the fluorine content decreased significantly

(see Fig. 4c–e), causing an increase in the surface energy of

the Ag-composite surface. While, surface fluorination lowers

the surface energy of the Ag-composite surface, which allows

the Ag-composite surface to regenerate its superhydrophobic

property and once more display high CAs with water droplets

placed at any location on its surface. It is clear from Fig. 4b that

the alternation of air-plasma and surface fluorination enables the

Ag-composite surface to switch its wetting characteristics from

completely water-wetting to nonwetting easily and reproducibly.

Furthermore, the Ag-composite surface can keep its switchable

wettability to water for at least 3 months under atmospheric

conditions, and the CAs for water are unchanged, which shows

its long-term stability. This approach to fulfill the switchable

hydrophobicity transition shows its advantages in simple-

operation and low-cost and may have significant applications

in smart devices such as micro-fluidic devices.

The mechanical stability of responsive surfaces is crucial

for their use in practical applications. Here, the mechanical

stability of the resulting responsive surface was evaluated by a

scratch test, and the methodology is illustrated in Fig. 5a.

Sandpaper (1200 mesh) served as an abrasion surface, with the

sample surface to be tested facing this abrasion material.

When a pressure of 6.8 kPa was applied on the sample, the

sample moved in one direction with a speed of 3 cm s�1.

Clearly, the sample surface remained superhydrophobic after

being scratched repeatedly. The CAs for water vary slightly

from 1561 in the initial state to 1521 after 10 abrasion cycles,

whereas the corresponding SA ranges from 31 to 221 after

10 scratch cycles (Fig. 5b). More importantly, as shown in

Fig. 5c, the Ag-composite still exhibits the reversible wettability

switching between superhydrophobicity and superhydrophilicity

by the alternation of air-plasma treatment and surface fluorina-

tion even after 10 scratch cycles. FESEM analysis demonstrates

that rough surface textures are retained after being scratched

repeatedly (Fig. 6a and b), indicating the mechanical robustness

of the responsive surface. In our present work, air-plasma

treatment and surface fluoration were used to alter the surface

composition and thereby to modulate the surface wettability. This

surface energy tunability when combined with the mechanically

stable surface structures gave rise to the longevity of reversible

wettability switching. The results of the scratch test demonstrate

that our created responsive surface is mechanically stable and thus

can withstand real world applications.

The surface of Ag-composite became smooth after repeated

abrasion with sandpaper, compared with the surface without

abrasion, and thus made the reversible wettability of the surface

decrease gradually. After a long abrasion process, the Ag-composite

lost its superhydrophobicity and thereby became unable to exhibit

the reversible switching between superhydrophobicity and super-

hydrophilicity. However, as the fabrication approach is facile and

time-saving, it provides an opportunity to construct a regenerative

surface. After repeating Ag deposition and surface fluoration

Fig. 5 (a) Schematic illustration of the methodology of the scratch test. (b) The switchable wettability of the surface is persevered even after

10 scratch cycles. (c) CAs and SAs as a function of number of abrasion cycles for the coated Ag-composite surface.

Fig. 6 FESEM images of the structured Ag-composite surface after (a) five and (b) ten abrasion cycles.

Publ

ishe

d on

19

Mar

ch 2

012.

Dow

nloa

ded

by U

nive

rsity

of

Pitts

burg

h on

28/

10/2

014

16:1

5:08

. View Article Online

1284 New J. Chem., 2012, 36, 1280–1284 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

process, the damaged composite surface was rendered with

superhydrophobicity again, with the value of CA being restored

to 1561. This allowed the composite surface to display the

reversible switching between superhydrophobicity and super-

hydrophilicity once more. The regenerated composite surface

was switched from a superhydrophobic state to a superhydro-

philic state (CA = 01) after air-plasma treatment, while

the plasma-treated surface retuned to a superhydrophobic one

(CA = 1561) after surface fluoration.

4. Conclusion

We have developed a facile way to fabricate a mechanically

robust responsive surface. Through consecutive air-plasma

treatment and surface fluoration, the wettability of the resulting

surface can be made to rapidly and easily switch between super-

hydrophobicity and superhydrophilicity for multiple cycles. It

was found that the resulting responsive surface still exhibited its

reversible wettability switching even after repeated abrasion

with sandpaper, indicating its good mechanical stability.

Additionally, the function of reversible wettability switching

of the surface can be restored for repeated use when failure of

this function occurred after a long-term abrasion. This study

represents an important addition to the field of responsive

surfaces and provides a new avenue to extend the lifetime of

the reversible wettability switching on surfaces.

Acknowledgements

This work was financially supported by the National Nature

Science Foundation of China (grant no. 50835009, and 51002162),

and the National 973 Project (grant no. 2007CB607601).

References

1 L. Feng, S. H. Li, Y. S. Li, H. J. Li, L. J. Zhang, J. Zhai, Y. L. Song,B. Q. Liu, L. Jiang and D. B. Zhu, Adv. Mater., 2002, 14, 1857.

2 P. Roach, N. J. Shirtcliffe andM. I. Newton, Soft Matter, 2008, 4, 224.

3 V. A. Ganesh, H. K. Paut, A. S. Nair and S. Pamakrishna,J. Mater. Chem., 2011, 21, 16304.

4 X. Zhang, F. Shi, J. Niu, Y. G. Jiang and Z. Q. Wang, J. Mater.Chem., 2008, 18, 621.

5 X. M. Li, D. Reinhoudt and M. Crego-Calama, Chem. Soc. Rev.,2007, 36, 1350.

6 F. Xia, Y. Zhu, L. Feng and L. Jiang, Soft Matter, 2009, 5, 275.7 D. J. Beebe, J. S. Moore, Q. Yu, R. H. Liu, M. L. Kraft,B.-H. Jo and C. Devadoss, Proc. Natl. Acad. Sci. U. S. A., 2000,97, 13488.

8 J. Lahann, S. Mitragotri, T. N. Tran, H. Kaido, J. Sundaram,I. S. Choi, S. Hoffer, G. A. Somorjai and R. Langer, Science, 2003,299, 371.

9 T. L. Sun and G. Y. Qing, Adv. Mater., 2011, 23, H55–H77.10 B. W. Xin and J. C. Hao, Chem. Soc. Rev., 2010, 39, 769.11 W. L. Song, F. Xia, Y. B. Bai, F. Q. Liu, T. L. Sun and L. Jiang,

Langmuir, 2007, 23, 327.12 N. J. Shirtcliffe, G. Mchale, M. I. Newton, C. C. Perry and P. Roach,

Chem. Commun., 2005, 3135.13 M. Im, D. H. Kim, J. H. Lee, J. B. Yoon and Y. K. Choi, Langmuir,

2010, 26, 12443.14 J. H. Chang and I. W. Hunter, Macromol. Rapid Commun., 2011,

32, 718.15 J. Y. Chung, J. P. Youngblood and C. M. Stafford, Soft Matter,

2007, 3, 1163.16 E. Stratakis, A.Mateescu,M. Barberoglou,M. Vamvakaki, C. Fotakis

and S. H. Anastasiadis, Chem. Commun., 2010, 46, 4136.17 S. T.Wang, H. J. Liu, D. S. Liu, X. Y.Ma, X. H. Fang and L. Jiang,

Angew. Chem., Int. Ed., 2007, 46, 3915.18 X. Y. Lu, J. Peng, B. Y. Li, C. C. Zhuang and Y. C. Han,Macromol.

Rapid Commun., 2006, 27, 136.19 G. Caputo, B. Cortese, C. Nobile, M. Salerno, R. Cingolani,

G. Gigli, P. D. Cozzoli and A. Athanassiou, Adv. Funct. Mater.,2009, 19, 1149.

20 X. T. Zhu, Z. Z. Zhang, X. H. Men, J. Yang and X. H. Xu, ACSAppl. Mater. Interfaces, 2009, 2, 3636.

21 X. T. Zhu, Z. Z. Zhang, X. H. Xu, X. H. Men, J. Yang, X. Y. Zhouand Q. J. Xue, Langmuir, 2011, 27, 10458.

22 A. B. D. Cassie and S. Baxter, Trans. Faraday Soc., 1944, 40, 546.23 I. A. Larmour, S. E. J. Bell andG. C. Saunders,Angew. Chem., Int. Ed.,

2007, 46, 1710.24 J. A. Chai, F. Z. Lu, B. M. Li and D. Y. Kwok, Langmuir, 2004,

20, 10919.25 X. T. Zhu, Z. Z. Zhang, X. H. Men, J. Yang, X. H. Xu and

X. Y. Zhou, Appl. Surf. Sci., 2011, 257, 3753.26 K. Tsougeni, D. Papageorgiou, A. Tserepi and E. Gogolides,

Lab Chip, 2010, 10, 462.

Publ

ishe

d on

19

Mar

ch 2

012.

Dow

nloa

ded

by U

nive

rsity

of

Pitts

burg

h on

28/

10/2

014

16:1

5:08

. View Article Online