Physica E 43 (2010) 81–84

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Electrowetting on a dielectric surface roughened with zinc oxidetetrapod nanocrystals

Jun Xia n, Jun Wu

School of Electronic Science and Engineering, Southeast University, Si Pai Lou 2#, Nanjing 210096, PR China

a r t i c l e i n f o

Article history:

Received 15 March 2010

Received in revised form

9 June 2010

Accepted 17 June 2010Available online 19 June 2010

77/$ - see front matter & 2010 Elsevier B.V. A

016/j.physe.2010.06.014

esponding author. Tel.: +86 25 83791911; fa

ail address: xiajun@seu.edu.cn (J. Xia).

a b s t r a c t

Changing the contact angle between a solid substrate and a droplet with an external voltage is

important in the field of microfluidics. On a flat surface, the range of the reversible contact angle is

about 901: from 301 to 1201. However, on a rough surface, such as superhydrophobic surface of carbon

nanotube, carbon nanofibers, silicon nanowires, ZnO nanorods, etc., the reverse transition from the

Wenzel state to Cassie state is usually prevented due to an energy barrier resulting in a lower contact

angle range. In this paper, we described the electrowetting on a rough surface of ZnO tetrapods. The

contact angle on ZnO tetrapod was as high as 1551, and a wide range of reversible contact angles

actuated by an electric potential was observed. Electrolysis, which was a problem in previous research

of nano-structured superhydrophobic surface, was avoided by applying a uniform dielectric layer

between the conductive ITO and the ZnO tetrapods. The proposed method has potential applications in

microfluidics devices with a large dynamic range of capillary force.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Electrowetting is basically tuning the contact angle between asolid substrate and a droplet of a fluid; the method is widely usedin microfluidics devices, such as lab-on-a-chip [1,2], liquid lenses[3,4], and reflective displays [5,6]. On a flat surface a thin and flatdielectric layer is normally applied between the conductivesubstrate and the droplet to prevent electrolysis: this technologyis usually called electrowetting on dielectric, EWOD [7], and thehighest contact angle of about 1201 for a sessile water droplet isobserved on hydrophobic material PTFE. Further increase of theinitial contact angle is preferred for most microfluidics devices,since it will not only increase the capillary force that is used tomove the droplet, but also extend the functional range of thecontact angle. A recent biomimic study shows that a super-hydrophobic surface can be realized by fabricating micro- andnano-scale structures on hydrophobic material [8,9]. The fabrica-tion methods include lithography, chemical deposition, sol–gelprocessing, polymer formation, etc [10–13]. However, electrowet-ting on a superhydrophobic surface (EWOS) is different fromEWOD. A sessile droplet on a superhydrophobic surface has twostable states, i.e. the Cassie state [14] and the Wenzel state [15],and the capacitor between the droplet and the substrate isinfluenced by the trapped air inside the nano-scale structures [16].

ll rights reserved.

x: +86 25 83363222.

A rapid transition from the Cassie to the Wenzel state is normallyobserved by applying external voltage in most investigations;however, the droplet is not reversible back from the Wenzel toCassie state due to a surface energy barrier, unless using externalagitation [17], heating [18], a drying process [19], etc. However evenworse, electrolysis is observed by further increasing the voltage inthe Wenzel state; therefore, the contact angle of the droplet cannotbe further reduced. A review of EWOS, including silicon nanoposts,expoxy microposts, carbo nanofibers, silicon nanowires, and carbonnanotubes, has been presented by Heikenfeld and Dhindsa [20].

We present a new method in this paper to generate super-hydrophobic surface by using ZnO tetrapod nanocrystals. Differ-ent from previous bottom–up fabrication methods that are usingcomplex chemical vapor deposition technology to grow nanos-tructures, we propose a top–down method to generate a roughsurface on conventional EWOD. The advantage of using thetetrapod geometry is that it has a three-dimensional topographyand always has one ‘arm’ directed normal to the plane ofsubstrate. Therefore a rough surface can easily be fabricated byscreen printing and spin coating with ZnO tetrapod nanocrystals.Electrowetting phenomena on the proposed surface were alsostudied and are described in this paper as well.

2. Experiment

ZnO tetrapods were synthesized through a vapor phasetransport and catalyst-free method in a horizontal tube furnace

J. Xia, J. Wu / Physica E 43 (2010) 81–8482

[21]. And then a ZnO tetrapod suspension was prepared by usingterpineol as solution. In order to increase adhesion and viscosity,ethyl cellulose was added to the solution: the content of ZnOtetrapods and ethyl cellulose in the suspension was 4 and 2.5 wt%,respectively. The suspension was stirred for 1 h at 60 %

oC to enable

complete dissolution of ethyl cellulose in terpineol.ITO-coated glass was used as substrate. It was ultrasonically

cleaned with ethanol and deionized water, successively. Toprevent electrolysis, a thin layer of polyimide (�1 mm) was firstspin coated on top of the ITO. The prepared polyimide layer had auniform flat surface. The micro- and nano-scale roughness wasthen created on top of the polyimide layer by spin coating the ZnOtetrapod suspension. The thickness of the ZnO tetrapod layer wascontrolled by the spin coating speed. For comparison reasons, wealso prepared a thick ZnO tetrapod layer by dip coating, whereasthe thin ZnO tetrapod layers were fabricated by spin coating at aspeed of 1000 rpm. After coating with the ZnO tetrapod suspen-sion, the substrates were baked at 300 1C for half an hour. Finally,a fluor-containg material Teflon AF 1601 (1 wt%) was used toreduce the surface tension of the prepared substrates by spincoating at a speed of 1000 rpm. The substrates were then baked at100 1C for half an hour.

A droplet of deionized water (�2 mL) was carefully positionedon the prepared substrates and the contact angle was recordedwith a digital camera. Electrowetting phenomena were studied byapplying a 1 kHz ac voltage with a square wave shape betweenthe water and the conductive ITO substrate by inserting a goldneedle in the water droplet. The ac voltage could be tuned from0 to 7120 V (the limit of the amplifier).

3. Results and discussion

Fig. 1(a) and (b) are SEM images of the ZnO tetrapod layersfabricated via dip coating and spin coating, respectively. Theimages of droplets on these two surfaces are also presented;the measured contact angles are 1551 and 1301, respectively. In

Droplet

ZnOtetrapods

Substrate

Fig. 1. SEM images of rough surfaces prepared by (a) dip coating ZnO tetrapod suspensi

are sketches to demonstrate the distribution of ZnO tetrapod on both the surfaces, res

the dip coated layer the ZnO tetrapods are closely stacked and areevenly overlapped as demonstrated in Fig. 1(c). This structureperfectly traps air between the ‘arms’. According to Cassie’s modeltrapped air dramatically increases the apparent contact angle andthus a spherical pearl (with an estimated diameter �1 mm) isobserved with a very small sliding angle (less than 21). In contrast,the ZnO tetrapods on the spin coated surface have a sparsedistribution. The droplet pushes the intermediate air away anddirectly contacts the polyimide substrate as shown in Fig. 1(d): itgets a lower energy and develops into a stable state, i.e. theWenzel state, with a smaller contact angle. In the Wenzel state,the droplet sticks to the rough surface even if we turn thesubstrate upside down.

On the dip coated substrate we do not observe any changes ofthe contact angle or other electrowetting phenomenon byapplying 120 V ac voltage. This is different from a recent studyon superhydrophobic ZnO nanorods [17], in which a transitionfrom the Cassie to Wenzel state is observed. The reason for thisdifference is that in our experiment the ZnO tetrapods areelectrically insulated from ITO by a polyimide layer. The distancebetween the droplet and the conductive ITO in our experiment ismore than 10 mm, which is the averaged length of ZnO tetrapod.In Ref. [17], however, ZnO nanorods are directly grown on ITOsubstrate and only the top of the nanorods is covered by a layer ofTeflon. The capacitor between the droplet and the ITO in ourexperiment is smaller than in Ref. [17] because of the largedistance.

In the Wenzel state the droplet is wetting the substrate andthe apparent contact angle is determined by the roughness of thesurface and the surface tension, as described below:

cosðyW Þ ¼ r cosðyF Þ ð1Þ

where r is the ratio of the total area of liquid–solid contact to theprojected area on the base, yF is the contact angle on a flat surface,and yW is the apparent contact angle in the Wenzel state. In ourexperiment the apparent contact angle on the surface with spincoated ZnO tetrapod is 1301. Comparing with the maximum

Droplet

ZnOtetrapods

Substrate

on and (b) spin coating ZnO tetrapod suspension at a speed of 1000 rpm. (c) and (d)

pectively.

Fig. 2. (a) and (b) are the captured images of deionized water (�2 mL) on ZnO tetrapod surface prepared by spin coating at external voltages 0 and 120 V, respectively.

60

80

100

120

140

Con

tact

ang

le (d

egre

e)

Voltage (volts)

flat surfacerough surface

0 50 100

Fig. 3. The measured contact angle as a function of applied external voltage for

both flat surface of polyimide and rough surface prepared by spin coating ZnO

tetrapod suspension at a speed of 1000 rpm.

J. Xia, J. Wu / Physica E 43 (2010) 81–84 83

contact angle on the flat surface of Teflon (which is 1151), we findthat the roughness ratio of the surface is 1.5.

Electrowetting is observed on the surface with spin coated ZnOtetrapod as depicted in Fig. 2(a) and (b). The apparent contactangle changes from 1301 to 851 by applying 120 V ac voltage,while the apparent contact angle returns to the initial state afterswitching off the external voltage. To make a comparison, we alsoshow the images of droplet actuated by an electric potential on aflat polyimide surface of which surface tension was modified byTeflon (1 wt%) in Fig. 2(a) and (b). It is clear that both the initialcontact angle and the apparent contact angle at 120 V ac voltageof a droplet on flat surface are lower than the ones on a roughsurface.

Electrowetting was not reversible in the Wenzel state in theprevious studies on superhydrophobic surface with nanostruc-tures [17–20]. Electrolysis is normally observed after furtherincreasing the voltage. The reason for this behavior is that in thoseprevious studies the nanostructures were directly grown on aconductive substrate, yielding an extremely high electrostaticfield on the tip of the nanostructures: this caused electrolysis. Inour experiment, the ZnO tetrapod is insulated from ITO by apolyimide layer. The three-dimensional ZnO tetrapod contacts thesubstrate with three ‘arms’, as can be seen in Fig. 1(d); therefore itonly increases the surface roughness and has only a minor effecton the electrostatic field distribution. Based on the energy-minimization approach [16,22], the electrowetting equation inour experiment may be described as

cosðyW Þ ¼ rcosðyF Þþð1�f Þe0edU2

2dslvð2Þ

where U is the external voltage, slv is the surface tension betweenthe liquid and the vapor, e0 and ed are the dielectric constants ofpolyimide layer, vacuum d is the distance between the dropletand the conductive substrate, and f is the fraction of the areacovered with ZnO tetrapods. f has a very small value in ourexperiment and could be neglected for a surface that is sparselycovered with ZnO tetrapod ‘arms’. This is confirmed by thecontact angle measurement experiment depicted in Fig. 3, inwhich the apparent contact angles as a function of the externalvoltage, both on the flat surface without ZnO tetrapod and therough surface with the spin coated ZnO tetrapod, are presented.

Hysteresis is observed on the rough surface with the spincoating ZnO tetrapod. Fig. 2(b) shows that the left and rightcontact angles are a little bit different. This could be caused by thenon-uniform distribution of the ZnO tetrapod. We assume thatthis hysteresis could be reduced by two ways: one is by uniformlydistributing the ZnO tetrapods and the other is by adding a nano-scale structure.

From the experiments described in this paper we concludethat a rough surface fabricated via spin coating ZnO tetrapods hasa wide range of reversible contact angle in the Wenzel state. Thisresult deviates significantly from recent studies on EWOS, whichfocus on the transition from the Cassie to Wenzel state. Thisdifference is explained by the presence of a uniform thin dielectriclayer positioned between the ZnO tetrapod and the conductivesubstrate. Our method may be successfully applied in micro-fluidics devices for tuning the capillary force over a widerdynamic range, which is vital to move a droplet.

Acknowledgements

This work is supported by the National High TechnologyResearch and Development Program of China (2007AA01Z303)and Innovation in Higher Education Disciplines IntroductionProgram (111 Program, B07027). The authors wish to thank Dr.Daniel den Engelsen for the fruitful comments and the help toprepare this paper.

References

[1] J. Zeng, T. Korsmeyer, Lab. Chip 4 (2004) 265.[2] J. Atencia, D.J. Beebe, Nature 437 (29) (2005) 648.[3] B. Bergea, J. Peseux, Eur. Phys. J. E 3 (2000) 159.[4] B.H.W. Hendriks, S. Kuiper, M.A.J. van, Opt. Rev. 12 (3) (2005) 255.[5] R.A. Hayes, B.J. Feenstra, Nature 425 (25) (2003) 383.[6] J. Heikenfeld, K. Zhou, E. Kreit, B. Raj, S. Yang, B. Sun, A. Milarcik, L. Clapp,

R. Schwartz, Nature Photon 3 (2009) 292.[7] C. Quilliet, B. Bergeb, Curr. Opin. Colloid Interface Sci. 6 (2001) 34.[8] L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, D. Zhu, Adv.

Mater. 14 (24) (2002) 1857.[9] P. Roach, N.J. Shirtcliffe, M.I. Newton, Soft Matter 4 (2008) 224.

[10] T. Baldacchini, J.E. Carey, M. Zhou, E. Mazur, Langmuir 22 (2006) 4917.[11] N. Zhao, F. Shi, Z.Q. Wang, X. Zhang, Langmuir 21 (2005) 4713.

J. Xia, J. Wu / Physica E 43 (2010) 81–8484

[12] H.M. Shang, Y. Wang, S.J. Limmer, T.P. Chou, K. Takahashi, G.Z. Cao, Thin SolidFilms 472 (2005) 37.

[13] H.Y. Erbil, A.L. Demirel, Y. Avci, O. Mert, Science 299 (28) (2003) 1377.[14] A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1944) 546.[15] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988.[16] V. Bahadur, S.V. Garimella, Langmuir 23 (2007) 4918.[17] J.L. Campbell, M. Breedon, K. Latham, K. Kalantar-zadeh, Langmuir 24 (2008)

5091.

[18] T.N. Krupenkin, J.A. Taylor, E.N. Wang, P. Kolodner, M. Hodes, T. Salamon,Langmuir 23 (2007) 9128.

[19] H.Z. Wang, Z.P. Huang, Q.J. Cai, K. Kulkarni, C.L. Chen, D. Carnahan, Z.F. Ren,Carbon 48 (2010) 868.

[20] J. Heikenfeld, M. Dhindsa, J. Adhes. Sci. Technol. 22 (2008) 319.[21] F.Z. Wang, Z.Z. Ye, D.W. Ma, L.P. Zhu, F. Zhuge, Mater. Lett. 59 (2005)

560.[22] W. Dai, Y.P. Zhao, J. Adhes. Sci. Technol. 22 (2008) 217.