Article Journal of Biomedical Nanotechnology

Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America

Vol. 9, 1250–1253, 2013

Analysis of Thickness of a Hydrophobic Fluoropolymer Film Based on Electrowetting Hyojin Ko1 , Minh Dinh Phan1 , Dibyendu Khatua1 , Chan-Hee Jung2 , Jae-Hak Choi2 , Oh-Sun Kwon1 ∗ , and Kwanwoo Shin1 ∗ 1

Department of Chemistry, Institute of Biological Interfaces, Sogang University, Seoul 121–742, Korea Research Division for Industry and Environment, Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea 2

The electrowetting of water drops on a dielectric fluoropolymer film was studied experimentally. The dependence of the contact angles of the water drops on the applied voltage has been well explained in the low-voltage limit by using the classical Young-Lippmann theory. With this theory, the thicknesses of films coated on glass substrates by using a spincoater were obtained indirectly by fitting the contact angle data and were confirmed by using X-ray reflectometry. The two sets of results showed a good agreement. In addition, we confirmed that the contact angle saturation at high voltage were consistent with Peykov’s model.

bySaturation, Ingenta to: University of South Carolina KEYWORDS: Electrowetting, Delivered Contact Angle EWOD, Reflectivity. IP: On: Mon, 11 Jul 2016 08:06:40 Copyright: American Scientific Publishers

INTRODUCTION Electrowetting on dielectrics (EWOD) is the electroactuated wetting and dewetting of a drop on a solid surface. The contact angle (CA, ) of a liquid drop on a dielectric film coated on a counter planar electrode can apparently be changed by applying an external electric field (Fig. 1).1–3 Very recently EWOD has been widely utilized in numerous bio and opto-related applications, such micro-fluidic lab-on-a-chip,4–6 liquid lenses7 and liquid displays8 in optoelectronic devices. The variation of CA with the applied electric field at the contact line has been well described by the classical Young-Lippmann equation: cos V  = cos Y +

1 CV 2 2lv


where Y , so called Young’s angle, is the contact angle at 0 V, lv is the interfacial tension between the liquid and the ambient vapor, C is the capacitance of the dielectric film, and V is the external voltage across the capacitance C. As the voltage increases, the CA decreases until a certain voltage after which it tends to asymptotically ∗

Authors to whom correspondence should be addressed. Emails: [email protected], [email protected] Received: 17 March 2012 Accepted: 16 May 2012


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approach a constant value; this is referred to as the contact angle saturation. Though this phenomenon is not yet completely understood theoretically, Peykov et al. proposed one thermodynamic model in 2000.9 They assumed that the saturation occurred when the liquid-solid interfacial energy became zero due to the presence of electrostatic energy. With this assumption, he introduced that the saturation angle sat would be given by cos sat =

c lv


where c is Zisman’s critical surface tension corresponding to ideal complete wetting ( = 0 . In this paper we present a simple method, allowing an exact thickness based on the curve fitting of EWOD experimental data. The thickness of a dielectric thin film layer obtained by fitting the data with Eq. (1) was in good agreement with the empirical value measured by using X-ray reflectometry. We could further confirm that the CA saturation was consistent with Peykov’s equation, Eq. (2).

EXPERIMENTAL DETAILS Polytetrafluoroethylene (PTFE,  = 193) which is an amorphous fluoropolymer (AF 1600) based on copolymers of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) 1550-7033/2013/9/1250/004


Ko et al.

Analysis of Thickness of a Hydrophobic Fluoropolymer Film Based on Electrowetting

Figure 1. Two different wetting states of a water drop on a dielectric film: (dot line) the initial hydrophobic state at 0 V, and (solid line) the hydrophilic state at non-zero applying DC voltage. Contact angle is a parameter to describe each state.

Figure 2. The wetting shape of a water drop on ITO/glass substrate coated with a hydrophobic AF 1600 film can be changed by applied voltage. The electrowetting no more progresses at near 32 V.

was purchased from Sigma-Aldrich. It was dissolved in function of squared voltage (Fig. 3(b)). From linear curve FC-40 (3M, Co.) solvent by stirring for four days at room fitting (solid blue line) we obtained the slope very easily. temperature. The solution with a concentration of 1.0 wt% In this curve fitting we excluded the CAs in two limits of was spin-coated onto the ITO/glass substrates at 2000 rpm CA, each of which responses to at too low voltage less for 120 seconds and was kept at 90  C for 10 minutes than 8 V and at too high voltage greater than 32 V corto evaporate the solvent, after which it was annealed at responding to the initial inert CAs and the CA saturation, 170  C, slightly higher than the glass transition tempera80 , respectively, in order to apply the linearity of Eq. (1) ture of 160  C, for 30 minutes. that obeys only at low voltage limit with an assumption The prepared substrate was placed on a contact of a model for the dielectric film as a simple Helmholtz anglemeter (SEO, Phoenix 100) and a water drop (∼3 L, capacitor: lv = 72 mN/m and resistivity 18.2 M · cmbyatIngenta 25  was Delivered to: University of South Carolina 0 (3) put on the substrate by using a micro-motorized syringe. IP: On: Mon, 11 Jul 2016 08:06:40C = d American Scientific Publishers A 50 m diameter platinum electrodeCopyright: (thin enough to where the dielectric constant of the fluoropolymer, AF not affect the surface tension of the drop) was immersed 1600, and the permittivity of air are  = 193 and 0 = into the drop and connected to a DC power supply 8854 × 10−12 F/m, respectively. Plug the slope obtained (Keithley 2400). When a voltage was applied to the drop, from the fitted curve into the Eqs. (1) and (3), C/2lv , we modified images of the liquid drop were captured by using finally were able to obtain the thickness of AF 1600 film a CCD zoom camera. The images were then with a Dropas 185 ± 10 nm. The uncertainty may occur heavily whenSnake plug-in by using ImageJ (NIH).10 11 ever the data in the low and high voltages are selectively chosen due to the non-linearity of these two regions.

RESULTS AND DISCUSSION The electrowetting experiments were performed by applying the electric potential with progressively increasing steps of 4 V between a water drop and a counter electrode coated an AF 1600 film, leading to a variation of advancing CA due to gradual spreading of the drop actuated by the electric potential (Fig. 2). The measured CAs are presented in Figure 3, which shows the variation of CA as a function of applied voltage. The CA curve apparently seemed to be well consistent to the classical Lippmann’s theory in Eq. (1) as shown in Figure 3(a). If we know the capacitance of the film that may subsequently require the thickness of the film, d (in Fig. 1), we can compare the CA data to the theoretical prediction directly. Conversely, by fitting the CA data and comparing the result to the theoretical prediction, we can obtain the thickness of the film indirectly. For the simplified fitting, the CA data were transformed to a linear form by representing the cosine of CA as a J. Biomed. Nanotechnol. 9, 1250–1253, 2013

Figure 3. Electrowetting measurement: (a)  verses. V and (b) cos  verses. V 2 for an AF 1600 film: The film thickness can be obtained from the slope of straight line. For linear curve fitting data must be selectively chosen.


Analysis of Thickness of a Hydrophobic Fluoropolymer Film Based on Electrowetting

Ko et al.

In order to confirm the reliability for the above estima1600, c = 157 mN/m,13 we obtained a constant satution of thickness in terms of the results of EWOD measureration of CA, sat = 78 , which is very good agreement ment, we measured the thickness of AF 1600 film directly with our observed the saturation CA as 80 . Although by using the non-destructive X-ray reflectometry. The meawe adopted Peykov’s thermodynamic model for our flusured X-ray reflectivity (circles) and the corresponding fit oropolymer dielectrics, that model is still insufficient to (line) for the AF 1600 film were presented in Figure 4, reveal any relationship between the CA saturation and in which many harmonic oscillations decayed exponenapplied voltage. tially as the X-ray momentum transfer increases represent Finally, it is worthy to note the observation of another the interferences of two reflected X-rays on two interfaces phenomenon in this EWOD experiment, the undesirable of air/AF 1600 and AF 1600/glass that contain the thickbubbles occurring inside the water drops. They appeared ness information. Fitting the X-ray reflectivity data with frequently even far before reaching to the saturation. one assumption of the uniform depth profile of electron Though they did not affect the apparent contact angle of scattering length density for the AF 1600 film produced the water drop, they should be suppressed or removed the thickness of the Teflon film to 180 ± 05 nm. This to produce the smooth linearity of CA variation. In most direct measured value is fairly in good agreement to that cases, because the bubbles were observed at the surfaces of obtained from the previous calculated from the EWOD of AF 1600 positioned just below the Pt electrode, we analysis for the CAs within the uncertainty, surely leading could assume that they were created by electrolysis. To the validation of the classical electrowetting theory at the determine what induced the electrolysis, we take images low voltage limit. of the AF 1600 surface by using atomic force microscopy On the other hand, the contact angle on the AF 1600 (AFM) in the tapping mode. As shown in Figure 5, we film was saturated at about 80 at the high voltage 32 V. observed many pinholes on the topography image. These This saturation has not been explained by any conventional pinholes enabled water to penetrate the AF 1600 film and theories, including the classical Young-Lippmann equamake contact with the underlying counter electrode, leadtion, Eq. (1), so far. Nevertheless, it implies that in high ing to electric breakdown and easily causing electrolysis voltage limit the capacitance in the last term of Eq. (1) (cf. the electric breakdown voltage of Teflon AF 1600 is cannot be a constant but has to be rather a function of 21 kV/mm).13 voltage so that it supports theDelivered exclusion by of Ingenta the saturated to: University Southsuch Carolina To of remove undesirable pinholes inherently disJul 2016 08:06:40 CAs for our linear curve fitting. IP: On: Mon, 11 tributed on the hydrophobic AF 1600 surface, we had betCopyright: American Publishers For an explanation of CA saturation in our samples of Scientific ter introduce a double layer for the dielectric by inserting low-dielectric amorphous polymer AF 1600, the Langevin another high-dielectric film, such as parylene, Al2 O3 , and model12 for the polarization of paraelectric molecules in SU-8, between the AF 1600 and the counter ITO electrode. dielectric film cannot be applied because the presence of Providing that a double-layer dielectric film is composed linearity at low voltage limit (see Fig. 3) reflects the fact of a very thin hydrophobic AF 1600 layer on thick other that the capacitance has already reached a constant that corresponds to complete saturation of the induced polarization. Therefore, instead of the Langevin model, we applied the thermodynamic Peykov’s model of Eq. (2). Applying the critical surface tension for solid Teflon AF

Figure 4. film.


Specular X-ray reflectivity for a 1.0 wt% AF 1600

Figure 5. AFM topographic image (3 × 3 m2 ) in the tapping mode of the surface of 1.0% AF 1600. J. Biomed. Nanotechnol. 9, 1250–1253, 2013

Ko et al.

Analysis of Thickness of a Hydrophobic Fluoropolymer Film Based on Electrowetting

dielectric layer, the thickness by using EWOD technique could be much easily obtained relatively to X-ray reflectivity method because of possibility of exclusion of tedious fitting work in the reflectivity for double layer. Whether or not the double-layer dielectrics may be applicable to the thickness measurement, the CA saturation dependent of either the thickness or the voltage is left as an interesting future work.

by the Ministry of Education, Science and Technology, and Sogang University Research Grant of 2010. REFERENCES

1. G. Lippmann, Relations entre les phénomènes électriques et capíllaires. Ann. Chim. Phys. 5, 494 (1875). 2. H. Moon, S. Cho, R. L. Garrell, and C.-J. Kim, Low voltage electrowetting-on-dielectric. J. Appl. Phys. 92, 4080 (2002). 3. O. Kwon, M. Kim, T. Kim, C. Lee, S. Han, J. Kim, J. Jung, J. Choi, and K. Shin, Revesibility of Electrowetting on Hydrophobic Surfaces and Dielectrics Under Continuous Applied DC Voltage. J. Nanosci. CONCLUSION Nanotechnol. 11, 7132 (2011). The electrowetting behavior of water drops on thin flu4. C. G. Cooney, C. Chen, M. R. Emerling, A. Nadim, and J. D. oropolymer films was studied focused on one applicaSterling, Electrowetting droplet microfluidics on a single planar surtion to determine the thickness of a hydrophobic film. face. Microfluid. Nanofluid. 2, 435 (2006). Film was prepared on the glass substrates by using the 5. H. Ko, J. S. Lee, C. Jung, J. Choi, O. Kwon, and K. Shin, J. Nanosci. Nanotechnol. In press. spin-coating the polytetrafluoroethylene resins, AF 1600. 6. M. Abdelgawad, P. Park, and A. R. Wheeler, Optimization of If the change in the contact angles of water drops dependevice geometry in single-plate digital microfluidics. J. Appl. Phys. dent on the applied voltage is studied, the thicknesses can 105, 094506 (2009). be predicted from the classical Young-Lippmann equation 7. B. Berge and J. Peseux, Variable focal lens controlled by an external for electrowetting. The prediction was a good agreement voltage: An application of electrowetting. Eur. Phys. J. E 3, 159 (2000). with the result obtained using the X-ray reflectivity tech8. R. A. Hayes and B. J. Feenstra, Video-speed electronic paper based nique. Thus our empirical method can be used for the on electrowetting. Nature 425, 383 (2003). quick supplemental estimation for the thickness of a thin 9. V. Peykov, A. Quinn, and J. Ralston, Electrowetting: A model for hydrophobic film. Additionally, we confirmed the fact that contact-angle saturation. Colloid Polym. Sci. 278, 789 (2000). the contact angle saturation was consistent to the Peykov’s 10. W. S. Rasband, ImageJ, U. S. National Institutes of Health, Bethesda, model rather than the Langevin model proposed in the Maryland, USA,, 1997 (2011). 11. A. F. Stalder, G. Kulik, D. Sage, L. Barbieri, and P. Hoffmann, Ref. [12]. Delivered by Ingenta to: University of South Carolina A snake-based approach to accurate determination of both contact IP: Juland 2016 08:06:40 points contact angles. Colloids Surf. A 286, 92 (2006). Acknowledgments: This work was supported by On: the Mon, 11 Copyright: American Scientific Publishers 12. J. Restolho, J. L. Mata, and B. Saramago, Electrowetting of ionic Nuclear R&D Program, Mid-career Researcher Program liquids: Contact angle saturation and irreversibility. J. Phys. Chem. (2011-0017539), GIST-NCRC grant (R15-2008-006) and C 113, 9321 (2009). Advanced Research Center for Nuclear Excellence funded 13. See website,

J. Biomed. Nanotechnol. 9, 1250–1253, 2013


Analysis of thickness of a hydrophobic fluoropolymer film based on electrowetting.

The electrowetting of water drops on a dielectric fluoropolymer film was studied experimentally. The dependence of the contact angles of the water dro...
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