Nanoscale Electrowetting Effects Studied by Atomic Force Microscopy

The AFM topography image of PMMA surface is shown in Figure 1. ... of adhesion force between tip and sample under different voltages, 0 V/m (a), .... ...
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J. Phys. Chem. C 2009, 113, 661–665

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Nanoscale Electrowetting Effects Studied by Atomic Force Microscopy Li Guan, Guicun Qi, Sheng Liu, Hui Zhang, Zhong Zhang, Yanlian Yang,* and Chen Wang* National Center for Nanoscience and Technology, Beijing 100190, P. R. China ReceiVed: July 24, 2008; ReVised Manuscript ReceiVed: October 29, 2008

Electric field effect on adhesive characteristics of the polymethyl methacrylate (PMMA) surface is studied by using force spectroscopy method of atomic force microscope (AFM). The adhesive interaction between the AFM tip and dielectric surface is obtained by monitoring the force-distance spectroscopy, which reflects the change of the surface tension under the influence of external electric field. Such changes in adhesion characteristics are attributed to the electrowetting effect at relatively low electrical field strength and the electrowetting saturation effect at high electrical field strength. It is also suggested that the force spectroscopy method has noticeably high stability in studying adhesion characteristics at nanometer scale. 1. Introduction The behavior of liquid spreading and wetting on solid surfaces has been extensively studied both on macroscopic and microscopic scales1-11 for their importance in many natural and industrial processes. Electrowetting effects are due to the enhancement of the wettability of a solid surface by applying an external voltage across the solid/liquid interface.3 The impact of external electric field on the contact angle has also been extensively pursued, with the aim of manipulating interface wettability without changing the chemical composition of the contacting phases. The endeavor can be manifested in the establishment of the Young-Lippmann equation (eq 1) for describing the voltage dependence of contact angle, which is prevalently adopted as the basis of discussions on electrowetting effects,3

cos θV ) cos θ0 +

C V2 2γLG

(1)

where θV, θ0 are the contact angles with and without electric potential V, and C is the capacitance between the electrodes. The above expression has been confirmed qualitatively by a number of contact angle experiments. Contact angle measurements are well established for characterizing the hydrophobicity of the materials surfaces. There are reports suggesting that the validity of the present electrowetting investigations is limited to the substrates with ideal dielectric properties.6 Other physical or chemical properties of substrate materials, such as piezoelectricity, charge-trapping characteristics and material defects, which could have significant influence on the electrowetting behavior, are yet to be explored.6 Considering the significant challenges in developing rigorous understanding of the electrowetting effects, the studies at microscopic scale would be keen for gaining detailed and complementary insights of the electrowetting mechanisms. As an alternative method to contact angle measurements, the electrowetting behavior could also be studied by using electrocapillary method, which could determine the variation of the surface tension by monitoring the shape changing of the substrate (electrode) as a function of the applied electric * To whom correspondence should be addressed. (C.W.) Telephone: 8610-82545609. Fax: 86-10-62656765. E-mail: [email protected].

potential.12,13 Beck14 reported the potential dependence on the interfacial tension of a gold ribbon in KCl solution. Variations in interfacial tension were calculated from the observed changes in the length of the ribbon. Fredlein and co-workers15 reported a modified version of this method, in which the electrode is in the form of a thin metal film deposited on a thin glass strip. However, the magnitude of shape changing of the electrode which takes place as a function of potential is extremely small to be measured by optical microscopy method. Therefore, the inevitable uncertainties in the capillary measurements could hamper the capability of obtaining quantitative relationship between the capillary force and external voltage. Furthermore, this method is subjected to the effect of bulk stress of the electrode in the measurements. In another related topical field, atomic force microscopy (AFM) is known to be an effective approach for studying localized surface structures and properties, such as viscoelasticity, friction and so on.16-18 Among many applications, local adhesion characteristics could be studied by force spectroscopy method.19,20 The adhesive interaction between the AFM cantilever apex and substrate surface, which could be in resemblance to the capillary interactions, could provide insightful information of the microscopic distribution of surface chemical composition, hydrophobicity, and mechanical properties, etc. It is therefore plausible to explore the feasibility of using force spectroscopy method to study the electrowetting behavior at microscopic scale. During the force spectrum measurement, the probe approaches sample surface and retracts afterward. It is well-known that the sample surface is covered with a thin water film in ambient conditions. As the tip lifts up, the water film will be pulled up to a certain height forming a liquid meniscus until breaking up. The force required to break up the water meniscus is attributed as the adhesion force (F), which can be considered as a reflection of the capillary force at microscopic scale. As proposed previously, the electrowetting behavior could be investigated by using capillary method,12-15 it is thus plausible to envision that force spectroscopy method has promising potentials for studying electrowetting behavior at nanometer scale with possibility of improved structural resolution and stability. In this work, we explored using force spectroscopy method to study the electrowetting behavior on the polymethyl meth-

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acrylate (PMMA) surface. The saturation effect in adhesion interaction can be clearly identified with the increasing electrical field strength. In addition, a dramatic decrease in adhesion interaction was observed when the electric field strength was beyond a threshold value. 2. Experimental Section In this study, a dielectric thin film was prepared by spin casting polymethyl methacrylate (PMMA) solution in cyclopentanone on substrate Si(100) doped with phosphorus (2-4 Ω · cm), with thickness of about 0.3 µm and then annealed in air at 105 °C. The force spectroscopy measurement was conducted using a scanning probe microscope (Nanoman II, Veeco Metrology) under ambient conditions. A silicon tip coated with Au was used as the top electrode, and the sample as the ground plate. The distance that tip travels before breaking up the water film is typically about 200 nm, and the pulling velocity was 14 µm/ s. Force spectra under different voltages were measured by using the same tip to avoid tip-dependent variations. The relative humidity was controlled between 40% and 50% by trial and error to ensure the existence of thin water films on the sample surface and the security of the results. When humidity is relatively low, the adhesion force between the tip and dielectric surface is very small since the condensed water is only bridging small asperities 21(which is also illustrated in Figure 64a of ref 21), which will cause appreciable error in the measurements. However, when humidity is relatively high, both the tip and the dielectric surface are completely wet which will cause some uncertainty due to the interaction between water film and environment. It was also observed that the adhesion force is far from stable under higher relative humidity in our previous experiment. This later situation was also proposed that the adhesion force is once again very small when the surface is completely wet. 21 The electric field strength is estimated by assuming the tip and sample as parallel plates for very crude estimation since quantitative description of tip geometry is impossible at this stage. This simplification would not affect the proportionality of the electric field strength to the exerted external voltage. For each voltage, 1000 force curves were surveyed to obtain the averaged tip-sample interaction force spectrum. Statistic histograms were obtained to characterize the distribution of the adhesion force F. A silicon tip and tapping mode were used to obtain the topography of PMMA surface. The water contact angle on PMMA surface was conducted using a contact angle measurement instrument (DSA100, Kru¨ss). 3. Results and Discussion The AFM topography image of PMMA surface is shown in Figure 1. It can be seen that the PMMA surface is featureless with a root-mean-square (rms) roughness around 0.39 nm. As has been discussed previously, there could be inevitable uncertainties in the contact angle measurements. As an example, the static contact angle without external voltage on PMMA surface was observed to decrease monotonically with time under ambient conditions, as shown in Figure 2a. Therefore, normalization of the measured results is needed for obtaining comparable results. The static contact angle decreases since the liquid drop in macroscopic scale could be affected by possible environmental conditions, such as evaporation of liquid, the interaction between liquid drop with substrate, the volume

Figure 1. Topography of the PMMA surface. The scan size is 2 µm × 2 µm, and the Z scale is illustrated as a color bar in the inset.

differences of the droplet, etc. 22 However, few quantitative results on these effects of static contact angle have been described in the literature. In contrast, this kind of fluctuations could be significantly reduced in the force spectrum measurements. Within the experimental area of nanometer scale probed by the AFM tip, the time-dependent uncertainty effects could be nearly completely excluded since the thin water film underneath the tip apex is in equilibrium state, as shown in Figure 2b, suggesting that the force spectroscopy method is highly stable for adhesion measurements. The difference between AFM force spectroscopy method and the reported contact angle measurements could be observed in several aspects. The time lapsed stability of force spectroscopy could be greatly improved compare to that of contact angle measurement. Furthermore, the influence of the contacting phases line effect and the environment effect on liquid drop in contact angle studies which are remarkable at macroscopic scale could be considerably reduced in the force spectroscopy method described in this work, considering that AFM tip radius is around tens of nanometers in comparison to the 2D continuous sample surface which can be approximated as infinite radius relative to the tip. Finally, high local electrical field strength could be applied in the electrowetting studies by force spectroscopy method. This later method could be particularly beneficial for studying the possible saturation behavior, which has been widely pursued as an important venue for underlying interfacial mechanisms. A number of force curves (1000 force curves under each potential condition) were surveyed to obtain the averaged tip-sample interaction force spectrum under different electric fields. From the distribution of the forces in the statistical histograms shown in Figure 3, the average adhesion forces were obtained under different electrical field strength through Gaussian fitting. Possible fluctuations caused by many factors such as the status of liquid bridge breaking and tip geometry could be minimized by the statistical treatment. The dependence of adhesion force (F) on the electrical field strength on PMMA surface is shown in Figure 4. According to the reported relationship of the capillary adhesion force for a small meniscus, 21 the capillary force F between a solid particle and solid surface covered with water film follows the expression:

F ) 4πRγLG cos θ

(2)

Here, R is the radius of the particle in contact with the surface, which is equivalent to the radius of the tip in AFM measurements, and γij is the surface tension between phase i and phase j. According to Young-Dupre equationγSG - γSL ) γLGcosθand

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Figure 2. (a) Time lapse of static contact angles with zero voltage under ambient conditions. (b) Time lapse of adhesion forces with zero voltage between the tip and the dielectric.

Figure 3. The histograms of adhesion force between tip and sample under different voltages, 0 V/m (a), 8 × 107 V/m (b), and 1.6 × 108 V/m (c)

Lippmann equationγSL(V) ) γSL(0) - (1)/(2)CV2, it can be obtained that for the case of an applied voltage:

F(V) ) 4πRγLG cos θ ) 4πR(γSG - γSL(V)) ) 1 4πR γSG - γSL(0) + CV 2 2

(

)

(3)

It should be noted that both γSG and γSL(0)could be considered as independent of the applied voltage.3 Thus it is plausible to assume that the relationship between adhesion force F(V) and the applied voltage V should follow the relationship:

F(V) ) a + bV2

(4)

Here F(V) is the adhesion force under applied voltage; a and b are constants.

It can be identified from Reg. 1 in Figure 4 that when the electrical field strength E is less than 4 × 107 V/m, the relationship between the adhesion force and voltage follows the quadratic characteristics, which coincide with Young-Lippmann equation (eq 1). The fitting result of this region is F ≈ 46.3 + 0.082V2 (the unit of the adhesion force F is nN). The voltage dependence of the contact angle is the keen issue in the electrowetting studies. One of the proposed mechanisms suggests that it is due to the line tension effect based on a thermodynamic analysis.23 Here, the line tension is in fact an electrostatic force originated from the excess charges at the three phases contact line. It was also proposed that a potential originated from an external source would influence the solid/ liquid interface only (because the gas phase is a good insulator)

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Figure 4. The magnitude of adhesion force versus voltage under ambient conditions. The black line with the solid black square is the force measurement results, and the red line is a polynomial fitting according to eq 4.

and therefore induce changes in γSL. Although there is no convincing theory on the mechanism of electrowetting processes, it is commonly perceived that the contact angle reduction is regarded as originated from the change of interfacial tension due to the induced electrical charge at the interface. In this study of microscopic scale electrowetting (usually the electric field E < 1 × 108 V/m), the charging at the interface leads to the decrease of surface tension γSL, hence increases the adhesion force measured by the AFM tip as illustrated in eq 3 (Reg. 1 in Figure 4). However, when the electrical field strength increases further, as shown in Reg. 2 in Figure 4, the measured adhesion force deviates significantly from the eq 4. As has been mentioned in the introduction, deviation of contact angle from Young-Lippmann equation is usually regarded as the saturation effect, which is reported in contact angle measurements,3 and is also observed by using the method of force spectroscopy at microscopic scale in this study. On the other hand, it is also coincident with the reported results that the Young-Lippmann equation is valid only when the voltage is limited in certain range.3 In this study, the saturation effect in adhesion interaction can be clearly identified with an approximate threshold electrical field strength about E ) 4 × 107 V/m. The measured adhesion force remains constant when the electrical field strength increases from E ) 8 × 107 V/m to E ) 1.1 × 108 V/m (Reg. 2 in Figure 4), and the nearly complete saturation phenomenon was achieved. Due to the inevitable uncertainties in contact angle measurements, it has been difficult to study the saturation effects at high electrical field strength which is important for the mechanistic studies. Moreover, the mechanism of the saturation phenomenon in electrowetting has not been fully clarified. The hypothesis such as charge trapping,24 gas ionization in the vicinity of the contact line and line instability,25 and so on, have been proposed. However, the influence of these proposed factors could be negligible at the nanometer scale. Therefore, the dominant effect of adhesion force could be possibly related to the interfacial tension. It was introduced previously that the change of the interfacial tension with potential is related to the surface charge density. With the polarization of dipoles in the dielectric film, the charge density which is closely connected with the capacitance reaches the threshold at high electric field. The capacitance of the system is mainly determined by the capacitance of the dielectric layer and is therefore constant when electric field reaches the threshold. Therefore the force between tip and

Guan et al. dielectric surface is invariable. This could accommodate the observed saturation behavior that adhesion force remains nearly unchanged when the electrical field strength is within the range of E ) 8 × 107 to 1.1 × 108 V/m (Reg. 2 in Figure 4). With even further increase of the electric field above 1.1 × 108 V/m (Reg. 3 in Figure 4), a decrease in the adhesion force can be observed which has not been reported by previous contact angle measurements. As illustrated in reference,1 polarization of water molecules by the external electric field becomes negligible below approximate field strength of E ) 1 × 108 V/m. However, when the electrical field strength E > 1 × 108 V/m, since the water molecule polarization is strongest at the interface, the water molecules attached to the interfaces could be aligned under the sufficiently strong electric field, which could affect the hydrogen bonding between surface molecules26-29 and hence the magnitude of γSG - γSL. Another possibility of the force decreasing could be related to the field-induced ionization in the liquid film that may cause enhanced leakage current between electrodes and eventually leads to breakdown of the capacitor formed by the two electrodes.8 As the result, the adhesion forces between tip and dielectric surface is reduced. It should be noted that the measured adhesion force between AFM tip and dielectric surface described by eq 3 is the total force that includes both the electrostatic force and capillary force. It is difficult to rigorously and quantitatively obtain the adhesion force in general situations. However, we consider that reliable analysis of the adhesion force could be performed under the experimental specifications in our study considering that the adhesion force is dominantly related to the interfacial tension under the approximation that the tip geometry and chemical specificity are unchanged. We wish to note that more rigorous researches should be carried out to investigate the eletrowetting mechanism at nanometer scale. 4. Conclusion We demonstrate here that electrowetting phenomenon can be investigated by using force spectroscopy method of AFM. Under relatively low electric field between tip and dielectric surface, the results are in good agreement with the reported ones by using contact angle method and the Young-Lippmann equation. Furthermore, the nearly complete electrowetting saturation phenomenon could be identified at high electric field which could be attributed to the capacitance effect between tip and dielectric surface. The study suggested that the nanoscale electrowetting behavior could provide complementary insights to the macroscopic contact angle studies. Acknowledgment. Financial support from the National Natural Science Foundation of China (90406019, 20673029) and National Key Project for Basic Research (Grant Nos. 2007CB936800,2006CB932100)arealsogratefullyacknowledged. Supporting Information Available: Text discussing the influence of environmental humidity on adhesive force and a figure showing the comparison of adhesion force on the PMMA surfce under different relative humidity conditions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Daub, C. D.; Bratko, D.; Leung, K.; Luzar, A. J. Phys. Chem C 2007, 111, 505. (2) Zhu, L.; Xu, J.; Xiu, Y.; Sun, Y.; Hess, D. W.; Wong, C.-P J. Phys. Chem. B. 2006, 110, 15945. (3) Quinn, A.; Sedev, R.; Ralston, J J. Phys. Chem. B. 2005, 109, 6268.

Nanoscale Electrowetting Effects (4) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Shu, Y. Langmuir 2004, 20, 3824. (5) Bienia, M; Quilliet, C.; Vallade, M. Langmuir 2003, 19, 9328. (6) Kang, K. H. Langmuir 2002, 18, 10318. (7) Decamps, C.; De Coninck, J. Langmuir 2000, 16, 10150. (8) Blake, T. D.; Clarke, A.; Stattersfield, E. H. Langmuir 2000, 16, 2928. (9) Janocha, B.; Bauser, H.; Oehr, C.; Brunner, H.; Gopel, W. Langmuir 2000, 16, 3349. (10) Prins, M. W. J.; Welters, W. J. J.; Weekamp, J. W. Science 2001, 291, 277. (11) Lee, J.; Moon, H.; Flower, J.; Schoellhammmer, T.; Kim, C. J. Sens. Actuators 2002, 95, 259. (12) Quinn, A.; Sedev, R.; Ralston, J. J. Phys. Chem. B. 2003, 107, 1163. (13) Morcos, I. J. Electroanal. Chem. 1975, 62, 313. (14) Beck, T. R. J. Phys. Chem. 1969, 73, 466. (15) Fredlein, R. A.; Damjanovic, A.; Bockris, J. O. M. Surf. Sci. 1971, 25, 261. (16) Christenson, E. M.; Anderson, J. M.; Hiltner, A.; Baer, E. Polymer 2005, 46, 11744.

J. Phys. Chem. C, Vol. 113, No. 2, 2009 665 (17) Paige, M. F. Polymer 2003, 44, 6345. (18) Vasilev, C.; Reiter, G.; Pispas, S.; Hadjichristidis, N. Polymer 2006, 47, 330. (19) Ratto, T. V.; Langry, K. C.; Rudd, R. E.; Balhorn, R. L.; Allen, M. J.; McElfresh, M. W Biophys. J. 2004, 86, 2430. (20) Wang, T.; Xu, J.; Qiu, F.; Zhang, H.; Yang, Y. Polymer 2007, 48, 6170. (21) Jacob, N.; Israelachvili, Academic Press 1985, 225. (22) Miyama, M.; Yang, Y.; Yasuda, H. K. Langmuir 1997, 13, 5494. (23) Digilov, R. Langmuir 2000, 16, 6719. (24) Verheijan, H. J. J.; Prins, M. W. J. Langmuir 1999, 15, 6616. (25) Peykov, V.; Quinn, A.; Ralston, J. Colloid Polym. Sci. 2000, 278, 789. (26) Bateni, A.; Laughton, S.; Tavana, H.; Susnar, S. S.; Amirfazli, A; Neumann, A. W. J. Colloid Interface Sci. 2005, 283, 215. (27) Luzar, A.; Svetina, S.; Zeks, B. Chem. Phys. Lett. 1983, 96, 485. (28) Lee, C. Y.; McCammmon, J. A.; Rossky, P. J. J. Chem. Phys. 1984, 80, 4448. (29) Luzar, A.; Svetina, S.; Zeks, B. J. Chem. Phys. 1985, 82, 5146.

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