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Langmuir 2006, 22, 5690-5696
Engineering the Electrocapillary Behavior of Electrolyte Droplets on Thin Fluoropolymer Films Jakub Kedzierski* and Shaun Berry Massachusetts Institute of Technology, Lincoln Laboratory, 242 Wood Street, Lexington, Massachusetts 02420 ReceiVed January 20, 2006. In Final Form: April 21, 2006 This study presents methods for engineering the electrocapillary behavior of fluoropolymer surfaces through the use of surfactants and an external insulating liquid. By the scaling of the appropriate surface energies, electrocapillary behavior is obtained at a record low voltage, with contact angle changes in excess of 100° at 4 V. A consistent description of electrocapillary saturation is presented, identifying three separate regimes: breakdown, thermodynamic instability, and relaxation. Methods for identifying and mitigating some of the saturation behaviors are discussed. Finally, the parameters influencing the observed voltage of zero charge are summarized.
Introduction The electrocapillary effect on thin dielectric films, also know as electrowetting, has recently attracted significant attention due to its ability to reversibly transport and shape fluids.1 A variety of microfluidic devices using the electrocapillary effect have already been demonstrated, including adjustable focus length fluidic lenses,2,3 droplet transport arrays for lab on a chip applications,4,5 and prototype displays.6 In essence, the electrocapillary effect results from the fact that the surface free energy of an interface is reduced by its capacitive free energy, and this capacitive energy can be adjusted by an electrode voltage. Thus, by simply applying a voltage across a proper film between an electrode and a conducting fluid, the contact angle and wetting behavior can be changed reversibly, causing fluids and interfaces to move in an electronically controlled manner. The simplicity of the electrocapillary effect (ECE) over other microfluidic techniques7,8 makes it attractive from an integration perspective. The fact that surface tension effects tend to be more important at small dimensions also makes ECE attractive from a scaling perspective. Unfortunately, ECE has several problematic characteristics. First, the voltages involved are typically high, often as high as several hundred volts.9 Efforts to lower the voltage through the use of thin dielectrics have met with some success;10 however, the lowest voltage at which a contact angle change of 60° has been observed is still over 200 * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Mugele, F.; Baret, J. C. J. Phys.: Condens. Matter 2005, 17, P705-R774. (2) Kuiper, S.; Hendriks, B. H. W. Appl. Phys. Lett. 2004, 85, 1128-1130. (3) Hendriks, B.; Kuiper, S. IEEE Spectrum 2004, 41, 32-36. (4) Fair, R. B.; Khlystov, A.; Srinivasan, V.; Pamula, V. K.; Weaver, K. N. Integrated chemical/biochemical sample collection, preconcentration, and analysis on a digital microfluidic lab-on-a-chip platform; Lab-on-a-Chip: Platforms, Devices, and Applications, Conf. 5591, SPIE Optics East, Philadelphia, Oct. 25-28, 2004. (5) Srinivasan, V.; Pamula, V. K.; Paik, P.; Fair, R. B. Protein Stamping for MALDI Mass Spectrometry Using an Electrowetting-based Microfluidic Platform; Lab-on-a-Chip: Platforms, Devices, and Applications, Conf. 5591, SPIE Optics East, Philadelphia, Oct. 25-28, 2004. (6) Hayes, R. A.; Feenstra, B. J. Nature 2003, 425, 383. (7) Huff, M. A.; Schmidt, M. A. Fabrication, Packaging and Testing of a Wafer-Bonded Microvalve; Technical Digest of the IEEE Solid State Sensor and Actuator Workshop, June 22-25 1992; pp 194-195. (8) Vieider, C.; Oehman, O.; Elderstig, H. A Pneumatic Actuated Micro Valve with a Silicon Rubber Membrane for Integration with Fluid-Handling Systems; Proceedings of Transducers 1995, 8th International Conference on Solid-state Sensors and Actuators, June 16-19 1995; pp 284-286. (9) Verheijen, H. J. J.; Prins, M. W. J. Langmuir 1999, 15, 6616-6620.
V. Second, at high enough voltage, the electrocapillary behavior always deviates from what is predicted theoretically, with the contact angle change smaller than expected. This effect, called electrocapillary saturation, is poorly understood, and no consistent picture has emerged as to its root cause or causes1,10 This paper addresses these problems and demonstrates that, through proper engineering of the fluids and surfaces involved, electrocapillary contact angle change of over 100° can be obtained with voltages as low as 4 V. An explanation of electrocapillary saturation is also presented, arguing that saturation consists of three unique regimes, one caused by the physical failure of the dielectric, a second caused by thermodynamic interface instability, and a third caused by temporal relaxation of the contact angle due to charge transfer from the electrolyte to the fluoropolymer surface. Experimental Methods Contact angle data for this experiment was obtained from a Rame´Hart goniometer, with an environmental fixture to hold the external oil phase. A typical experimental setup is shown in Figure 1. With the exception of one experimental split, the ground electrode consisted of a phosphorus-doped Si wafer with a thermally grown oxide of thickness tox, and a spun-on amorphous fluoropolymer (aFP) layer of thickness taFP. The fluoropolymer used was M-grade CYTOP with special terminations to promote SiO2 adhesion. For one split, a Pt ground electrode was used, with aFP spun directly on top. After the aFP layer was spun, the ground electrode was baked in air at 180 °C for 5 min unless otherwise indicated. The ground electrode was placed in an external insulating liquid ambient, the O-phase. A droplet of the electrolyte, W-phase, was placed on the aFP surface and contacted with a Pt wire electrode. During electrocapillary testing, a voltage, Vapp, was applied to the Pt wire electrode, while the ground electrode was held at zero volts. The W-phase was composed of 0.1 M NaCl aqueous solution and a chemical additive to lower the surface tension to the O-phase. Two additives were used, 1-propanol and sodium dodecyl sulfate (SDS), although not in conjunction. The O-phase consisted of either decane or dodecane. Since propanol is somewhat soluble in decane and dodecane, the W-phase and O-phase were stored in the same jar and allowed to equilibrate for at least 24 h prior to measurement. Note that when composition of W-phase and O-phase is given, it refers (10) Moon, H.; Cho, S. K.; Garrell, R. L.; Kim, C.-J. J Appl. Phys. 2002, 92, 4080-4087.
10.1021/la060204e CCC: $33.50 © 2006 American Chemical Society Published on Web 05/26/2006
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( ) ∂γWE ∂V
µ
)-σ
and a capacitive behavior of
oxaFP0 ∂σ ) ∂V oxtaFP + aFPtox
Where γWE is the surface free energy of the electrolyte/aFP interface, V is the applied voltage, µ is the electrolyte chemical potential, σ is the surface charge density in the electrolyte, and ox, aFP, tox, and taFP are the dielectric constants and thickness of the SiO2 and fluoropolymer films, respectively. Assuming that at some applied voltage (V ) Vzc) the surface charge is zero, σ(Vzc) ) 0 and γWE(Vzc) ) γzc WE, leads to Figure 1. Measurement setup: the O-phase was an insulating liquid, the W-phase was a conducting electrolyte. The ground electrode was covered by a SiO2 layer of thickness tox and an amorphous fluoropolymer (aFP) layer of thickness taFP. The top electrode consisted of a Pt wire, biased by an external voltage source, Vapp. The contact angle, CA, was measured in the W-phase. The relevant surface free energies are γWO, γOE, and γWE for the O-phase-toW-phase surface energy, O-phase-to-aFP surface energy, and W-phase-to-aFP surface free energy, respectively. to the initial composition of these phases; after reaching equilibrium with each other, the composition changes slightly. The measurement of the O-aFP surface energy, γOE, was done on a 40 nm aFP film in air. First, aFP surface energy was obtained using a geometric mean two-liquid method,11 at 18.3 mJ/m2. Using this value and the contact angle for decane and dodecane droplets on the same aFP film, γOE was calculated. The value of γOE was found to be 1.5 and 2.2 mJ/m2 for decane and dodecane interfaces, respectively. Note that a significant error may exist in these measurements since a small error in the contact angle or surface energy of aFP produces a large variation in γOE. Four types of electrocapillary measurements were performed in this experiment. In the DC electrocapillary measurement, a constant voltage (Vapp) was applied to the Pt wire electrode, the contact angle was measured after a 1 s stabilization time, and the Pt wire electrode was returned to zero bias. This process was carried out for Vapp values from 0 to the positive saturation value and then from 0 to the negative saturation value. The AC electrocapillary measurement was performed in a similar way, except the applied signal was a 1 kHz square wave of voltage Vapp and -Vapp. The contact angle was measured as |Vapp| increased from 0 V to the saturation voltage. In the relaxation measurement, a step voltage function was applied to the Pt wire electrode and the contact angle was measured as a function of time. Finally, in the capacitive measurement, the width of the electrolyte droplet at its base, as well as the capacitance formed between the droplet and the ground electrode, was measured as a function of applied signal amplitude, Vapp. For the capacitance measurement, the applied signal was a 1 kHz sine wave taken from the CV meter.
Model Simple theoretical models of the experimental setup shown in Figure 1 have been presented before9,10,12 as γWE - γ0WE ) - (0.5)CV2 where γ0WE is the surface free energy of the electrolyte/aFP interface at V ) 0. However, this relation is not valid at low voltages because the surface charge of capacitors with heterogeneous electrodes (in this case an electrolyte and solid) is not necessarily zero at zero applied bias. An extension of the model that accounts for this effect can be derived simply from the Lippman equation: (11) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (12) Decamps, C.; De Coninck, J. Langmuir 2000, 16, 10150-10153.
γWE(V) ) γzc WE -
oxaFP0 2(oxtaFP + aFPtox)
(V - Vzc)2
This relation in addition to the Young’s equation, γWO cos(θW) ) γWE(V) - γOE, allows for the calculation of the expected contact angle between the W-phase and the aFP surface given the applied potential. Implicit in the derivation of this model is the assumption that the chemical potentials that are relevant to the surface energy of the electrolyte are not changed by the applied electrical potential. This approximation may not be accurate in cases where ionic surfactants are present at the surface of the electrolyte; however, it was accurate enough that the above model fit all experimental data obtained in this study fairly well, even in cases where ionic surfactants were used. In general, when the above model was fit to the experimental contact angle data the following methodology was used. Values for tox and taFP were measured ellipsometrically. The value of γOE was measured by methods outlined in the previous section, when dodecane was used γOE ) 2.2 mJ/m2. The parameters of γWO, γzc WE, and Vzcwere used to fit the contact angle data. Each parameter was used to fit a different aspect of the electrocapillary parabola, and thus, each could be obtained with little error. γWO was used to fit the parabola curvature, Vzc was used to fit its vertex potential, and finally γzc WE was used to fit the contact angle maximum at that potential.
Electrocapillary Scaling It is highly desirable to lower the voltages used in the electrocapillary effect. Voltages of up to 200 V are often used to actuate an electrolyte with a micrometer or more thick insulating film.9 Scaling studies that have been performed before have largely focused on the electrocapillary behavior of water in an air ambient10 and relied on scaling of the insulator thickness. However, scaling the insulator thickness and voltage down by the same factor reduces the electrocapillary contact angle change, often below what is usable due to contact angle hysteresis. The contact angle change decreases because it is dependent on the square of the voltage and only inverse of insulator thickness. Scaling insulator thickness down linearly and voltage by the square root solves this problem but causes the electric field in the insulator to increase, and an increase in the field can lead to increases in parasitic behaviors such as saturation and breakdown. To scale the electrocapillary effect at a constant field γWO and γzc WE have to be scaled down together with the dielectric thickness and voltage. It is possible to scale γWO to about 20 mJ/m2 by using W-phase surfactants and air as the external phase. However, to scale γWO further, the external phase must be an insulating liquid. By using an external oil phase (O-phase) and surfactants to control surface tension, γWO and γzc WE can be scaled
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down together with the applied voltage and insulator thickness to produce high-quality electrocapillary behavior at low voltage. Figure 2 shows AC electrocapillary data taken for a W-phase (propanol-H2O based) droplet in an O-phase (decane), with γWO of only 1.5 mJ/m2. A contact angle change of 160° f 60° was achieved at 7 V, with a field of 0.04 V/nm, and an aFP thickness of 150 nm. Without scaling the W-phase surface energy and using just pure water on aFP in air, one would need a 230 V bias and an aFP thickness of 6.9 µm to achieve a contact angle change from 120° f 60° at the same electric field. Scaling surface energies in the above manner has several advantages in addition to those obtained as a direct result of constant field scaling. First, using an external oil phase instead of air replaces a poor insulator (air) with a much higher quality one and prevents evaporation. Second, the use of surfactants allows for further engineering of the electrolyte/aFP interface, which turns out to be critically important in reducing parasitic saturation effects described in subsequent sections. Third, the γOE of aFP and oil interface tends to be significantly lower than the surface energy of the aFP in air. As a consequence, the initial contact angle at V ) Vzc is higher, typically as high as 150° T 170° instead of 120°. Formally, scaling rules can be written using a scale factor R as
γWO t V and V′ ) , t′ ) , γ′WO ) R R R γ′WEzc - γ′OE )
γzc WE - γOE R
Note that the last two equations are somewhat coupled through phase composition and difficult to vary independently. However, in cases where γWO is dominated by the polar component of the p W-phase (γWO = γpW), and γzc WE is composed of γW plus the zc dispersive interaction of the W-phase and aFP (γWE ) γpW + γdWE), as is the case for data presented in this paper, scaling γWO approximately satisfies the last relation as well. This is because γOE ) γdOE for nonpolar hydrocarbon liquids and is close to the same value as γdWE. Actually, even without surfactants or a fluoropolymer interface when the decane/air and water/air interfaces are considered, their dispersive surface energy components are similar, 23.9 and 21.6 mJ/m2, respectively.13 Replacing air with aFP lowers these values by an order of magnitude, but their ratio appears to stay about the same. When considering the scaling of electrocapillary devices, all the dimensions are also scaled by the factor R. Such scaling causes the radii of curvature for the relevant phase boundaries to also scale. As a result, capillary pressures, which are proportional to surface energies divided by the radius of curvature, stay constant.
Saturation There are a multitude of nonideal effects associated with electrocapillary behavior. Normally, as the applied voltage, V, is increased, at some point the contact angle either stops increasing or the expected contact angle change is not as great as predicted theoretically. This behavior, commonly referred to as electrocapillary saturation, is still not well understood. Many theories have been proposed including surface charging,9 ionization of ambient material,14 surface free energy of zero,15-17 and material (13) Fowkes, F. M. J. Phys. Chem. 1963, 67, 2538. (14) Vallet, M.; Vallade, M.; Berge, B. Eur. Phys. J. B 1999, 11, 583. (15) Peykov, V.; Quinn, A.; Ralston, J. Colloid Polym. Sci. 2000, 278, 789793.
Figure 2. AC electrocapillary data with γWO of 1.5 mJ/m2. The picture shows a snapshot of the W-phase droplet shape at |Vapp| ) 0 and 7 V. The W-phase consisted of 75% propanol and 25% water 0.1 M NaCl by volume. The O-phase consisted of decane. The insulator was made from 11 nm of thermal SiO2 topped by 150 nm of amorphous fluoropolymer (aFP). Error bars indicate standard deviation at each data point for three subsequent measurements.
defects.18 In this experiment, several distinct saturation mechanisms were observed: they are categorized as dielectric breakdown, thermodynamic instability, and relaxation. Dielectric breakdown is caused by failure of the insulating properties of the dielectric films between the ground electrode and the electrolyte. Thermodynamic instability and relaxation appear to be related mechanisms for charge transfer between the electrolyte and the surface of the fluoropolymer, occurring at γWE e 0 and γWE > 0, respectively.
Dielectric Breakdown The insulator stack most carefully investigated in this experiment consisted of 11 nm of thermal SiO2, covered by a certain thickness of aFP. The complete breakdown of the dielectric stack causes direct current to flow from one electrode to the other; it is also accompanied by hydrolysis at the W/aFP interface. This is a signature behavior of breakdown since rapid hydrolysis does not occur in other saturation mechanisms. A complete breakdown of the dielectric stack occurred consistently at V ) 10 V, with taFP ) 150 nm and tox ) 11 nm. However, for a stack with taFP ) 20 nm and tox ) 11 nm, breakdown occurred more intermittently anywhere between 7 and 8 V. Films with and tox ) 30 nm did not show breakdown up to 25 V. Therefore, the dielectric breakdown is mostly (16) Quinn, A.; Sedev, R.; Ralston, J. J. Phys. Chem. B 2003, 107, 11631169. (17) Quinn, A.; Sedev, R.; Ralston, J. J. Phys. Chem. B 2005, 109, 62686275. (18) Seyrat, E.; Hayes, R. A. J. Appl. Phys. 2001, 90, 1383-1386.
Electrocapillary BehaVior of Electrolyte Droplets
Figure 3. Capacitance measurement for a propanol-based W-phase on a 36 nm aFP film. The left figure shows the acquired capacitance and droplet contact area data, while the right figure shows the calculated electrical aFP thickness. This thickness is equivalent to an aFP insulator thickness that would produce the measured capacitance, provided charge would exist at the very surface of the doped Si and W-phase. Any dodecane that exists between the W-phase and the aFP surface was counted as a part of the capacitive aFP thickness.
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Figure 4. DC contact angle data for SDS-based W-phases in dodecane. Three SDS concentrations were used, 3%, 0.1%, and 0.03 wt %. The 3% SDS W-phase was measured on 20 and 165 nm aFP films; the other W-phases were measured on the 20 nm aFP films only.
determined by the silicon dioxide thickness, which has a higher breakdown field. Several attempts were made at using fluoropolymer coats without any under layer, and such dielectric stacks were much more prone to breakdown at low voltage.
Capacitance Measurements The other two saturation mechanisms appear to be due to charge transfer. To investigate the location of charge during saturation, the capacitance and the contact area of the droplet were measured as a function of applied AC voltage amplitude. On the basis of these measurements, it was possible to calculate the mean distance between the top of the oxide dielectric and the average charge distribution in the W-phase by simply assuming that the total capacitance is caused by a known oxide capacitor in series with an aFP capacitor. Since the dielectric constant of dodecane is very close to that of aFP, any dodecane film that exists between the W-phase and the aFP is counted as a part of the electrical aFP thickness determined in this measurement. Figure 3 shows the electrical aFP thickness for a propanol-based W-phase for a voltage range that covers saturation, which occurs at 2.8 V for this system. The charge remains above or on the aFP film. If charge had penetrated the aFP to a significant distance on a 1 ms time scale, the electrical aFP thickness would be dip below the physical aFP thickness. It is possible to conclude that the reduction of useful electrocapillary charge visible in saturation, in the regime where dielectric breakdown does not occur, is due to charge transfer between the W-phase and the surface of the aFP film.
Thermodynamic Instability The electrocapillary effect lowers γWE with increasing voltage, eventually, provided that dielectric breakdown or relaxation processes do not dominate, γWE approaches 0. It has been argued by other research groups15-17 that saturation occurs in the regime where γWE e 0 due to the inherent instability of the interface. Data obtained in this study indicates that γWE e 0 is usually a sufficient condition for saturation. Figure 4 shows the DC contact angle data for several SDS W-phases in dodecane. The aFP thicknesses were 20 and 165 nm, and the SDS concentrations were 0.03, 1, and 3 wt %. Saturation occurred for each W-phase at a slightly different contact angle, but it was approximately symmetric for negative and positive bias. Best fit values for γWO, γzc WE, and Vzc at each concentration of SDS were obtained by fitting the model to the contact angle curve. The W-O surface energy,γWO, varied slightly with concentration, taking the values of 6.0, 4.6, and 3.6 mJ/m2
Figure 5. Surface energy between the electrode and the SDS-based W-phases as a function of DC voltage, values calculated from data shown in Figure 3. Saturation occurred near the condition where γWE ) 0, except when the dielectric breaks down. The surface energy γWE was calculated from modeled values of γWO, γzc WE, and Vzc and measured values of γOE, tox, and taFP using the Young’s equation.
as the concentration of SDS increased. The values for γzc WE were ∼2 mJ/m2 higher than their corresponding γWO values, indicating that γdWE = 2 mJ/m2. Using the extracted model parameters and the contact angle data, it was possible to calculate γWE for each bias point. Figure 5 shows the calculated γWE values for the three SDS concentrations. Saturation of γWE occurs very close to γWE ) 0 independent of SDS concentration, unambiguously supporting thermodynamic instability as the cause of electrocapillary saturation in this regime. The nature of how thermodynamic instability affects the aFP film is still under investigation. It appears that charge is rapidly transferred onto the surface of an aFP film from the W-phase when γWE e0. This likely occurs due to adsorption of ions from the W-phase to the aFP surface. Strong evidence of such a charge transfer has already been observed in previous studies.9 However, surface charging is not always limited to the condition where γWE e 0; for other W-phase compositions surface charging occurs at a variety of contact angles and surface energies, as described in the next section.
Relaxation Relaxation is a phenomenon that causes the contact angle to change over time even though the applied DC bias is held constant. Typically, relaxation of the electrocapillary effect on fluoropolymer films causes the contact angle to increase with time. A simple measure of relaxation is a plot of the contact angle with respect to time at a constant DC electrode bias. Figure 6 shows such a
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Figure 6. Relaxation behavior of a propanol-based W-droplet on a 20 nm aFP film. Relaxation of the negative 2.5 V step occurs faster than the positive, but relaxation is relatively fast in both cases with the droplet fully relaxing in ∼10 s.
Figure 8. DC contact angle data for propanol-based W-phases in dodecane. The propanol W-phase, 75% propanol by weight, was measured on aFP layers of thickness 165 and 20 nm. The modeled γWO energy was 2.6 mJ/m2.
Figure 7. Relaxation behavior of a propanol-based W-droplet on a 165 nm aFP film. Relaxation of the negative 7.5 V step occurs faster than the positive. Relaxation times are significantly slower than on the 20 nm film shown in Figure 5.
Figure 9. Surface energy between the electrode and the propanolbased W-phase as a function of DC voltage, values calculated from data shown in Figure 8. Saturation occurred asymmetrically for negative and positive excitations with γWE . 0 in each case. The surface energy, γWE, was calculated from modeled values of γWO, γzc WE, and Vzc and measured values of γOE, tox, and taFP using the Young’s equation.
plot for the propanol-based W-phase on a thin 20 nm aFP film. Contact angles relax so quickly that they never achieve their theoretical values even within the short measurement window of 0.3 s; subsequently, they relax fully to the original 0 V value of ∼160° even though the applied voltage is maintained. Relaxation appears to also be caused by charge transfer to the aFP surface, perhaps also through ionic adsorption. Although the mechanism may be similar, relaxation differs from thermodynamic instability in three important aspects, it occurs at γWE significantly above 0, it has a less abrupt dependence on voltage, and it generally proceeds more slowly. Like thermodynamic instability, relaxation can cause saturation. If the contact angle can relax on the time scale of the measurement, the contact angle will appear saturated. This can be clearly seen by comparing Figures 6-8. Figures 6 and 7 are relaxation plots of a propanolbased W-phase on two different aFP films, one with 20 nm thickness, and the other with 165 nm thickness. Finally, Figure 8 is the DC electrocapillary plot for the same films and phases. For all four saturation conditions, the contact angle to which the droplet relaxes in ∼2 s was approximately the contact angle of DC saturation. This is because each DC point measurement takes approximately 2 s, so if the contact angle can relax significantly on this time scale, it appears saturated. In Figure 9, the γWE is plotted after the relevant parameters are fitted to the electrocapillary data shown in Figure 8; it is clear that saturation due to relaxation occurs at energies well above 0 mJ/m2. It is possible to change the experimental time scale from 1 s to 1 ms by using a 1 kHz square wave as the excitation signal. Figure 10 shows the AC electrocapillary response of the thin, 20 nm aFP film from Figure 8, subject to a such a signal. Saturation
Figure 10. AC measurement of the propanol-based W-phase on the 20 nm aFP film. Relaxation-induced saturation occurs significantly later with a 1 kHz excitation than with a DC measurement where the relevant time scale is ∼1 s.
occurred later at 2.8 V and 85°, still below the thermodynamic instability limit, so it can be inferred that relaxation occurs at 1 ms time scale at this condition. Obviously, in order for thermodynamic instability to lead to electrocapillary saturation, relaxation must be slow enough not to cause saturation first. For SDS-based W-phases, relaxation is not significant at a 1 s time scale. Figures 11 and 12 show the relaxation behavior of SDS-based W-phases on the 20 and 165 nm aFP films used in the previous section, with relaxation times many orders of magnitude slower. The importance of surfactants in controlling and limiting surface charging rates is difficult to
Electrocapillary BehaVior of Electrolyte Droplets
Figure 11. Relaxation behavior of the 3 wt % SDS W-phase on a 20 nm aFP film. The relaxation time is significantly longer than with the propanol-based W-phase on the same film.
Figure 12. Relaxation behavior of the 3 wt % SDS W-phase on a 165 nm aFP film. Relaxation times are significantly longer than with the propanol-based W-phase on the same film. The positive excitation relaxes faster than the negative one.
overstate. The effect is dramatic, as can be seen by comparing Figures 6 and 11, which show relaxation data for the same film with different W-phases. For a negative voltage excitation, at a contact angle of ∼90°, with a 20 nm aFP film, the characteristic surface charging time is at least 6 orders of magnitude longer for the SDS-based W-phase than for the propanol-based W-phase.
Voltage of Zero Charge (Vzc) The electrocapillary maximum does not necessarily occur at zero applied voltage. Classic work for mercury-electrolyte interfaces shows nonzero electrocapillary maximums.19 However, up to now, a nonzero Vzc has not been observed for electrocapillary behavior on solid interfaces, mainly because contact angle changes at low voltages have been too small. In this experiment, a small positive Vzc was obtained for SDS W-phases. The observed Vzc is caused in part by the dipole formed between the layer of negatively charged SDS molecules and the screening Na+ ions on the surface of the W-phase. Figure 13 shows Vzc as a function of SDS W-O interface energy, as expected a correlation exists with higher surface concentrations of SDS leading to higher Vzc. However, the use of ionic surfactants is not the only way to change Vzc. Figure 14 shows the influence of a change in the ground electrode material from n-Si to Pt. Vzc changes by 0.5 V from 0 to -0.5 V with the use of a Pt ground electrode. Although the direction of the shift is consistent with the work function difference of the two materials, its magnitude is not, indicating that some Fermi-level pinning may be occurring, most likely due to charge transfer at the Pt-aFP interface. (19) Grahame, D. C. Chem. ReV. 1947, 41, 441.
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Figure 13. Voltage of zero charge, Vzc, plotted as a function of SDS W-phase, γWO energy. SDS concentration was varied from 3 to 0.03 wt %. Lower SDS concentration lead to higher γWO energy and lower Vzc, suggesting that an important component of Vzc for W-phases with ionic surfactants is the surface surfactant concentration.
Figure 14. DC electrocapillary data for a propanol-based W-phase on two different ground electrodes. The n-Si electrode shows a Vzc very close to 0, while the Pt ground electrode shifts the Vzc by half a volt. The direction of shift corresponds to the work function difference between the two materials, suggesting that material work function is a component of Vzc.
Conclusion Through the use of surfactants and an external insulating liquid, the electrocapillary effect on thin amorphous fluoropolymers was scaled to show more than 100° contact angle change at a voltage excitation of 4 V. Further gains are possible, at a constant electric field, by simultaneously reducing insulator thickness, voltage, and interface energy. By varying the concentrations and type of surface-active species in the water phase, insight has been gained into the nature of electrocapillary saturation. Three different conditions, each sufficient for saturation, were found in the experimental space examined: breakdown, thermodynamic interface instability, and relaxation. Breakdown is caused by a physical failure of the insulator and can be identified by the onset of hydrolysis due to direct current flow from the water phase into the electrode in the neighborhood of the failure. Limiting dielectric breakdown is possible by using a silicon oxide underlayer. Thermodynamic interface instability occurs when the interface energy between the water phase and the polymer reaches 0 mJ/m2. After this condition is met, further increases in voltage do not cause further changes in contact angle. The voltage condition for thermodynamic interface instability can be calculated from measured surface energy components. Relaxation causes the contact angle to change slowly with time even when the applied potential is held constant, typically relaxing back to its original value at V ) 0. If relaxation occurs sufficiently quickly on the experimental
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scale, it causes saturation. A strong dependence of saturation voltage on the frequency of the bias applied is an indicator of relaxation. Relaxation was found to be caused by charge transfer from the water phase to the surface of the fluoropolymer and strongly dependent on the type of surfactant used. For one condition, the characteristic charging time was increased by at least 6 orders of magnitude by using SDS as the surface active species. Finally, the voltage at which the electrocapillary contact angle maximum occurs can be modified by using ionic surfactants or by changing electrode materials. Proper engineering of surfaces and fluids involved in the electrocapillary behavior can significantly lower the voltages needed to drive microfluidic devices, expanding their potential base of platforms and enabling miniaturization. The parasitic effects that cause variation from theoretically predicted behavior can also be controlled to a very large degree by proper surface
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and fluid engineering, increasing microfluidic device reliability and lifetime. Acknowledgment. The authors thank the Advanced Concepts Committee at MIT’s Lincoln Laboratory for funding this study. Thanks also to Ted Fedynyshyn for his insight on molecular arrangements in thin polymer films. Many thanks to Rod Kunz and Prof. Behrouz Abedian for their helpful discussions. Thanks also to Craig Keast for his help with the purchase of the measurement equipment. This work was sponsored by the United States Air Force under Contract No. FA8721-05-C-002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government. LA060204E
