Langmuir 2008, 24, 10549-10551
10549
Electrowetting-Based Microdrop Tensiometer Arun G. Banpurkar,†,§ Kevin P. Nichols,‡ and Frieder Mugele*,† Physics of Complex Fluids and Mesoscale, MESA+ Institute for Nanotechnology, and Institute for Mechanics Process and Control Twente (IMPACT), UniVersity of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands, and Center for AdVanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, UniVersity of Pune, Pune-411 007, India ReceiVed May 20, 2008. ReVised Manuscript ReceiVed August 6, 2008 We performed electrowetting (EW) contact angle measurements to determine the interfacial tension between aqueous drops laden with various inorganic and organic solutes and various ambient oils. Using low frequency AC voltage, we obtained interfacial tensions from 5 to 72 mJ/m2, in close agreement with macroscopic tensiometry for drop volumes between 20 and 2000 nL. In addition to the conventional EW geometry, we demonstrate the possibility of performing “contact-less” measurements without any loss of accuracy using interdigitated coplanar electrodes.
1. Introduction Interfacial tensions play a key role in the generation, manipulation, and stability of liquid droplets in various applications including lab-on-chip (LoC) devices, emulsification, foams, dairy products, cosmetics, and pharmaceuticals.1-3 Conventionally, interfacial tensions in such systems, which are characterized by the presence of buffer, surfactants, and biomolecules, are measured using macroscopic techniques such as Wilhelmy plate, Du Nou¨y ring, pendant drop, and so forth.4 If, however, only small amounts of precious solutions are available, these techniques are not suitable. Few attempts are reported in the literature to measure interfacial tensions for drop volumes compatible with amounts of sample typically available in microfluidic applications.5-7 All of these approaches are based on microfluidic channels into which the liquid of interest has to be injected. In this letter, we describe a simple technique to measure the interfacial tension γ between immiscible liquids based on the electrowetting (EW) effect (for a review, see ref 8). Monitoring the voltage-dependent decrease of the contact angle of sessile drops, we determine interfacial tensions for various combinations of liquids from the decrease in contact angle with increasing voltage. For drop volumes ranging from 20 to 2000 nL and drops containing various solutes including surfactants as well as bioproteins, we measure interfacial tensions between 5 and 72 mN/m, in good agreement with conventional macroscopic measurements. To avoid direct contact with a wire (as required in the generic EW configuration), we also present measurements * To whom correspondence should be addressed. E-mail: f.mugele@ utwente.nl. † Physics of Complex Fluids, MESA+, and IMPACT, University of Twente. ‡ Mesoscale, MESA+, and IMPACT, University of Twente. § University of Pune.
(1) Teh, S. Y.; Lin, R.; Hung, L. H.; Lee, A. P. Lab Chip 2008, 8, 198–220. (2) Song, H.; Chen, D. L.; Ismagilov, R. F. Angew. Chem., Int. Ed. 2006, 45, 7336–7356. (3) Fair, R. B. Microfluidics Nanofluidics 2007, 3, 245–281. (4) Lyklema, J. Fundamentals of Interface and Colloid Science, Vol. III: Liquid fluid interfaces; Academic Press: San Diego, 2000. (5) Hudson, S. D.; Cabral, A. T.; Goodrum, W. J.; Beers, K. L. Appl. Phys. Lett. 2005, 87, 081905. (6) Nguyen, N. T.; Lassemono, S.; Chollet, F. A.; Yang, C. IEE Proc.: Nanobiotechnol. 2006, 153, 102–106. (7) Liu, J.; Li, H.; Lin, J. M. Anal. Chem. 2007, 79, 371–377. (8) Mugele, F.; Baret, J.-C. J. Phys.: Condens. Matter 2005, 17, R705–R774. (9) Seyrat, E.; Hayes, R. A. J. Appl. Phys. 2001, 90, 1383–1386.
Figure 1. (a) Schematic showing a conventional EW setup, with a platinum wire plunged into a conductive liquid drop and connected to external potential. The side-view of a sessile drop at U ) 0 and at a finite voltage are depicted as filled and a line profile, respectively. (b) Top view of the alternative wire-free configuration with a substrate with embedded interdigitated electrodes.
Figure 2. ∆ cos θ versus U 2/γwo for several systems with interfacial tensions between 5 and 38 mJ/m2. Inset: raw data (cos θ versus U 2) for the same measurements as in the main panel.
using coplanar substrates with buried interdigitated electrodes (IDE).
2. Experimental Section We used two kinds of electrode geometries for studying EW. The first kind, the generic EW geometry shown in Figure 1a, consists of a planar electrode uniformly coated with a dielectric layer. For the present experiments, we used a conductive indium tin oxide (ITO) layer on a microscopic cover glass plate. The ITO surface was covered with an approximately 4 µm thick layer of Teflon AF (1600) (Dupont) by dip coating from a 6% solution following a standard recipe.9 For the second kind of geometry, we patterned the ITO
10.1021/la801549p CCC: $40.75 2008 American Chemical Society Published on Web 08/23/2008
10550 Langmuir, Vol. 24, No. 19, 2008
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Table 1. Interfacial Tension for Aqueous/Oil Obtained Using an EW-Based Tensiometer on Both Planar Electrodes and IDEsa γwo (EW drop tensiometer) (mN/m)
a
liquid interface (T ) 23 °C)
planar electrode
interdigitated electrode
γwo (Du Nou¨y ring) (mN/m)
H2O/silicone oil (θY ) 169°) H2O/ mineral oil (168°) H2O/air (62°) H2O/n-hexadecane (169°) aq gelatin (2%)/mineral oil (160°) aq gelatin (2%)/silicone oil (167°) aq SDS (0.7%)/silicone oil (165°) aq CTAB (0.1%)/mineral oil (164°) aq Triton X-100 (0.1%)/silicone oil (163°) aq glycerin (50%)/silicone oil (169°) milk/silicone oil (167°) aq bioprotein (1%)/silicone oil (167°)
calibration 48.8 ( 0.5 72.4 ( 0.6 52.9 ( 0.7 24.1 ( 0.6 20.1 ( 0.8 7.5 ( 0.3 6.5 ( 0.5 4.7 ( 0.3 32.8 ( 0.8 19.4 ( 0.7 20.0 ( 0.5
calibration 47.3 ( 0.7 72.0 ( 0.2 48.5 ( 0.6 24.8 ( 0.7 20.4 ( 0.8 7.5 ( 0.8 7.5 ( 0.8 5.6 ( 0.5 35.0 ( 0.7 18.6 ( 0.9 15.7 ( 0.7
38 ( 0.2 48.0 ( 0.2 72.6 ( 0.5 53.3 ( 0.5 23.3 ( 0.4 20.2 ( 0.5 7.0 ( 0.4 7.1 ( 0.4 4.4 ( 0.4 33.0 ( 0.5 17.9 ( 0.1 18.7 ( 1.0
Young’s contact angle θY is given in brackets. The γwo values from bulk tensiometry are also listed.
layer into IDEs, as schematically shown in Figure 1b, using standard photolithography in combination with ion beam etching. The present IDEs had an electrode width w ) 22.5 µm and a pitch of 30 µm. For these samples, a 6.1 µm thick layer of SU8 was spin-coated to produce a uniform dielectric layer, which was then covered with a thin spin-coated Teflon AF (1600) layer to attain the desired hydrophobicity and chemical inertness. Aqueous solutions in deionized water were prepared using analytical grade reagents in a weight percentage. NaCl (0.1 mM) was added to the drop to increase the conductivity to about 2.5 mS/cm. Immiscible oils such as mineral (Sigma; surface tension, γ ) 22.3 mN/m; viscosity, µ0 ≈ 30 mPa s), silicone (Wacker AK5; γ ) 19.2 mN/m; µ0 ≈ 5 mPa s), and n-hexadecane (J.T. Baker; γ ) 27.5 mN/m, µ0 ≈ 3 mPa s) were used as the ambient phase. The aqueous drops were gently placed on the substrate once the system was submerged in the ambient oil. For the standard EW configuration, a platinum (Pt) wire (radius r ) 25 µm) was cleaned in a flame and plunged into the drop to provide an electrical connection. The contact angle θ(U) was measured as a function of the applied voltage amplitude ramp (U ) 0-110 Vrms, frequency ) 10 kHz, and ramp frequency ) 0.02-0.005 Hz) using an optical contact angle goniometer (OCA-15+, Data Physics, Germany) with built-in image analysis software. For comparison, we determined the interfacial tensions of all liquid combinations also with a commercial tensiometer (K-11, Kru¨ss, Germany) using the standard Du Nou¨y ring method.
3. Results and Discussion We characterized conductive aqueous drops laden with surfactant, gelatin, bioprotein, and so forth in different ambient oils to demonstrate the potential of EW-based tensiometry. All drops display the expected decrease in the apparent contact angle θ(U) upon increasing the amplitude of the AC voltage U. In the inset of Figure 2, we show for a few examples that cos θ(U) follows the generic EW behavior described by the electrowetting equation,
cos θ(U) ) cos θY +
C 2 U 2γwo
(1)
for all systems (except for a slight negative curvature toward high voltage that we will disregard in the following). The decrease in contact angle is thus determined by the dimensionless quantity (CU 2/2)/γwo, frequently denoted as the electrowetting number, which measures the ratio between the electrostatic energy per unit area (of the drop substrate interface) and the water-oil interfacial tension γwo.8 Here, the capacitance C per unit area of planar substrate depends on the dielectric permittivity (ε0ε) and thickness (d) of the dielectric layer and the geometry of the electrode. For a homogeneous planar electrode, it reduces to the familiar parallel plate capacitor formula C ) ε0ε/d. (In principle, an additional contribution to the capacitance, in series with the
dielectric layer, can arise from thin oil films that can be entrapped between the dielectric layer and the aqueous drops.3,10 For the slow ramp times investigated here, this contribution turns out to be negligible.) Note that Young’s angle θY (U ) 0) is always very high (close to 170°) for such oil-aqueous systems, irrespective of the specific solutes. This is convenient from a practical perspective, since it gives rise to a particularly large tuning range of the contact angle. From the inset of Figure 2, we can see that the slope R of the EW curves is the smallest for water, which has the highest interfacial tension, and the largest for the Triton-X surfactant solution, which has the lowest interfacial tension, as observed also in earlier EW studies on the influence of surfactants.11,12 All curves are very reproducible over hundreds of ramping cycles and display a hysteresis smaller than the symbol size in Figure 2. Since all liquids display the same generic voltage-dependence according to eq 1, we can rescale all the data on top of each other using the macroscopically measured interfacial tensions. As shown in the main panel of Figure 2, the variation of the cosine of the contact angle, ∆ cos θ ) cos θ(U) - cos θY, plotted versus U 2/γwo indeed collapses to a single master curve with a unique slope that depends only on the drop-substrate capacitance C. The absolute values of the interfacial tensions are readily extracted from the values of R once C is known. While C can be determined in a variety of independent ways (for the planar electrode geometry for instance by simply measuring the thickness of the insulator), in practice it turned out that it is determined most easily and reliably in a calibration measurement with a liquid combination of known interfacial tension, such as (salt) water-silicone oil (γwo ) 38 mN/m). In the present experiments, this procedure yielded typical values of C ) 3.2-4.0 µF/m2, depending on the dielectric thickness of the specific sample. Obviously, this calibration procedure is also applicable for the IDEs, for which a calculation of the capacitance based on the thickness and the electrode geometry is not straightforward. Having determined C for a given substrate, the interfacial tensions are determined from the slopes Ri of the EW curves for small voltage: (γwo)i ) C/2Ri. The results of all measurements performed are listed in Table 1. The largest deviations between the EWbased value and the macroscopic measurements are observed for milk and for bioprotein (∼8%). For the other systems, the deviation is typically less than 5%. In addition to the various (10) Staicu, A.; Mugele, F. Phys. ReV. Lett. 2006, 97, 167801. (11) Raccurt, O.; Berthier, J.; Clementz, P.; Borella, M.; Plissonnier, M. J. Micromech. Microeng. 2007, 17, 2217–2223. (12) Berry, S.; Kedzierski, J.; Abedian, B. J. Colloid Interface Sci. 2006, 303, 517–524. (13) Zhao, Y. J.; Cho, S. K. Lab Chip 2007, 7, 273–280.
Letters
oil-water systems, we also measured the water-air surface tension using a captive air bubble method as in ref 13, again in good agreement with the expectations. To avoid the usage of the wire, which may be a source of contamination, we performed also measurements using the IDE sample (Figure 1b). When a voltage is applied between the electrodes, the drop assumes an intermediate potential through capacitive coupling, implying that a higher voltage has to be applied to achieve the same variation of θ(U), because the drop-substrate capacitance is reduced. Except for this effect, the EW curves display the same behavior as those for the conventional geometry. It is worth noting that the drop should cover several periods of the electrode pattern to obtain a reliable measurement. For the present, relatively wide electrodes, this limits the minimum drop size to approximately 20 nL (which is still smaller than what could be achieved with an immersed wire of 50 µm diameter). Furthermore, one should note that the IDE substrate is anisotropic. As a consequence, the EW curves look slightly different when viewed along the stripe electrodes (y-direction) and perpendicular to it (x-direction) as shown in Figure 3. While the overall slope is almost the same, the curves obtained from views along the electrodes contain a number of consecutive steps. These steps are due to pinning of the three phase contact line along the (effectively more hydrophobic) gaps between the electrodes. At each of the steps, the contact line jumps from one gap to the next.14 This artifact can be avoided by measuring approximately perpendicular to the electrodes. Along that direction, the contact line can move freely and hence the equilibrium contact angle can be measured reliably. With these precautions in mind, we repeated the measurements for all the systems investigated before. Again, we calculated the effective capacitance from a calibration measurement with a water-silicone oil drop of known interfacial tension, and we extracted the interfacial tensions of all other liquids from the (14) Brinkmann, M.; Lipowsky, R. J. Appl. Phys. 2002, 92, 4296–4306.
Langmuir, Vol. 24, No. 19, 2008 10551
Figure 3. ∆ cos θ versus U 2 for water in silicone oil on IDE substrate. The black dotted (red solid) line refers to approximate measurements performed along the viewing direction indicated by the black dotted (red solid) arrow, that is, the along the x (y)-direction. The images show overlaid snapshots of a small (20 nL) and large (500 nL) surfactantladen drop at zero and at high voltage, respectively, free of any distortion by an immersed wire.
slopes R at small voltage. The results listed in Table 1 display the same accuracy as those for the conventional EW geometry. In conclusion, we demonstrated that electrowetting is a useful tool for rapid, quantitative measurements of interfacial tensions for nanoliter sample volumes. The technique can be easily implemented in standard contact angle goniometry setups without requiring additional microfluidic channels. Furthermore, it will be straightforward to extend the method to dynamic surface tension measurements, which will be limited by the hydrodynamic response time of the system. Acknowledgment. A.G.B. acknowledges the BOYSCAST program from the Indian government for financial support. We thank Siva Vanapalli for comments on the manuscript. LA801549P
