Electrowetting based microliter drop tensiometer Arun G. Banpurkar* Kevin Nichols and Frieder Mugele Physics of Complex Fluids, Faculty of Science and Technology, IMPACT and MESA+ Institute, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands * Also at: Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune 411 007, India We use electrowetting on dielectric for rapid characterization of interfacial tension of conductive liquids. A microliter drop placed on dielectric beneath a planar electrode and the change in sessile drop contact angle was measured as a function of continuous ac voltage amplitude ramps. The liquids including surfactant laden drops as well as aqueous solutions of protein and gelatin were investigated with interfacial tensions ranging between 5-72 mnm -1. For all liquids, we find excellent agreement between the interfacial tensions obtained from electrowetting tensiometer and a conventional macroscopic tensiometer. In addition, we demonstrate the possibility of performing contact-less measurements without any loss of accuracy using interdigitated coplanar electrodes. 1
Interfacial tension plays a key role in the manipulation of liquid droplets through the microfluidic and lab-on-chip (LOC) devices. Aqueous solutions such as, buffer, surfactant, bio-molecules and body fluids are integral to these fluidic and micro total analysis systems (µtas) [see Ref. (1,2) and reference therein]. Interfacial tension, γ defined as surface energy per unit area of the interface thus governs interface dynamics, surface wetability, detergency, and emulsification 3. Their precise measurement therefore becomes central to several such scientific and technological applications. Static and dynamic measurements of γ can be realized using classical 3 and recently developed microfluidic based tensiometers 4,5. However, in above systems as well as in pharmaceuticals and forensic examination desires a minimal uses of the reagents in determination of γ, which motivates newer methods in the microdrop tensiometry. In this letter we describe a simple means to rapidly measure γ using generic technique so called electrowetting on dielectric (EWOD). This method utilizes about ~0.5-2 µl of conductive liquid. The change in sessile drop contact angle was optically measured as a function of external potential between drops and planar electrode beneath a dielectric layer. In the recent past, electrowetting (EW) has been effectively used for both fundamental experiments on wetting studies 6-9 and variety of technological applications including fluid transport and mixing in LOC devices 1,2,10-12, liquid lenses 13 displays 14. The EW on dielectric is a variant of classical electro-capillarity phenomenon by Lippmann 15. When a partially wetting liquid drop was deposited on the plane dielectric substrate forms spherical cap with equilibrium Young s contact angle θ Y related to various interfacial energies: Y ( γ sv γ sl ) γ cos θ =, where, γ IJ, designates interfacial tension between the phases I and J 16. If external electric potential is applied (see Fig. 1(a)) between conductive drop and a planar electrode underneath dielectric layer then due to charging of capacitor there is gain in electrostatic energy 6. Effective solid-liquid interface energy is thus decreases, which is given by, γ eff sl C ( U ) = γ sl U 2 2. Here, C is capacitance per unit area of substrate and depends on dielectric permittivity (ε 0 ε) and thickness (d) of dielectric layer and the geometry of electrode. For a homogeneous planar electrode, it reduces to the familiar parallel plate capacitor formula C = ε 0 ε /d. A macroscopic contact angle θ(u) (Fig 1 (a) line profile) of the sessile drop is then given by Lippmann-Young s equation: cosθ ( U ) cos + C U 2 = θy, where, θ(u) is apparent 2 γ macroscopic contact angel measured on the scale of drop size 17. This equation is found to 2
hold true as long as voltage is not too high and also valid for dc and low frequency ac voltages 6. The frequency range < 0.1-30 khz for ac field are so chosen that perfect conductor model holds such that electric fields are completely screened from the interior of the liquid drop and does not contribute to electrophoretic body force 18,19. It can be learned from Lippmann-Young s equation that cosine of contact angle θ(u) varies linearly with U 2 with constant slope, m = C 2γ depend only on substrate capacitance C and interface energy γ of the liquid with respect to their surrounding environment. Therefore from EW response of sessile drop of known γ, the dielectric substrate capacitance C can be determined without difficulty. Subsequently such substrates could be employed for EW response of aqueous conducting liquids drop to assess their interfacial tension. In our study, we used two kinds of electrode geometries, the generic one, which requires the wire in the drop 6, and wire-free one with interdigitated patterned electrodes. The generic setup (Fig. 1(a)) comprises a planar bottom electrode, made of conductive indium tin oxide (ITO) layer on a cover glass plate. To get dielectric layer ITO surface was coated to a desired uniform thickness of about 4.5 µm using 6% Teflon AF (1600) amorphous fluoropolymer solution following a standard recipe 20. Most of the aqueous solutions partially wet to the Teflon surface with equilibrium Young s angle θ Y ~ 170-160 O and 120-110 O in oil and air environment, respectively. In the second kind of experiments, interdigitated patterned electrodes were used as alternative geometry (see Fig.1 (b)). An ac voltage is applied between electrodes underneath a dielectric layer makes contact free measurement. Herein conductive liquid drop assumes an intermediate potential through capacitive coupling between liquid and electrodes. Fabrication of interdigitated electrodes invoed a deposition of thin conducting layer of molybdenum on glass substrates using physical vapor deposition. Electrodes were then dry etched in a desired pattern (pitch = 30 and electrode width = 22.5 µm), further whole surface was spin coated using 6.1 µm thick SU8 dielectric subsequently, 10 diluted Teflon AF (1600) solutions was spin coated to achieve the desired hydrophobicity. 3
Pt wire (a) (b) U U θ U θ Y ITO Glass substrate Teflon FIG. 1. (color online) (a) A generic EWOD set-up. Platinum wire plunge into conductive liquid drop and connected to ac potential as shown. Sessile drop contact angle (sold profile; U= 0) and at applied voltage U (line profile) is also shown (b) EWOD for interdigitated patterned electrode. An ac potential is applied across the insulated electrodes and contact angle was measured by side view along electrode direction (arrow). We used analytical grade surfactants to prepare aqueous solutions in de-ionized water and their conductivity was increased to about ~ 2.5 ms/cm by adding NaCl salt. Mineral oil (Sigma; µ 0 30 mpas), silicone oil (Wacker AK5; µ 0 5 mpas), n-hexadecane (J T Baker; µ 0 3 mpas) were used as ambient media. Aqueous drop of volume 0.5-2 µl using micropipette was gently deposited on to a planar dielectric substrate placed in cubic cell filled with immiscible oil. Platinum (Pt) wire (radius=25 µm) cleaned on alcohol flame to avoid contaminations was plunged into the drop. An ac voltage amplitude ramp from oscillatoramplifier (U = 0-110 V rms and f =10 khz; ramp period τ = 200 sec) was applied as shown in Fig. 1(a). Real time contact angle θ(u) of a sessile drop was measured using optical contact angle goniometer (OCA-15+, Data Physics, Germany) with built-in SCA-20 software 21. Interfacial tension, γ for all the test solution using Du noüy ring was also measured on bulk tensiometer (K-11, Krüss, Germany) at constant temperature of 23 o C. We covered wide range of aqueous solution to demonstrate the potential of EW based tensiometer. At the outset we observed EW response for water drop in silicone oil environments for which γ = 38 mnm -1. Figure 2 show plot of variation in contact angle vs U 2. From the linear regression fit to this data points the slope m, was obtained and using the expression: m = C 2γ, normal value of substrate capacitance was found to be C=4.11 µfm - 2. Subsequently, EW response for different aqueous interfaces using the above dielectric substrate was observed and variation in sessile drop contact angles with applied voltages amplitude U 2 were plotted in Fig. 2. Now from data points corresponding to each interface, slope value was obtained from linear regression fit. Substituting capacitance value the above equation is soed to get interfacial tension γ for the corresponding aqueous interfaces which are listed in Table I. Notable interfacial tension of water with their vapor was obtained by 4
captive air bubble tenchnique 12. Air bubble was trapped at water-substrate interface and potential is applied between surrounding water and planar electrode and contact angle was measured inside water phase between tangent at three phase contact line and flat substrate. 1.6 aqua. glycerin/mineral oil H 2 O/silicone Oil aqua. glycerine/silicone oil aqua gelatine/mineral oil aqua Bioprotein/silicone oil milk/silicone oil aqua SDS/silicone oil aqua CTAB/ mineral oil aqua. Triton-X/silicone oil H 2 O-n-Hexadecane cos θ(u)-cos θ Y 1.2 0.8 0.4 0.0 2000 4000 6000 8000 10000 12000 U 2 (V 2 ) FIG. 2: (color online) Electrowetting response for various aqueous/oil interfaces on planar substrate. Linear regression fit to the corresponding experimental data points shown by line. TABLE I. Interfacial tension of aqueous solution obtained using EW based tensiometer on both planar and interdigitated patterned electrodes along with bulk tensiometer result. Liquid Interface (Temperature = 23 o C) γ (EW drop tensiometer (mn/m) γ (Du noüy ring) Planar Interdigitated (mn/m) Electrode Electrode H 2 O/silicone oil Calibration Calibration 38± 0.2 H 2 O/ mineral oil 48.8± 0.5 47.3± 0.7 48.0± 0.2 H 2 O/ air 72.4 ±0.6 -- 72.6± 0.5 H 2 O/ n-hexadecane 52.9±0.7 48.5±0.6 53.3±0.5 Aqu. Gelatin (2 % w)/ mineral oil 24.1±0.6 24.8±0.7 23.3 ± 0.4 Aqu. Gelatin (2 %w)/silicone oil 20.1±0.8 20.4±0.8 20.2 ± 0.5 Aqu. SDS (0.7 %w)/ silicone oil 7.5± 0.3 7.5± 0.8 7.0± 0.4 Aqu. CTAB (0.1 %w)/mineral oil 6.5± 0.5 7.5± 0.8 7.1 ± 0.4 Aqu. Triton X-100 (0.1 %w)/ silicone oil 4.7±0.3 5.6±0.5 4.4 ± 0.4 Aqu. Glycerin (50% w)/ silicone oil 32.8±0.8 35.0±0.7 33.0 ± 0.5 Milk / silicone oil 19.4±0.7 18.6±0.9 17.9± 0.1 Aqu. Bio protein (1% w)/ silicone oil 20.0±0.5 15.7±0.7 18.7± 1.0 5
cos θ(u)-cos θ Y 0.6 0.5 0.4 0.3 0.2 0.1 H 2 O-si. oil aqua CTAB-si oil milk-si. oil aqua. gelatin-si. oil bioprotein-si. oil aqua. SDS-si. oil aqua. Triton-si. oil aqua. glycerine-si oil 0.0 0 2000 4000 6000 8000 10000 12000 FIG. 3: (color online) Electrowetting response for various aqueous/oil interfaces on interdigitated pattern electrode. The linear regime was used to Lippmann-Yong s model to get effective capacitance and γ. U 2 (V 2 ) The Pt wire contact to the drop in the generic EW may be the potential source of contamination, especially for bio fluids. In the second set of experiment, we demonstrate contact less EW using interdigitated patterned substrate. While drop was deposited on to surface, an ac voltage was applied across interdigitated electrodes and EW response was recorded for a continuous voltage amplitude ramp (U = 0-110 V rms ; τ =200 sec). Figure 3 shows EW response for sessile drop of different liquids (Table I). In general a linear behavior confirms Lippmann-Young type variation in contact angle with U 2 despite the complex (and in detail not known) distribution of the electric field. The plot does reveal a series of discrete jumps though, which reflect discontinuous displacements of the contact line as the drop switches between metastable configurations covering progressively more (increasing U) or less (decreasing U) electrode stripes. For drops covering initially at least 10 electrode stripes, however, the overall linear behavior is preserved and the interfacial tensions for liquid interfaces were extracted as follow: EW response for 0.5-2 µl water drop in silicone oil (γ = 38 mnm -1 ) was observed. Effective capacitance of substrate was evaluated using m eff = C 2γ which is found to about C eff = 0.678 µfm -2. From slope value obtained by linear regression fit to corresponding data points (Fig. 3) and using: m eff = C 2γ, the γ were obtained and are listed in Table I. Thus we presented the measurement of γ ranging from 5-72 mnm -1 for simple to complex aqueous solutions like milk. Smaller value of γ can always be studied as it shows larger contact angle variation for relatively low voltage. On the other hand maximum 6
measurable γ is mainly decided measurable in the range of dielectric breakdown field. The γ values using EW tensiometer in both planar and interdigitated patterned electrode geometries are in good agreement to those obtained on standard macroscopic tensiometer and the published data 22. The measurement accuracy and precession ranges from 2 to 7 % for simple and complex solution, respectively. In addition EW based measurement derived from average slope value over different voltage amplitude cycles on the same drop hence facilitates accurate γ values. In conclusion we demonstrate electrowetting as dominant tool for the rapid characterization of interfacial tension of conductive liquids with remarkable accuracy. In this electrowetting experiment, macroscopic variation in shape and contact angle of sessile droplet as a function of applied ac voltage amplitude was employed to estimate static interfacial tension. These measurements also work equally well using interdigitated patterned substrates with no direct electrical connection to test droplet. AGB acknowledges the BOYSCAST program from Indian government, Siva Vanapalli for comments on ms. 7
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