Deborah J.
Lomax
a,
Pallav
Kant
b,
Aled T.
Williams
a,
Hollie V.
Patten
a,
Yuqin
Zou
a,
Anne
Juel
b and
Robert A. W.
Dryfe
*a
aSchool of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK. E-mail: robert.dryfe@manchester.ac.uk
bMCND and School of Physics & Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
First published on 22nd September 2016
The control of wetting behaviour underpins a variety of important applications from lubrication to microdroplet manipulation. Electrowetting is a powerful method to achieve external wetting control, by exploiting the potential-dependence of the liquid contact angle with respect to a solid substrate. Addition of a dielectric film to the surface of the substrate, which insulates the electrode from the liquid thereby suppressing electrolysis, has led to technological advances such as variable focal-length liquid lenses, electronic paper and the actuation of droplets in lab-on-a-chip devices. The presence of the dielectric, however, necessitates the use of large bias voltages (frequently in the 10–100 V range). Here we describe a simple, dielectric-free approach to electrowetting using the basal plane of graphite as the conducting substrate: unprecedented changes in contact angle for ultra-low voltages are seen below the electrolysis threshold (50° with 1 V for a droplet in air, and 100° with 1.5 V for a droplet immersed in hexadecane), which are shown to be reproducible, stable over 100 s of cycles and free of hysteresis. Our results dispel conventional wisdom that reversible, hysteresis-free electrowetting can only be achieved on solid substrates with the use of a dielectric. This work paves the way for the development of a new generation of efficient electrowetting devices using advanced materials such as graphene and monolayer MoS2.
Herein we return to the canonical droplet configuration pioneered by Frumkin to demonstrate a robust and versatile approach to reversible and hysteresis-free EWOC, see Fig. 1(a), which does not require the application of an alternating voltage or voltage pulses to overcome hysteresis. Reversible wetting is shown to occur on a laminar conductor, the basal plane of highly oriented pyrolytic graphite (HOPG). HOPG can be readily refreshed by mechanical cleavage, it possesses macroscopic (mm scale) lateral domains containing only microscopic (sub-micron scale) steps21 and a high equilibrium contact angle (CA) for aqueous droplets (ca. 64° for water in air, with considerably higher values for aqueous solutions immersed in organic phases),22,23 from which EWOC can be performed. We note that low-voltage EWOC on graphite has recently been reported,24 although in this case, the process was dependent on ion-intercalation, i.e. wetting involved graphitic edges (as opposed to the basal-plane), hence the notable contact angle hysteresis observed.
A range of aqueous electrolyte solutions were prepared for liquid/air electrowetting: ≤6 M LiCl, 3 M MgCl2, 3 M CsCl, 3 M LiOH, 3 M KOH, ≤3 M KF (all supplied by Sigma Aldrich) and 3 M KCl (Fluka). All solutions were made using ultrapure water (18.2 MΩ cm, “PURELAB” Ultrafiltration system, Elga Process Water).
Immiscible phases used in the liquid/liquid experiments were hexadecane (99%, Sigma Aldrich) or 1,2-dichlorobenzene (DCB, 99%, Aldrich). The latter was used to prepare organic electrolyte solutions of 0.02 M bis(triphenylphosphoranylidene)ammonium tetrakis(4-chlorophenyl)borate (BTPPATPBCl). The synthesis of the BTPPATPBCl was adapted from the published procedure,25 involving metathesis of bis(triphenylphosphoranylidene)ammonium chloride (Aldrich) and potassium tetrakis(4-chlorophenyl)borate (Sigma Aldrich) in 2:1:1 acetone:ethanol:water, followed by recrystallisation in 1:1 acetone:ethanol.26
Electrowetting was performed with an Autolab PGSTAT302N potentiostat (Metrohm Autolab, Utrecht, Netherlands) using a three electrode set-up, where the HOPG acted as the working electrode.
In the standard electrowetting configuration, droplets were deposited on the HOPG using a microinjector (PV820 Pneumatic PicoPump) to expel the electrolyte solution from a micropipette (drawn from borosilicate capillaries with a Sutter P-97 Flaming/Brown Micropipette Puller to give a tip with inner and outer diameters of approximately 0.5 microns and 2 microns, respectively). Where necessary to prevent evaporation, humid conditions were maintained by placing the HOPG within a glass cell containing a pool of ultra-pure water.
Both the HOPG and the micropipette were controlled using manual micro-positioners, so that the micropipette could be brought in close proximity to the surface and the smoothest regions of HOPG could be targeted. The micropipette also served as the electrolyte reservoir within which the auxiliary electrodes were located, ≈3 cm from the HOPG surface. These comprised of a Pt wire counter electrode (0.10 mm diameter, 99.99+%, Advent Research Materials) and a partially exposed polyester-insulated Pt wire pseudo-reference electrode (0.125 mm diameter, 99.99%, Goodfellow Cambridge Ltd, UK).
For the liquid/liquid configuration, the HOPG was immersed within the surrounding phase in a glass cell, followed by droplet placement with the microinjector. This configuration also allowed pipette-free electrowetting with application of the potential through the surrounding electrolyte phase, using a Pt mesh counter electrode (52 mesh per inch, 99.9%, Advent Research Materials) and Pt wire pseudo-reference electrode (0.3 mm diameter, 99.99%, Advent Research Materials).
The droplet shape during electrowetting was determined from side-on images primarily captured using a CCD camera (Infinity, Lumenera) with the droplet backlit using an LED light source. High speed imaging was performed at 104 frames per second (Fastcam SA3, Photron Ltd, Tokyo, Japan), coupled with a xenon light source (Xenon Nova 300, Karl Storz GmbH, Tuttlingen, Germany).
Contact angle values were extracted from the images using the gradient of the droplet edge near the contact line, following the methodology of Neumann et al.27,28 The images were processed with MATLAB™ (MathWorks Inc., Natick, MA, USA) to first perform background subtraction and then to find the droplet edge using the in-built Canny edge detection algorithm. The contact angle was extracted from the arcs representing the droplet edge near the contact line, implemented by fitting a 4th order polynomial to the Canny-determined edge. Calculation of θ then followed from the derivative of the polynomial at z = 0 where z is the distance from the surface, i.e. from the gradient at the surface:
(1) |
Electrical impedance spectroscopy (EIS) was performed with 6 M LiCl over a frequency range of 10–105 Hz. The effective capacitance (Ceff) was determined using the method advocated by Tribollet et al. from graphical analysis for capacitance with a constant phase element contribution (when the exponent α ≠ 1).29
A value for α was calculated by performing a linear fit to a plot of the log of imaginary impedance (Zj) (Ohm) vs. the log of frequency (f) (Hz):
(2) |
(3) |
The surface tension of the 6 M LiCl(aq.)/air interface was measured using an Attension Theta Lite Optical Tensiometer (Biolin Scientific AB, Stockholm, Sweden) with analysis performed with the corresponding OneAttension software: a value of 83.3 ± 0.1 mN m−1 was measured at 295 K.
Atomic force microscopy (AFM) was performed in PeakForce QNM tapping mode with a Multimode8 (Bruker) using silicon nitride SNL-10 cantilevers. Image analysis was performed with Nanoscope Analysis (v1.6, Bruker). All images were processed using the 2nd order “Flatten” procedure before analysis using the “Section” tool to determine step heights and the “Roughness” tool to find Rq and Ra, the root-mean-square (RMS) roughness and mean roughness, respectively, where z is the feature height and N is the number of measured features:
(4) |
(5) |
Fig. 2 (A) Change in apparent CA for E ≥ Epzc, showing averages and standard deviations of between 5 and 23 experiments. Solutions with equivalent chloride concentration show similar wetting. (B) Percentage change in the droplet footprint with E, including droplets with a fourfold variation in initial diameter. (C) Cosine of CA versus electrowetting number for the data presented in (A). Inset: same plot with potential correction for the space-charge region within the graphite in η. (see Experimental and ESI,† Fig. S2 for details of capacitance and surface tension measurements). (D) Effect of electrolyte identity on EWOC: the change in CA with potential (relative to the PZC of each electrolyte) for a variety of concentrated (3 M) aqueous electrolytes: LiCl, LiOH, KCl, CsCl and KF. |
Classical electrowetting theory describes the dependence of the cosine of θ on the applied potential, known as the Young–Lippmann equation:12,13
(6) |
A direct comparison of liquid/air EWOC with eqn (6) is presented in Fig. 2(C) by plotting the cosine of the CA data from Fig. 2(A) against η. Note that the interfacial capacitance is often treated as constant, using a Helmholtz model of the electrical double layer, where the only contribution to capacitance is ascribed to the layer of counter-ions adjacent to the surface. Whilst the resultant quadratic dependence on potential describes much of the extant EWOD data well, the approximation of constant capacitance is unrealistic for the EWOC case, due to the variation in electrode/electrolyte capacitance even over moderate excursions of potential.30 The capacitance of such conductor/electrolyte interfaces is readily measured, with alternating voltage measurements of the frequency-dependent current response yielding the interfacial capacitance via the imaginary component of the total impedance. The capacitance is normally dominated by the solution side of the interface,30 which can be decomposed into a Stern layer, consisting of ions specifically adsorbed on the surface (and dictated by the size of the constituent ions), and a diffuse double-layer component, however the latter term can be neglected at the higher electrolyte concentrations employed herein. C(E) was accordingly measured via AC impedance (see ESI,† Fig. S2) and η was evaluated using the trapezoid rule at each value of E.
Fig. 2(C) shows that the measurements are consistent with the prediction of eqn (6) (solid line) at low bias. At higher biases, the observed CA change falls below the prediction based on the total capacitance, although the agreement is substantially improved if the potential is corrected for the space-charge region within the graphite (see inset of Fig. 2(C) and ESI,† Fig. S2).31 The overall consistency between the calculated and experimental data implies that the EWOC phenomenon can be rationalized in terms of the capacitance of the electrical double-layer formed at the HOPG/droplet interface. This, in turn, highlights the role of the dielectric in inhibiting performance. Capacitance is inversely proportional to the thickness of the charged layer, this is up to several microns thick for a typical dielectric in EWOD,32 compared with an electrical double layer in the high electrolyte EWOC configuration on the order of 1 nm.30 Hence eqn (6) implies that a 100-fold increase in potential (given the approximate square dependence) is required for EWOD to compensate for the 104-fold decrease in capacitance associated with the presence of the dielectric.
The EWOC phenomenon on graphite is robust, being observed for a range of aqueous electrolytes, from molar to sub-millimolar concentrations, see Fig. 2(D) and ESI,† Fig. S3. The subtle differences between electrolytes are attributed to specific ion-graphite surface interactions, manifested as asymmetry of the CA dependence with respect to Epzc, and shifts in the value of Epzc. To correct for the effect of the latter, the data in Fig. 2(D) is plotted relative to Epzc for each electrolyte (see Table S1 of ESI†). Overall, the data of Fig. 2 reveals another key property of HOPG surfaces that is essential to their function in EWOC, namely the low electrochemical activity of the HOPG basal plane, particularly for electrolytic processes requiring a catalytic function.21 Metallic surfaces are much more susceptible to the formation of surface oxides and/or electrocatalytic processes associated with water decomposition, which reduce the zone of stability of metal/solution interfaces with respect to electrolysis.10,30 The lower activity of graphite, coupled with its layered (hence smooth, see ESI,† Fig. S4) structure explains why hysteresis-free wetting can be achieved at low voltages without solution electrolysis, as the current–voltage data (ESI,† Fig. S1 and S5) shows. ESI,† Fig. S4 also presents an analysis of the influence of the height of steps within the HOPG basal plane on the wetting response: steps larger than ca. 100 nm appear to pin the droplets. For the grade of HOPG used, domains of 10–100 microns exist with steps below this threshold height, hence this range of droplet sizes was employed for the wetting experiments.
Fig. 3 (A) Change in CA as a function of E in steps of ±0.1 V. Blue and red symbols indicate E > Epzc and E < Epzc, respectively. Changes in CA of up to 90° below the electrolysis threshold for E > Epzc are followed by saturation, see ESI† for electrolysis data. For E < Epzc, CA decreases monotonically. (B) Percentage change in the footprint diameter of the droplet with E. (C) Pipette-free configuration used to illustrate generality of liquid/liquid EWOC, (D) change in CA with E, and (E) cycling data for steps in E between −0.2 V and +0.6 V (right). |
The ability to perform EWOC in both liquid/air and immiscible liquid configurations should provide new insight into the outstanding question of CA saturation with potential in EWOD, which is frequently seen at θ ≥ 45°.12,32 CAs as low as 10° are reached in the liquid/air configuration with only minimal evidence of CA saturation (Fig. 2), whereas saturation is seen at θ ≃ 47° in the liquid/liquid configuration (Fig. 3). The higher voltages required for liquid/liquid EWOC (supported by the wider potential range before electrolysis, see ESI,† Fig. S3(f)) are consistent with the presence of an ultra-thin layer of organic liquid between the droplet and the substrate,33,34 likening this configuration to EWOD, albeit at ultra-low voltages. This suggests that CA saturation may be associated with the presence of a dielectric layer, therefore supporting dielectric breakdown as underlying saturation.35,36
Much of the interest in the phenomenon of electrowetting stems from its applications,2,6,8 for which reproducibility and the timescale of CA response are critical. Fig. 4(A) shows that highly reproducible changes (to within 1% over 450 cycles) of CA and droplet diameter are obtained for the liquid/air case. Moreover, pronounced CA hysteresis between spreading and de-wetting cycles has been observed in prior approaches to EWOD12,13 and EWOC16 whilst here we find a remarkably low hysteresis of <2° for E ≤ 0.7 V, Fig. 4(B–D). Increased hysteresis (up to 7°) is seen when E is extended to 0.8 V, due to droplet pinning at the associated low CA (see Fig. 4(B)), although the low-hysteresis response is recovered when E is lowered to 0.5 V.
Fig. 4 (A) Constant CA and drop diameter of aqueous 6 M LiCl in air (to within ±1%) over 450 cycles of potential steps between E = −0.2 V and +0.6 V, step duration = 0.25 s. (B) Comparison between CA on stepping E from an initial E = −0.2 V (static measurements, see Fig. 2), and incrementing between −0.2 V and +0.7 V in 0.1 V steps (wetting), then −0.1 V (de-wetting). (C) Same as in b with E incremented to +0.8 V. (D) Same comparison of diameter variation. (E) Change in footprint diameter (from 290 μm to 219 μm) with time over three cycles of step-change from E = −0.2 V to +0.7 V. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6sm01565d |
This journal is © The Royal Society of Chemistry 2016 |