AC Electrowetting Modulation of Low-Volatile Liquids Probed by XPS: Dipolar vs Ionic Screening Pinar Aydogan Gokturk, Burak Ulgut, and Sefik Suzer* Department of Chemistry, Bilkent University, Bilkent, 06800 Ankara, Turkey *S Supporting Information ABSTRACT: X-ray photoelectron spectroscopic (XPS) data have been recorded for a low-molecular-weight poly(ethylene glycol) microliter-sized sessile liquid drops sitting on a dielectric covered planar electrode while imposing a ±6 V square-wave actuation with varying frequencies between 10 −1 and 10 5 Hz to tap into the information derivable from (AC) electrowetting. We show that this time-varying XPS spectra reveal two distinct behaviors of the device under investigation, below and above a critical frequency, measured as ∼70 Hz for the liquid poly(ethylene glycol) with a 600 Da molecular weight. Below the critical frequency, the liquid complies faithfully to the applied bias, as determined by the constant shift in the binding energy position of the XPS peaks representative of the liquid throughout its entire surface. The liquid completely screens the applied electrical field and the entire potential drop takes place at the liquid/ dielectric interface. However, for frequencies above the critical value, the resistive component of the system dominates, resulting in the formation of equipotential surface contours, which are derived from the differences in the positions of the twinned O 1s peaks under AC application. This critical frequency is independent of the size of the liquid drop, and the amplitude of the excitation, but increases when ionic moieties are introduced. The XP spectra under AC actuation is also faithfully simulated using an equivalent circuit model consisting of only resistors and capacitors and using an electrical circuit simulation software. Moreover, a mimicking device is fabricated and its XP spectra are recorded using the Sn 3d peaks of the solder joints at different points on the circuit to confirm the reliability of the measured and simulated AC behaviors of the liquid. These new findings indicate that in contrast to direct current case, XPS measurements under variable frequency AC actuation reveal (through differences in the frequency response) information related to the chemical makeup of the liquid(s) and brings the laboratory- based XPS as a powerful complimentary arsenal to electrochemical analyses of liquids and their interfaces. ■ INTRODUCTION Electrowetting on dielectrics (EWOD) is a process of changing the contact angle of a liquid droplet sitting on a dielectric- covered conducting electrode by application of external electric field(s). Spreading of a liquid drop under an electric field can be observed on both dielectric coated and conductive surfaces as a result of decreasing contact angle by additional electrical force(s). However, the dielectric layer prevents the passage of Faradic current and eliminates the possibility of electrolysis during the process. Hence, larger amplitudes of contact angle changes are achieved. This activation can be realized under both direct current (DC) and alternating current (AC) fields with variable frequency. 1−6 In fact, it has been reported that the contact-angle hysteresis is smaller in AC and DC, which is why AC is preferred in many applications. 7,8 One of the major obstacles in both DC and AC actuations is related to the saturation of the contact angle at high voltages, and numerous techniques have been developed to circumvent it. 9−11 Furthermore, many experimental results have shown that the contact angle saturation occurs at higher effective voltages for the AC when compared to the DC electrowetting. 6,12,13 Wettability of solid surfaces has great importance for both natural and industrial processes. For this reason, contact angle tunability with application of external bias is crucial and electrowetting offers an excellent option for manipulating small amounts of liquids. Indeed, this technology has been used in various applications such as lab on a chip, 14 liquid-based lenses, 15 electronic paper(s), 16 and electrofluidic displays. 17 Electrowetting on dielectric behavior is conventionally modeled by an analogy to equivalent electric circuits adopted from the model(s) used for solid-sate devices, one of which is depicted in Figure 1, together with our experimental setup. 1,3,18,19 In this model, the liquid drop is represented by a parallel resistor and a capacitor, whereas the dielectric layer(s) as another capacitor, and the two electrical double layers formed at the liquid/top and the liquid/bottom electrodes are usually not taken into account. This model is also supported by numerous electrical impedance spectro- scopic data. 20 Received: December 9, 2018 Revised: February 9, 2019 Published: February 15, 2019 Article pubs.acs.org/Langmuir Cite This: Langmuir 2019, 35, 3319-3326 © 2019 American Chemical Society 3319 DOI: 10.1021/acs.langmuir.8b04099 Langmuir 2019, 35, 3319−3326 Downloaded via BILKENT UNIV on November 18, 2019 at 13:36:46 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.