DC Electrowetting of Nonaqueous Liquid Revisited by XPS

Abstract

Liquid poly(ethylene glycol) (molecular weight, ∼600 Da) with a low vapor pressure is used as droplets in an ultrahigh-vacuum X-ray photoelectron spectrometer (XPS) chamber with traditional electrowetting on dielectric (EWOD) device geometry. We demonstrate that, using XPS data, independent of the sign of the applied voltage, the droplet expands on the substrate with the application of a nonzero voltage and contracts back when the voltage is brought back to zero. However, the main focus of the present investigation is about tracing the electrical potential developments on and around the droplet, using the shifts in the binding energy positions of the core levels representative of the liquid and/or the substrate in an noninvasive and chemically specific fashion, under imposed electrical fields, with an aim of shedding light on numerous models employed for simulating EWOD phenomenon, as well as on certain properties of liquid/solid interfaces. While the lateral resolution of XPS does not permit to interrogate the interface directly, we explicitly show that critical information can be extracted by probing both sides of the interface simultaneously under external bias in the form of potential steps or direct current. We find that, even though no potential drop is observed at the metal-wire electrode/liquid interface, the entire potential drop develops across the liquid/solid-substrate interface, which is faster than our probe time window (∼100 ms) and is promptly complying with the applied bias until breakdown. No indication of band bending nor additional broadening can be observed in the C 1s peak of the liquid, even under electrical field strengths exceeding 10 V/m. Moreover and surprisingly, the liquid recovers within seconds after each catastrophic breakdown. All of these findings are new and expected to contribute significantly to a better understanding of certain physicochemical properties of liquid/solid interfaces.

X-ray Photoelectron Spectroscopy with Electrical Modulation Can Be Used to Probe Electrical Properties of Liquids and Their Interfaces at Different Stages

“…When an external dc bias is applied to the liquid drop on the dielectric, no measurable current passes (<10 pA), even under dc voltages of as large as 180 V. As a result, both the C 1s and also the O 1s peaks of PEG display trivial potential-dependent shifts of 180.0 ± 0.1 eV in their corresponding measured binding energies as shown explicitly in Figure . The estimated breakdown voltage, 180 ± 20 V (see SI section for details), for this system is consistent with our measurement, which is also consistent with our previously reported measurement on a similar PEG drop on CYTOP. , …”

“…The present work is a continuation of our previous work, where we directly measure the electrical potential developments via recording XPS spectra of a pristine PEG drop on a Teflon-coated SiO 2 /Si substrate under both dc and ac (using square-wave pulses, SQW) low-voltage (10 V) actuations. Afterward, we expose the liquid drop, within the spectrometer, to higher-voltage dc actuations until it undergoes several breakdown events around 200 V. What is different in this work from our previous papers , is that we characterize the postbreakdown system again using the low-voltage actuations, which reveals yet a different asymmetric property with respect to the polarity of the actuation. This postbreakdown asymmetric behavior is different from the prebreakdown contact-angle saturation, often reported in the literature, which was also mentioned above.…”

“…The observation of voltage-driven asymmetric ion movement through the pores of a multilayer graphene electrode was another novel application of the operando-XPS . We have recently utilized Operando XPS to probe the electrowetting properties of ionic and other low-volatility liquids under both dc and ac actuation in order to derive chemically resolved electrical information on such systems, indispensably guided by electrical modeling …”

Lab-based operando x-ray photoelectron spectroscopy for probing low-volatile liquids and their interfaces across a variety of electrosystems

“…[76][77][78] Within the last three years, we have reintroduced XPS as a powerful analytical arsenal for probing electrical potential developments of liquids and their interfaces with dielectric substrates to extract novel information about them, under in situ and in operando conditions. [37][38][39][40] In order to probe impedance type properties of the samples under investigation, it is necessary to impose a varying frequency (AC) actuation for which the natural choice is a sine-wave. However, as we said earlier, the sine-wave distorts the form of the XP signal, see Figs.…”

“…33 We, then, extended it to probing the nature of an electrochemical corrosion process between the ionic liquid and Au electrodes, 34 and finally to detect, in situ, an electrochemically generated carbene formation 35 and its CS 2 adduct. 36 Most recently, we have utilized o-XPS to probe electrowetting properties of ionic and other low-volatile liquids under both DC, 37 and AC (Ref. 38) actuations, in order to extract chemically resolved electrical information of such systems, guided by modeling.…”