Spontaneous electrification of fluoropolymer–water interfaces probed by electrowetting

Banpurkar, Arun; Sawane, Yogesh B.; Wadhai, Sandip M.; Murade, C.U.; Sîretanu, Igor; Ende, Dirk van den; Mugele, Friedrich Gunther

doi:10.1039/c6fd00245e

Cited by 46 publications

(60 citation statements)

References 35 publications

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“…54,55 By using their contact angle deviations, they determined this voltage shift to U* = 35 V at U b =140 V to represent the energy loss during charging. More recently, Banpurkar et al 56 and Bonfante et al 67 have extended this approach to shed further light on the charging/polarization mechanisms of materials at interfaces under both dc and ac actuations, again using contact angle and electrical measurements.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

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

et al. 2019

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Operando X-ray photoelectron spectroscopy (o-XPS) has been used to record the binding energy shifts in the C 1s peak of a pristine poly(ethylene glycol) (PEG) liquid drop in an electrowetting on dielectric (EWOD) geometry and after exposing it to several high-voltage breakdown processes. This was achieved by recording XPS data while the samples were subjected to 10 V dc and ac (square-wave modulation) actuations to extract electrical information related to the liquid and its interface with the dielectric. Through analysis of the XPS data under ac actuation, a critical frequency of 170 Hz is extracted for the pristine PEG, which is translated to a resistance value of 14 MΩ for the liquid and a capacitance value of 60 pF for the dielectric, by the help of simulations using an equivalent circuit model and also by XPS analyses of a mimicking device under similar conditions. The same measurements yield an increased value of 23 MΩ for the resistance of the liquid after the breakdown by assuming that the capacitance of the dielectric stays constant. In addition, an asymmetry in polarity dependence is observed with respect to both the onset of the breakdown voltage and also the leakage behavior of the deteriorated (PEG + dielectric) system such that deviations are more pronounced at positive voltages. Both dc and ac behaviors of the postbreakdown system can also be simulated, but only by introducing an additional element, a diode or a polarity-and magnitude-dependent voltage source (VCVS), which might be attributed to negative charge accumulation at the interface. Measurements for a liquid mixture of PEG with 8% ionic liquid yields an almost 2 orders of magnitude smaller resistance for the drop as a result of the enhanced conductivity by the ions. Coupled with modeling, XPS measurements under dc and ac modulations enable probing unique electrochemical properties of liquid/solid interfaces.

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Section: ■ Results and Discussionmentioning

confidence: 99%

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

et al. 2019

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“…The latter was claimed to be the consequence of permanent trapping of charge carriers at the fluoropolymer surface, which increased with aging of substrates in water as well as in various nonaqueous polar liquids. 41 Hence, the nonaqueous PEG on the CYTOP layer, used in this work, also exhibits an asymmetric EW response. Similar visual changes are also observable when the device is in the XPS chamber.…”

Section: ■ Experimental Sectionmentioning

confidence: 94%

DC Electrowetting of Nonaqueous Liquid Revisited by XPS

2018

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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.

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“…Electrowetting experiments revealed charge deposition on hydrophobic surfaces. 33 Drops that condense on a surface will also ''jump'' from this surface with a net charge. 34,35 Furthermore, water drops ejected from a nozzle are usually charged, leaving behind the opposite charge in the nozzle.…”

Section: Introductionmentioning

confidence: 99%