Research on the Asymmetric Phenomenon of Voltage Polarity Based on Dielectric Wetting

The present research investigated the voltage polarity asymmetry phenomenon based on dielectric wetting. In an ITO-hydrophobic layer-droplet setup, three reagents with different pH values (3.96, 7.0, and 10.18), two types of hydrophobic materials (AF1601 and 6%T6), and two different thicknesses (340 nm and 2.5 μm) of each material were systematically investigated. The results show that the thickness of the hydrophobic dielectric layer and the pH of the droplets had a significant impact on the droplet contact angle variation with the voltage. The contact angle on the thick hydrophobic dielectric layer followed the Lippmann-Young equation as the voltage changed. The angle of the thin hydrophobic dielectric layer was affected by its own properties and the type of droplet, which led to the occurrence of voltage polarity asymmetry of the electrowetting phenomenon. After further investigation of this phenomenon, it was found that it mainly accounted for the decrease in electric field strength at both ends of the droplet, which was caused by electrochemical reactions and changes in circuit resistance. The leakage current is an important indicator, and this phenomenon can be prevented by increasing the thickness of the hydrophobic dielectric layer.

Switchable Optical Properties of Dyes and Nanoparticles in Electrowetting Devices

The optical properties of light-absorbing materials in optical shutter devices are critical to the use of such platforms for optical applications. We demonstrate switchable optical properties of dyes and nanoparticles in liquid-based electrowetting-on-dielectric (EWOD) devices. Our work uses narrow-band-absorbing dyes and nanoparticles, which are appealing for spectral-filtering applications targeting specific wavelengths while maintaining device transparency at other wavelengths. Low-voltage actuation of boron dipyromethene (BODIPY) dyes and nanoparticles (Ag and CdSe) was demonstrated without degradation of the light-absorbing materials. Three BODIPY dyes were used, namely Abs 503 nm, 535 nm and 560 nm for dye 1 (BODIPY-core), 2 (I2BODIPY) and 3 (BODIPY-TMS), respectively. Reversible and low-voltage (≤20 V) switching of dye optical properties was observed as a function of device pixel dimensions (300 × 900, 200 × 600 and 150 × 450 µm). Low-voltage and reversible switching was also demonstrated for plasmonic and semiconductor nanoparticles, such as CdSe nanotetrapods (abs 508 nm), CdSe nanoplatelets (Abs 461 and 432 nm) and Ag nanoparticles (Abs 430 nm). Nanoparticle-based devices showed minimal hysteresis as well as faster relaxation times. The study presented can thus be extended to a variety of nanomaterials and dyes having the desired optical properties.

Tuning the electrowetting behavior of quantum dot nanofluids

HYPOTHESIS

The electrowetting behavior of droplets can be altered by the inclusion of salts, surfactants, or nanoparticles. We propose that varying the properties of cadmium selenide/zinc sulfide quantum dots will affect the electrowetting behavior of fluorescent nanofluids. Information gathered will allow for greater control of fluid properties when designing a colloidal system in an electrowetting environment.

EXPERIMENTS

Aqueous-based quantum dots were functionalized with mercaptocarboxylic acid ligands of various chain length and binding motifs by a room temperature phase transfer method. The size and concentration of the quantum dot were varied, and droplets of the resulting nanofluids were exposed to increasing amounts of voltage. The change in contact angle was evaluated and correlated to the surface chemistry, size, and concentration of the quantum dots.

FINDINGS

Quantum dot nanofluids with longer alkyl chains have the most pronounced change in contact angle and were the most stable under applied voltage. The size of the nanoparticles does not significantly impact the electrowetting behavior at low concentration (3 µM), but nanofluids containing smaller diameter quantum dots show enhanced electrowetting behavior at higher concentration (27 µM). The fluorescent properties of the QD nanofluids studied were not affected after repeated electrowetting cycles.

Approximately symmetric electrowetting on an oil-lubricated surface

As the most widely used insulator materials in the electrowetting (EW) systems, amorphous fluoropolymers (AFs) provide excellent hydrophobicity, dielectric properties and chemical inertness; however, they suffer from charge trapping during electrowetting with water and the consequent asymmetric phenomenon. In this study, an ultra-thin oil-lubricated AF surface was proposed to release the charge trapping in the dielectric layer and further suppress the polarity-dependent asymmetry during electrowetting. The negative spontaneously trapped charges gathering on the dielectric/water interface with aging time were characterized by various measurements and calculations, which explained the polarity dependence of the asymmetric electrowetting. Approximately symmetric EW curves withstanding water aging were obtained for the oil-lubricated AF surface, confirming the blocking effect on charge trapping induced by the lubricated surface. The improved reversibility of EW with low contact angle hysteresis brought by the oil-lubricated surface was also demonstrated. This study reveals the mechanism behind the asymmetric EW phenomenon and offers an attractive oil-lubricated EW material system for suppressing the charge trapping on the dielectric/water interface, which can significantly improve the manipulation of the EW devices.

Modes and break periods of electrowetting liquid bridge

In this paper, we propose a microscale liquid oscillator using electrowetting-on-dielectric (EWOD). Specifically, a mesoscale liquid bridge (LB) between two horizontal surfaces with EWOD is considered. When EWOD is applied, the solid surface becomes more hydrophilic, and hence the contact angle (CA) is reduced. Following the activation of EWOD, the LB can remain connected or it can break into either symmetric or asymmetric shapes depending on the initial liquid volume and wettability of the two surfaces. The LB dynamics activated by EWOD is studied using the multibody dissipative particle dynamics (MDPD) method. Our numerical results show that the behavior of an LB under EWOD can be interpreted via three modes. In the first mode, the LB does not break after applying EWOD. In the second mode, the LB breaks and does not reform. The third mode happens when, depending on the interplay of the volume of the liquid and CA manipulation, the LB continuously breaks, recoils, and reforms. For asymmetric cases, it was observed that the LB may completely detach from one surface and may not reform. It was also observed that decreasing the wettability of a surface, for cases with a continuous breaking and reformation behavior, increases the connecting time interval and decreases the breaking time interval in one break-reform cycle. The results provided in this investigation facilitate fundamental understanding of LB dynamics and their application for the design of microscale liquid oscillators using EWOD.

DC Electrowetting of Nonaqueous Liquid Revisited by XPS

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 10⁷ 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.

Droplet Translation Actuated by Photoelectrowetting

In traditional electrowetting-on-dielectric (EWOD) devices, droplets are moved about a substrate using electric fields produced by an array of discrete electrodes. In this study, we show that a drop can be driven across a substrate with a localized light beam by exploiting the photoelectrowetting (PEW) effect, a light-activated variant of EWOD. Droplet transport actuated by PEW eliminates the need for electrode arrays and the complexities entailed in their fabrication and control, and offers a new approach for designing lab-on-a-chip applications. We report measurements of the maximum droplet speed as a function of frequency and magnitude of the applied bias, intensity of illumination, volume of the droplet, and viscosity and also introduce a model that reproduces these data.

Low Voltage Electrowetting on Ferroelectric PVDF-HFP Insulator with Highly Tunable Contact Angle Range

We demonstrate a consistent electrowetting response on ferroelectric poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) insulator covered with a thin Teflon AF layer. This bilayer exhibits a factor of 3 enhancement in the contact angle modulation compared to that of conventional single-layered Teflon AF dielectric. On the basis of the proposed model the enhancement is attributed to the high value of effective dielectric constant (εeff ≈ 6) of the bilayer. Furthermore, the bilayer dielectric exhibits a hysteresis-free contact angle modulation over many AC voltage cycles. But the contact angle modulation for DC voltage shows a hysteresis because of the field-induced residual polarization in the ferroelectric layer. Finally, we show that a thin bilayer exhibits contact angle modulation of Δθ (U) ≈ 60° at merely 15 V amplitude of AC voltage indicating a potential dielectric for practical low voltage electrowetting applications. A proof of concept confirms electrowetting based rapid mixing of a fluorescent dye in aqueous glycerol solution for 15 V AC signal.

Role of space charges inside a dielectric polymer in the electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP)

Mushroom-shaped structures with a high aspect ratio are fabricated based on the action of space charges inside the dielectric polymer.

Progress in low voltage reversible electrowetting with lubricated polymer honeycomb substrates

Electrowetting of silicone oil lubricated PC + EC substrates. (A)U= 0 V; (B)U= 55 V.