Impact of substrate elasticity on contact angle saturation in electrowetting

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.

Nanofluid Droplet Impact on Rigid and Elastic Superhydrophobic Surfaces

Ice accumulation on cold surfaces is a common and serious phenomenon that exists in numerous industrial fields, such as power transmission, wind turbines, and aircraft. Despite recent efforts in mitigating ice accumulation on the cold surface, it remains a challenge to achieve robust anti-icing on the cold surface in terms of nanofluid droplet. Here, we report a rigid superhydrophobic Cu surface and an elastic polydimethylsiloxane (PDMS) superhydrophobic surface to enhance water-repellency performance, characterized by a significant reduction in contact time and a decrease in the spreading ratio. As for the rigid superhydrophobic Cu surface, the underlying mechanism is ascribed to the existence of stable air cushions between the micropillar array, which reduce the contact area and further suppress the heat conduction. As for the elastic PDMS superhydrophobic surface, the rapid detachment of the nanofluid droplet relies on superior surface elasticity, which can further suppress the nanofluid droplet splashing at a high impacting velocity. We believe that this work can provide a new view for the improvement of water-repellency for a wide range of applications.

Dielectric charge injection (DCI)-enabled contactless droplet wetting modulation for droplet-surface material interchange

HYPOTHESIS

Electrowetting-on-dielectric (EWOD) employs direct droplet-electrode contact to generate electric fields across the dielectric layer to modulate droplet wetting. Because the charged surface state drives this process, it should be possible to accomplish a contactless modulation of droplet wetting by charge injection onto the dielectric surface where a droplet is situated.

EXPERIMENTS

We present our technique, dielectric charge injection (DCI), to contactlessly modulate droplet wetting via corona discharge-based physics. We study the ability of droplets on nonwetting surfaces to transition to a wetting state under DCI, quantify contact angle (CA) in relation to applied voltage, and examine reversibility under regimes with and without charge injection. The observed phenomena are applied to enable droplet-surface material interchange.

FINDINGS

Using DCI, we induce wetting of a deionized water droplet on a non-wetting polydimethylsiloxane (PDMS) surface immersed in hexadecane, with tunable CA modulation based on applied voltage. Upon simple removal of the voltage and/or conductor, droplet fully recovers the initial non-wetting state. We combine these capabilities to enable droplet-surface material interchange of two modes: material deposition (droplet-to-surface) and material recovery (surface-to-droplet). DCI presents a unique strategy for contactless, reversible wetting state modulation that is simple yet powerful for applications such as integrating droplet microfluidics to mass spectrometry.

Large tuning in the electrowetting behaviour on ferroelectric PVDF-HFP/Teflon AF bilayer

Electrowetting (EW) response on a dielectric depends on its permittivity value, Young contact angle and voltage amplitude. We present a large change in EW contact angle, from 163° to 80°, on the bilayer dielectric made up of ferroelectric PVDF-HFP with a thin layer of fluoropolymer. The thickness values of both layers were separately optimized for high effective capacitance essential for the large EW response. It reveals that the bilayer with ~ 500 nm thick PVDF-HFP layer and ~ 50 nm thin layer of Teflon results in the maximum value of effective dielectric constant, ε ≈ 8. Besides this gain, dc-voltage EW response exhibits hysteresis mainly due to polarization in the ferroelectric layer such that, hysteretic offset voltage was found to depend on the applied voltage amplitude and thickness of the dielectrics. Finally, bilayer was subjected to ac-voltage EW in silicone oil for ambient temperature ranging from - 25 to 70 °C. The consistent EW response in this ambient without any degradation/delamination of polymer surface confirmed the durability of the bilayer on the transparent ITO electrodes.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10853-021-06308-z.