Special Issue "Electrowetting and Smart/Programmable Liquid Interfaces in Microfluidics"

A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "C:Chemistry".

Deadline for manuscript submissions: closed (31 October 2020).

Special Issue Editor

Prof. Athanasios G. Papathanasiou
E-Mail Website
Guest Editor
School of Chemical Engineering, Section II, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece
Interests: interfacial phenomena; electro-magnetofluidics; wetting on complex structures; nonlinear phenomena-stability analysis

Special Issue Information

Dear Colleagues,

The realization of electrostatically addressable wetting of solid surfaces by liquids renders electrowetting (EW) an ideal technique for dynamically manipulating the shape and position of liquid/solid interfaces in the micron scale. Although the electrically assisted modification of wettability is quite an old idea concept (around 1870) of the renowned physicist, G. Lippmann, it was the revolutionary idea of B. Berge (around 2000), that boosted EW applications and related scientific research. Berge introduced a dielectric layer between conductive liquids and conductive solid electrodes that limited the electric current losses and considerably increased the magnitude of the electrostatic energy that could be stored at the liquid/dielectric solid interface. This configuration, known as ElectroWetting On Dielectric (EWOD), is already implemented in many droplet-based microfluidic commercial applications: from dynamically tunable liquid lenses, and lab-on-a-chip systems (droplet transport and mixing of reagents) to low consumption displays, and energy harvesting shoes.

Not only the development of novel EWOD applications but also the optimization and improvement of the established ones are strongly related to the understanding of fundamental electrohydrodynamic phenomena coupled with the electrochemical or electromechanical properties of materials used. Very high electric fields which are desirable for achieving submicron scale curved liquid interfaces require the development, or deposition, of thin dielectric stacks which should resist, mechanically or electrically, under repeated (tens of thousands) actuation cycles. Reversibility of EWOD actuation, as well as other fundamental limiting phenomena, such as the well-known contact angle saturation, are closely related to material surface structure, and its mechanical or chemical properties. The objective of this Special Issue is to highlight research papers, short communications, and review articles that concern novel EW applications, as well as progress on fundamental phenomena that are related to the efficient handling of liquid interfaces via EW.

Prof. Athanasios G. Papathanasiou
Guest Editor

Manuscript Submission Information

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Keywords

  • Lab-on-chip
  • Contact angle saturation
  • EW display
  • Liquid lens
  • Wetting of complex substrates
  • Superhydrophobic surfaces
  • Liquid infused surfaces
  • Electrocapillarity
  • Electrospreading
  • Droplet microfluidics
  • Modeling spreading dynamics
  • Energy and water harvesting

Published Papers (4 papers)

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Research

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Open AccessArticle
Dielectrowetting Control of Capillary Force (Cheerios Effect) between Floating Objects and Wall for Dielectric Fluid
Micromachines 2021, 12(3), 341; https://doi.org/10.3390/mi12030341 - 23 Mar 2021
Viewed by 324
Abstract
A capillary interaction between floating objects and adjacent walls, which is known as “Cheerios effect”, is a common phenomenon that generates capillary attraction or repulsion forces between them depending on their wettabilities, densities, geometries, and so on. This paper deals with controlling the [...] Read more.
A capillary interaction between floating objects and adjacent walls, which is known as “Cheerios effect”, is a common phenomenon that generates capillary attraction or repulsion forces between them depending on their wettabilities, densities, geometries, and so on. This paper deals with controlling the capillary forces, specifically, acting on objects floating on a dielectric (non-conductive) fluid. A key control input parameter is the wettability (contact angle) of the sidewall adjacent to the floating object. By introducing dielectrowetting to the sidewall and actively changing the contact angle on the sidewall, the capillary force is controlled and easily reversed between attraction and repulsion. In this reversing process, the tilting angle of the sidewall is another critical parameter. A theoretical relation taking the titling angle into account is compared and in good agreement with experimental results obtained from the trajectory of the floating object. Finally, a continuous motion of the floating object is demonstrated using this control where an array of dielectrowetting electrode pads is sequentially activated. Full article
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Open AccessArticle
Surface Wettability and Electrical Resistance Analysis of Droplets on Indium-Tin-Oxide Glass Fabricated Using an Ultraviolet Laser System
Micromachines 2021, 12(1), 44; https://doi.org/10.3390/mi12010044 - 01 Jan 2021
Viewed by 508
Abstract
Indium tin oxide (ITO) is widely used as a substrate for fabricating chips because of its optical transparency, favorable chemical stability, and high electrical conductivity. However, the wettability of ITO surface is neutral (the contact angle was approximately 90°) or hydrophilic. For reagent [...] Read more.
Indium tin oxide (ITO) is widely used as a substrate for fabricating chips because of its optical transparency, favorable chemical stability, and high electrical conductivity. However, the wettability of ITO surface is neutral (the contact angle was approximately 90°) or hydrophilic. For reagent transporting and manipulation in biochip application, the surface wettability of ITO-based chips was modified to the hydrophobic or nearly hydrophobic surface to enable their use with droplets. Due to the above demand, this study used a 355-nm ultraviolet laser to fabricate a comb microstructure on ITO glass to modify the surface wettability characteristics. All of the fabrication patterns with various line width and pitch, depth, and surface roughness were employed. Subsequently, the contact angle (CA) of droplets on the ITO glass was analyzed to examine wettability and electrical performance by using the different voltages applied to the electrode. The proposed approach can succeed in the fabrication of a biochip with suitable comb-microstructure by using the optimal operating voltage and time functions for the catch droplets on ITO glass for precision medicine application. The experiment results indicated that the CA of droplets under a volume of 20 μL on flat ITO substrate was approximately 92° ± 2°; furthermore, due to its lowest surface roughness, the pattern line width and pitch of 110 μm exhibited a smaller CA variation and more favorable spherical droplet morphology, with a side and front view CA of 83° ± 1° and 78.5° ± 2.5°, respectively, while a laser scanning speed of 750 mm/s was employed. Other line width and pitch, as well as scanning speed parameters, increased the surface roughness and resulted in the surface becoming hydrophilic. In addition, to prevent droplet morphology collapse, the droplet’s electric operation voltage and driving time did not exceed 5 V and 20 s, respectively. With this method, the surface modification process can be employed to control the droplet’s CA by adjusting the line width and pitch and the laser scanning speed, especially in the neutral or nearly hydrophobic surface for droplet transporting. This enables the production of a microfluidic chip with a surface that is both light transmittance and has favorable electrical conductivity. In addition, the shape of the microfluidic chip can be directly designed and fabricated using a laser direct writing system on ITO glass, obviating the use of a mask and complicated production processes in biosensing and biomanipulation applications. Full article
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Open AccessArticle
Oil Conductivity, Electric-Field-Induced Interfacial Charge Effects, and Their Influence on the Electro-Optical Response of Electrowetting Display Devices
Micromachines 2020, 11(7), 702; https://doi.org/10.3390/mi11070702 - 20 Jul 2020
Cited by 1 | Viewed by 503
Abstract
A pixel in an electrowetting display (EWD) can be viewed as a confined water/oil two-phase microfluidic system that can be manipulated by applying an electric field. The phenomenon of charge trapping in the protective dielectric and conductivity of the oil phase reduce the [...] Read more.
A pixel in an electrowetting display (EWD) can be viewed as a confined water/oil two-phase microfluidic system that can be manipulated by applying an electric field. The phenomenon of charge trapping in the protective dielectric and conductivity of the oil phase reduce the effective electric field that is required to keep the three-phase contact line (TCL) in place. This probably leads to an oil-backflow effect which deteriorates the electro-optical performance of EWD devices. In order to investigate charge trapping and conduction effects on the device electro-optical response, an EWD device was studied, which was fabricated with a black oil, aiming for a high-contrast ratio and color-filter display. For comparison, we also prepared a device containing a purple oil, which had a lower electrical conductivity. As anticipated, the black-oil device showed faster backflow than the purple-oil device. A simple model was proposed to explain the role of oil conductivity in the backflow effect. In addition, the rebound and reopening effects were also observed after the voltage was switched to zero. The above observations were strongly dependent on polarity. By combining observations of the polarity dependence of the oil conductivity and assuming that negative charges trap more strongly in the dielectric than positive charges, our experimental results on rebound and reopening can be explained. In the AC optical response, the pixel closing speed decreased in time for intermediate frequencies. This is likely related to the phenomenon of charge trapping. It was also found that the periodic driving method could not suppress the backflow effect when the driving frequency was above ~10 kHz. Our findings confirm the significance of the above charge-related effects of EWD devices, which need to be investigated further for better understanding in order to properly design/use materials and driving schemes to suppress them. Full article
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Review

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Open AccessReview
Progress in Advanced Properties of Electrowetting Displays
Micromachines 2021, 12(2), 206; https://doi.org/10.3390/mi12020206 - 18 Feb 2021
Viewed by 458
Abstract
Electrowetting display (EWD) has promising prospects in the electronic paper industry due to it having superior characteristics, such as the ability to provide a comfortable reading experience and quick response. However, in real applications, there are also problems related to dielectric deterioration, excess [...] Read more.
Electrowetting display (EWD) has promising prospects in the electronic paper industry due to it having superior characteristics, such as the ability to provide a comfortable reading experience and quick response. However, in real applications, there are also problems related to dielectric deterioration, excess power consumption, optical instability and narrow color gamut etc. This paper reviewed the existing challenges and recent progress made in terms of improving the optical performance and reliability of EWD. First, the principle of electrowetting applied in small and confined configurations is introduced and the cause of the failure of the dielectric layer is analyzed. Then, the function of the pixel structures is described to avoid display defects. Next, electric signal modulations are compared in terms of achieving good image quality and optical stability. Lastly, the methods are presented for color panel realization. It was concluded that multi-layer dielectrics, three-dimensional pixel structures, proper electric frequency-and-amplitude modulation and an RGB color panel are expected to resolve the current limitations and contribute to designing advanced reflective displays. Full article
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