(A) study on electrowetting devices for efficient digital microfluidics

다국어 초록 (Multilingual Abstract)

Digital microfluidics (DMF) is an alternative technology for lab-on-a-chip systems, in which the microfluidic functions of discrete fluid droplets could be performed without microchannel networks or mechanical moving parts. Electrowetting is an attractive method to manipulate liquid droplets in DMF because it allows all the microfluidic functions of dispensing, transporting, splitting, merging, and mixing the fluid droplets with the advantages such as the absence of heat generation, rapid switching response, and low power consumption. The objectives of this study are to review electrowetting components and configurations for DMF and to improve efficiency of the electrowetting devices for low-voltage operation, simple fabrication, or high electrowetting force generation.
An electrowetting device consists of four compulsory components of electrodes, a conducting liquid droplet surrounded by an insulating medium, a dielectric insulator, and a hydrophobic layer. Design criteria for successful droplet transport by electrowetting were described. The electrode width and spaces of permanent hydrophobic regions between two control electrodes should be considered with the droplet size. The required voltage can be reduced by decreasing the liquid-vapor interfacial tension using surfactant-assisted droplets or surrounding medium immiscible with the liquid droplet. However, the choice of surfactant or surrounding medium should be considered from the targeted application. The properties of dielectric materials have a remarkable impact on electrowetting performance. When selecting dielectric material, its electrical, mechanical, thermal, chemical properties should be considered.
Atomic layer deposition (ALD) is one of the best methods to deposit a pinhole-free and thin conformal dielectric layer. The ALD Al2O3 was employed as the dielectric layer for low voltage electrowetting. After the control electrode array of 1 mm × 1 mm squares with 50 μm spaces was patterned on a glass substrate, 127 nm thick Al2O3 (εr = 10.4) film was grown by ALD system. Then, a 20 μm wide reference electrode line was patterned on the Al2O3 film and 30 nm thick Teflon AF was spin-coated. The minimum voltage to move 2 μL water droplet could be as low as 3 V, which is the lowest threshold voltage reported so far in electrowetting researches. This result opens a possibility of manipulating droplets without any surfactant or oil treatment at only a few volts using ALD Al2O3 as the electrowetting dielectric.
The hydrophobic surface is essential for enlarging the contact angle variation with high inherent contact angle and reducing contact angle hysteresis, which makes the droplet manipulation more efficient in electrowetting devices. The thinner hydrophobic layer is coated on the dielectric layer, the better electrowetting efficiency is when using a thin dielectric layer. An effective and efficient condition for Teflon AF spin-coating was obtained experimentally, and the resulting minimum acceptable Teflon AF concentration and thickness were 0.2 wt% and 9 nm respectively for large inherent contact angle of about 116° and small contact angle hysteresis of 4-6°.
Electrowetting configurations for DMF are divided into roughly two-plate and single-plate configurations. The two-plate electrowetting device is suitable for a wide range of droplet operations such as dispensing, transporting, splitting, and merging. In the ground-type two-plate configuration, the electrowetting force is generated only on the bottom plate. When the top plate is replaced by the same as the bottom plate, the electrowetting force for droplet transport can be generated on both plates. However, the applied voltage is divided into two dielectric layers. Therefore, the force in the unground-type two-plate electrowetting decreases. Single-plate configurations also can be divided into ground-type and unground-type. As well as the two-plate configurations, the ground-type is more efficient in electrowetting performance than the unground-type, but it requires an additional fabrication step for patterning the top reference electrode on the dielectric layer. The proposed configuration for single-plate electrowetting devices allows more simplified fabrication process without any decrease of the electrowetting performance. The control electrode array and the reference electrode were patterned simultaneously on the same plane. Then, polyimide as the dielectric layer was patterned for opening the reference electrode and then Teflon AF was coated. The droplet movement of diluted methylene blue by electrowetting was successfully demonstrated.
A novel two-plate electrowetting configuration was proposed to enhance the electrowetting efficiency from the configurational point of view. This twin-plate configuration, a combination of two same ground-type single-plate devices, allows applying the voltage across each of the dielectric layers on both plates without any voltage division, while keeping the droplet grounded with the reference electrodes. Therefore, the total electrowetting force can increase 2-fold compared to a typical two-plate configuration theoretically. For making the twin-plate electrowetting device, two same fabricated ground-type single-plate devices were attached with a gap between them after aligning their control electrode arrays to face each other with one of them upside down. This twin-plate electrowetting device shows significantly faster droplet velocity than that of the two-plate device, and hence the required voltage for generating a desired electrowetting force can be reduced to below 80%.
Although the contact angle changes when an electrowetting force is applied to the droplet on the control electrode array, the droplet cannot move onto the activated control electrode with small applied voltage below a certain threshold because of the adhesive friction. In order to transport the droplet successfully, sufficient contact angle difference between the advancing side and the receding side should be generated by an applied voltage over the threshold voltage. The dependence of the threshold voltage for droplet transport on the contact angle hysteresis was described theoretically and experimentally for three different ground-type electrowetting configurations. Analyzing the contact angles at the electrowetting threshold voltage theoretically, it was found that the threshold voltage in either the single-plate or twin-plate configuration equals to the hysteresis voltage and it becomes higher than that of the two-plate configuration by factor of square root of 2. The fabricated devices using 2.5 μm thick polyimide (εr = 3.3) as the dielectric layer and thin Teflon AF as the hydrophobic layer (θ0 = 116° and α = 4-6°) showed the threshold voltages of 28, 40, and 28 V for droplet transport initiation, and the threshold voltages of 35, 50, and 35 V for stable droplet transport in the single-plate, two-plate, and twin-plate devices, respectively. All the results are in very good agreement with theoretical expectations.

목차 (Table of Contents)

• Abstract
• List of Tables
• List of Figures
• List of Symbols and Abbreviations
• Table of Contents
• Chapter 1 Introduction
• 1.1 Digital microfluidics
• 1.2 Electrowetting
• 1.3 Outline and objectives
• Chapter 2 Electrowetting Components
• 2.1 Electrodes
• 2.1.1 Electrode materials in electrowetting devices
• 2.1.2 Electrode shapes for electrowetting droplet manipulation
• 2.2 Droplet and surrounding medium
• 2.2.1 Liquids used for electrowetting actuation
• 2.2.2 Droplet size for effective electrowetting transport
• 2.2.3 Surrounding medium for electrowetting
• 2.3 Dielectric layer
• 2.3.1 Dielectric materials used for electrowetting
• 2.3.2 Dielectric thickness for efficient electrowetting
• 2.3.3 Low voltage electrowetting on ALD Al2O3
• 2.4 Hydrophobic layer
• 2.4.1 Contact angle saturation
• 2.4.2 Contact angle hysteresis
• 2.4.3 Teflon AF coating for efficient electrowetting
• Chapter 3 Electrowetting Configurations
• 3.1 Two-plate configurations
• 3.2 Single-plate configurations
• 3.3 Twin-plate configuration
• 3.4 Electrowetting threshold for droplet transport
• Chapter 4 Conclusions
• References
• Abstract in Korean
• Resume