Electrically controlled localized charge trapping at amorphous fluoropolymer-electrolyte interfaces

Charge trapping is a long-standing problem in electrowetting-on-dielectric (EWOD), causing reliability reduction and restricting its practical applications. Although this phenomenon has been investigated macroscopically, the microscopic investigations are still lacking. In this work, the trapped charges are proven to be localized at three-phase contact line region by using three detecting methods -- local contact angle measurements, electrowetting (EW) probe, and Kelvin Probe Force Microscopy (KPFM). Moreover, we demonstrate that this EW-induced charge trapping phenomenon can be utilized as a simple and low-cost method to deposit charges on fluoropolymer surfaces. Charge density near the three-phase contact line up to 0.46 mC/m2 and the line width with deposited charges ranging from 20 to 300 micrometer are achieved by the proposed method. Particularly, negative charge densities do not degrade even after harsh testing with a water droplet on top of the sample surfaces for 12 hours, as well as after being treated by water vapor for 3 hours. These findings provide an approach for applications which desire stable and controllable surface charges

Electrowetting: from basics to applications

Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects¿rather than a unique one¿are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices

Characterization of advanced materials for low-frequency Vibrational Energy Harvesting (VEH)

openNowadays sensors are among the most exploited systems in everyday life, with several applications stimulating an increasing amount of research. They generally require external power, thus adding issues such as maintenance and size constraints. The most promising energy harvesting (EH) technology for miniaturization is Reverse Electro wetting on Dielectric (REWoD). It can provide high power density by exploiting the mechanical modulation of the capacity at the liquid/dielectric interface attaining, without any external bias, power densities of µW/cm2. With respect to other EH techniques, REWoD harvests energy from low frequency vibrations (< 10Hz, human motion). I exploited low-cost materials as proof of concept of the feasibility of vibrational EH, suitable for wearable devices, using highly hydrophobic Al and PVDF coated electrodes in combination with polyacrylamide (PAAm) hydrogels loaded with LiCl solutions. The morphology at the sub-micrometer scale and the composition of the outer layers of Al have been studied as a function of the chemical etching time and have been correlated with the surface wettability. The etched Al surfaces exhibit binary structures with nanoscale block-like convexes and hollows, providing more space for air trapping. The analysis shows not only that the change in wetting behaviour correlates with the amount of Al hydroxide at the surface, but also confirms the essential role of the adsorption of airborne carbon compounds. The hydrophobic behaviour depends therefore on the combined effects of surface morphology and surface chemical composition. To compensate for the degradation of the hydrogels with time due to the microstructure of the external oxide layer, an alternative bare Al electrode covered with PVDF has been tested: PAAm hydrogels show now no degradation with time while being able to provide, at frequencies lower than 10 Hz, a peak power/unity of 0.6 Watt, higher than 0.25 Watt, obtained by using the Al oxide electrode.openXXXIII CICLO - SCIENZE E TECNOLOGIE DELLA CHIMICA E DEI MATERIALI - Scienza e tecnologia dei materialiPaolini, Giuli

Investigation of Microfluidic Kelvin Water Dropper and Its Applications in Contact and Contactless Electrowetting

A typical Kelvin water dropper is a device that can convert gravitational potential energy to a high voltage electrostatic. This device consists of two inductors, two collectors, tubes, and electrical connections. A Kelvin water dropper is able to generate extremely high voltage by separating ions using two positive feedback loops. A Kelvin water dropper provides a low cost solution for the applications in which high voltage is needed. In the present research, low cost Microfluidic Kelvin Water Droppers (MKWDs) were developed and built in house for electrowetting applications. Two MKWDs with different tube inner diameters (254 and 508 μm) were constructed to evaluate their appropriate power output for electrowetting. It was demonstrated that higher flow rate led to higher voltage generated, whereas the MKWD with larger tube diameters generated less voltage. Thereafter, contact electrowetting using the homemade MKWD was studied. Electrowetting is a process used to manipulate deformation of liquid droplets on a dielectric surface using an external electric field. In contact electrowetting, the droplet is in contact with a working electrode that applies the voltage. The electric field in the present research was applied using the MKWD. It was demonstrated that contact electrowetting of water droplets can be controlled using the MKWD. Then, a computational model was built to simulate the contact electrowetting using the MKWD. The model was successfully validated by comparing the experimental and simulation results. Finally, contactless electrowetting using the MKWD was investigated. As compared to contact electrowetting, the working electrode was separated from the water droplets. When applying an electric field using the MKWD, it was observed that the water droplet first corrugated due to electrostatic attraction, and then collapsed due to electrowetting. Unlike contact electrowetting, two processes were involved in contactless electrowetting. To simulate two processes, the first model was built based on the theory of electrostatics, and the second model was based on the conventional electrowetting. Both models were validated by a nice agreement between the experimental and simulation data

ELECTRIC FIELD INDUCED DROPLET MANIPULATION

In this thesis, we explore several droplet manipulation concepts on different length scales for a surface cleaning application. The design evolution to transfer these techniques from laboratory conditions to a chaotic environment, such as on the road, is an evolving engineering challenge where reliability and performance are equally important. Electrowetting and liquid dielectrophoresis are techniques by which an electromechanical response from an applied electric field enables precise droplet manipulation. This thesis presents several contributions to these technologies, focusing primarily on scalability, simplicity, and reliability. The control of surface wettability using the electric methods attracts much attention due to their fast response (milliseconds), exceptional durability (hundreds of thousands of switching cycles) and low energy consumption (hundreds of microwatts). Furthermore, their superior performance and reliable nature have prompted a vast amount of literature to expand their application. They are widely used in several scientific and industrial fields, including microfluidics, optical devices, inject printing, energy harvesting, display technologies, and microfabrication. Droplet actuation using electric methods has been a long-standing interest in microfluidics, and most often, it is limited by high operating voltages. The first actuation method explored in this thesis is based on interdigitated electrodes to generate a dielectrophoretic response. In order to apply an effective electrostatic force for droplet manipulation, the geometry of the electrodes must be optimised, which similarly leads to a lower operating voltage (as low as 30 V). Furthermore, microscale electrodes can be iteratively combined to realise larger arrays to move larger droplets. The iterative approach was developed for a large-scale device to manipulate droplets of varying sizes while keeping the actuation process simple. In the second actuation method, a pair of microelectrodes separated by a variable gap distance generated an electrostatic gradient to produce a continuous droplet motion along the length of the electrode pad. The novel actuation method transported droplets of different sizes without active control. The droplet actuation was demonstrated on a larger scale using several platforms, including radial-symmetric, linear, and bilateral-symmetric droplet motion. An automated self-cleaning platform was tested in laboratory conditions and on the road. The technology has significant potential in the automotive sector to clean body parts, camera covers, and scanning sensors. The electrostatic force applied across the droplet was calculated by placing a continuously moving droplet on a tilted platform and measuring the critical angle at which the droplet’s gravity overcomes the opposing applied electric force. Several electrode designs were also considered to evaluate the effect of electrode geometry on the actuation force. The droplet actuation was also modelled using an analytical approach to estimate the critical signal frequency, maximum electrostatic energy, and maximum electrostatic force. Lastly, a tilting micromirror platform investigated the dielectrophoretic response without measuring the droplet contact angle. The mirror platform is also suitable for other optical applications as it provides three axes of movement for beam steering. The tilting platform enabled an angular coverage of up to 0.9° (± 0.02°), with a maximum displacement of 120 μm. We also explored the feasibility of using a microhydraulic actuator based on liquid dielectrophoresis for a microfluidic application. The actuation method opens new possibilities for positioning and manipulating particles and components. These could be hazardous medical materials or even radioactive substances, where direct contact should be avoided

Experimental study of the influence of an electric field on the shape of a droplet

The electrowetting effect has emerged as an important research topic in recent years. Manipulation of droplets by electric fields has been extensively studied employing different setups, the most common being the configuration in which the electric field is created inside the droplet. However, such a configuration has been mostly employed using conductive liquids, whereas non-conductive droplets have been barely tested under the effect of an electric field using this setup. On the other hand, the influence of an electric field on the shape of a droplet has also been of considerable interest using an alternative configuration in which the electric field is applied between two electrodes with no contact with the drop – the Taylor cone configuration. However, the effects observed employing this last setup are rather different to those obtained using the aforementioned design. While the most important effect in those experiments in which the electric field is created inside the drop is the reduction of the contact angle, the main effect found using the second configuration is the droplet elongation in the electric field direction. In order to demonstrate if dielectric and almost perfectly wetting liquid droplets are affected in the same way as other fluids already tested, the effect of an electric field on the shape of an HFE-7500 droplet has been examined under two different experimental setups by means of interferometric techniques. For the setup where the internal electric field is created, no statistically relevant reduction was measured. Nevertheless, shape deviations have been found at a certain distance of the wire through which the electric field is applied. Moreover, the existence of such deviations depends strongly on the AC frequency. Regarding the second design, it should be mentioned that the initial parallel plate design was working correctly for water+salt droplets. However, no remarkable effects were observed applying strong electric fields of up to 10 kV/cm on HFE-7500 droplets. With the goal of increasing the field strength, two improvised configurations of a metal ring and a needle as a counter-electrodes have also been implemented. In these cases, a strong non-uniform electric field was created which did lead to shape changes of the droplets

Development of a Digital Microfluidic Toolkit: Alternative Fabrication Technologies for Chemical and Biological Assay Platforms

This thesis proposes the development of a digital microfluidics (DMF) device using alternative fabrication methods and materials for application in chemical and biological assays. DMF technology which relies on electrowetting-on-dielectric (EWOD) mechanism, offers several advantages such as reduced sample volume, faster analysis, device flexibility, and portability. It is however not without shortcomings as the fabrication of DMF devices is expensive while the reliability of such devices is reduced due to surface contamination when highly concentrated biomolecular samples (e.g. protein and cells) are used. The first experimental work in this thesis aims to reduce the cost of electrode patterning of DMF devices by investigating the use of inkjet printing method in conjunction with several combinations of conductive ink and substrate. It has been found that EWOD device made of PEDOT:PSS, a type of conductive polymer ink printed on Melinex®, a polyethylene terephthalate substrate presents the most reliable droplet actuation performance with velocity comparable to the standard chrome-on-glass device. Two types of inkjet-printed PEDOT:PSS-on-Melinex® device have been fabricated; one is a 3D 4 × 4 electrode array device and the other is a magnetic micro-immunoassay device establishing the feasibility of the proposed method. The 3D 4 × 4 electrode array device which utilises both sides of the substrate (i.e. top and bottom surfaces) for electrode patterning allows for future construction of multi-level DMF devices with large functional area. Implementation of such electrode design increases throughput as it made multiple parallel assays possible. The second inkjet-printed device demonstrates the possibility of employing the PEDOT:PSS-on-Melinex® device in heterogeneous immunoassay by successfully performing mixing and merging of two droplets and more importantly the magnetic beads separation operation. The second experimental investigation concerns the search for substitute materials for the dielectric and hydrophobic components of EWOD device using off-the-shelf products. For the dielectric component, the best performing material in terms of electrowetting reversibility is Rust-Oleum® Polyurethane Finish while for the hydrophobic surface is Top Coating of NeverWet® superhydrophobic material. Both are low-cost materials which employ a very simple spraying technique as their fabrication method. The NeverWet® superhydrophobic material has been selected for detailed investigation due to its other potential function as an anti-biofouling surface to either eliminate or minimise the biomolecules adsorption problem. The superhydrophobic material has shown great potential by demonstrating droplet contact angle reversibility and low roll-off angle for highly concentrated protein solution indicating low adsorption of protein on its surface. A superhydrophobic EWOD device has been fabricated using the Top Coating of NeverWet® as the actuating surface and the device has reliably transported concentrated protein droplets across its surface. It is hoped that the findings in the thesis will assist towards the future realisation of low-cost and robust DMF devices for a wide range of biological and chemical assays applications outside of conventional laboratory environment

Water Droplet Impact on Functional Surfaces

The impact and spreading of picolitre-sized water droplets on a substrate is of importance in many applications such as rapid cooling, delayed freezing, crop spraying, and inkjet printing. In this thesis, the effects of substrate chemistry, roughness, hardness, charge, and porosity on such droplet impact are studied. The effect of roughness was investigated through the use of superhydrophobic CF4 plasma fluorinated polybutadiene. Comparison of the maximum spreading ratio and droplet oscillation frequencies with literature models shows that both are found to be lower than theoretically predicted. Further study of the effect of multiple types of surface topography was carried out via the CF4 plasma texturing of honeycomb surfaces, leading to hierarchical surfaces with roughness on two length scales. This led to the discovery that surfaces with similar static contact angles can give rise to different droplet impact dynamics, governed by the underlying surface topography. The effect of the mechanical properties of the substrate upon picolitre droplets can be important in microfluidics. The oscillatory dynamics of picolitre droplets following impact were found to depend upon the thickness and elasticity of the substrate. Higher oscillation frequencies are measured for softer and thicker films, which are correlated to larger surface deformations around the contact line. Static buildup during inkjet printing is known to affect print quality. The role of surface charge on picolitre droplet impact onto polymer substrates is found to give rise to increased droplet impact velocities. Higher surface potentials can result in unexpected behaviour such as droplet bouncing or increased contact area diameters leading to a decrease in print resolution. Printing on porous materials is important as porosity can aid ink adhesion and durability. CF4 plasma fluorination of porous membranes can inhibit droplet spreading laterally over a surface, with little change in the imbibition behaviour in the material, leading to printing that is more highly defined. These hydrophobic membranes remain oleophilic and could also find use in oil–water separation. Similarly, a hydrophilic–oleophobic switching surface can be beneficial in a range of applications such as anti-fogging, self-cleaning, and oil– water separation. Polelectroyle–fluorosurfactant complexes were found to exhibit excellent switching, resulting in a surface that quickly becomes hydrophilic whilst remaining oleophobic