Optical Fluid-based Photonic And Display Devices

Conventional solid-state photonic devices exhibit an ultra-high optical performance and durability, but minimal adaptability. Recently, optical fluid-based photonic and display devices are emerging. By dynamically manipulating the optical interface formed by liquids, the optical output can be reconfigured or adaptively tuned in real time. Such devices exhibit some unique characteristics that are not achievable in conventional solid-state photonic devices. Therefore, they open a gateway for new applications, such as image and signal processing, optical communication, sensing, and lab-on-a-chip, etc. Different operation principles of optical fluidbased photonic devices have been proposed, for instance fluidic pressure, electrochemistry, thermal effect, environmentally adaptive hydrogel, electro-wetting and dielectrophoresis. In this dissertation, several novel optical fluid-based photonic and display devices are demonstrated. Their working principles are described and electro-optic properties investigated. The first part involves photonic devices based on fluidic pressure. Here, we present a membrane-encapsulated liquid lens actuated by a photo-activated polymer. This approach paves a way to achieve non-mechanical driving and easy integration with other photonic devices. Next, we develop a mechanical-wetting lens for visible and short-wavelength infrared applications. Such a device concept can be extended to longer wavelength if proper liquids are employed. In the second part, we reveal some new photonic and display devices based on dielectrophoretic effects. We conceive a dielectric liquid microlens with well-shaped electrode for fixing the droplet position and lowering the operating voltage. To widen the dynamic range, we demonstrate an approach to enable focus tuning from negative to positive or vice versa in a single dielectric lens without any moving part. The possibility of fabricating microlens arrays iv with different aperture and density using a simple method is also proposed. Furthermore, the fundamental electro-optic characteristics of dielectric liquid droplets are studied from the aspects of operating voltage, frequency and droplet size. In addition to dielectric liquid lenses, we also demonstrate some new optical switches based on dielectrophoretic effect, e.g., optical switch based on voltage-stretchable liquid crystal droplet, variable aperture or position-shifting droplet. These devices work well in the visible and near infrared spectral ranges. We also extend this approach to display and show a polarizer-free and color filter-free display. Simple fabrication, low power consumption, polarization independence, relatively low operating voltage as well as reasonably fast switching time are their key features

Actuation Of Droplets Using Transparent Graphene Electrodes For Tunable Lenses And Biomedical Applications

Variable focal length liquid microlenses are the next candidate for a wide variety of applications. Driving mechanism of the liquid lenses can be categorized into mechanical and electrical actuation. Among different actuation mechanisms, EWOD is the most common tool for actuation of the liquid lenses. In this dissertation, we have demonstrated versatile and low-cost miniature liquid lenses with graphene as electrodes. Tunable focal length is achieved by changing both curvature of the droplet using electrowetting on dielectric (EWOD) and applied pressure. Ionic liquid and KCl solution are utilized as lens liquid on the top of a flexible Teflon-coated PDMS/parylene membrane. Transparent and flexible, graphene allows transmission of visible light as well as large deformation of the polymer membrane to achieve requirements for different lens designs and to increase the field of view without damaging of electrodes. Another advantage of graphene compared to non-transparent electrodes is the larger lens aperture. The tunable range for the focal length is between 3 and 7 mm for a droplet with a volume of 3 μL. The visualization of bone marrow dendritic cells is demonstrated by the liquid lens system with a high resolution (more than 456 lp/mm). The Spherical aberration analysis is performed using COMSOL software to investigate the optical properties of the lens under applied voltages and pressure. We propose a prototype of compound eye with specific design of the electrodes using both tunable lenses and tunable supporting membrane. The design has many advantages including large field of view, compact size and fast response time. This work maybe applicable in the development of the next generation of cameras, endoscopes, cell phones on flexible platform. We also proposed here the design and concept of self-powered wireless sensor based on the graphene radio-frequency (RF) components, which are transparent, flexible, and monolithically integrated on biocompatible soft substrate. We show that a quad-ring circuit based on graphene transistors may simultaneously offer sensing and frequency modulation functions. This battery-free and transparent sensors based on newly discovered 2D nanomaterials may benefit versatile wireless sensing and internet-of-things applications, such as smart contact lenses/glasses and microscope slides

Electrohydrodynamic printing of a dielectric elastomer actuator and its application in tunable lenses

Optical lenses driven by dielectric elastomer (DE) actuators with tunable focal lengths are presented here. They are inspired by the architecture of the crystalline lens and the ciliary muscle of the human eye and have prompted a growing interest. The most commonly used DEs in tunable lenses have often required highly transparent films and also the need to encapsulate clear liquid silicone to act as the lens. There is a restriction on the properties of the tunable lens imposed by materials limitations. Here, the fabrication of a fully 3D printed tunable lens with an inhomogeneous structure is described. It exhibited a 29% change in focal length from 33.6 mm to 26.1 mm under a dynamic driving voltage signal control. Furthermore, it displayed excellent stability when the focal length was tuned from far to near (30.1 mm to 25.3 mm) for 200 cycles. The tunable lens obtained mimics the working principle of the human eye in auto adjusting the focal length and has evident potential applications in imaging, information storage, beam steering and bifocal technology

Electrotunable liquid sulfur microdroplets.

Manipulating liquids with tunable shape and optical functionalities in real time is important for electroactive flow devices and optoelectronic devices, but remains a great challenge. Here, we demonstrate electrotunable liquid sulfur microdroplets in an electrochemical cell. We observe electrowetting and merging of sulfur droplets under different potentiostatic conditions, and successfully control these processes via selective design of sulfiphilic/sulfiphobic substrates. Moreover, we employ the electrowetting phenomena to create a microlens based on the liquid sulfur microdroplets and tune its characteristics in real time through changing the shape of the liquid microdroplets in a fast, repeatable, and controlled manner. These studies demonstrate a powerful in situ optical battery platform for unraveling the complex reaction mechanism of sulfur chemistries and for exploring the rich material properties of the liquid sulfur, which shed light on the applications of liquid sulfur droplets in devices such as microlenses, and potentially other electrotunable and optoelectronic devices

Electrohydrodynamic Printing of a Dielectric Elastomer Actuator and Its Application in Tunable Lenses

Optical lenses driven by dielectric elastomer (DE) actuators with tunable focal lengths are presented here. They are inspired by the architecture of the crystalline lens and the ciliary muscle of the human eye and have prompted a growing interest. The most commonly used DEs in tunable lenses have often required highly transparent films and also the need to encapsulate clear liquid silicone to act as the lens. There is a restriction on the properties of the tunable lens imposed by materials limitations. Here, the fabrication of a fully 3D printed tunable lens with an inhomogeneous structure is described. It exhibited a 29% change in focal length from 33.6 mm to 26.1 mm under a dynamic driving voltage signal control. Furthermore, it displayed excellent stability when the focal length was tuned from far to near (30.1 mm to 25.3 mm) for 200 cycles. The tunable lens obtained mimics the working principle of the human eye in auto adjusting the focal length and has evident potential applications in imaging, information storage, beam steering and bifocal technology

Ultrasonic actuation of a fine-needle improves biopsy yield

Despite the ubiquitous use over the past 150 years, the functions of the current medical needle are facilitated only by mechanical shear and cutting by the needle tip, i.e. the lancet. In this study, we demonstrate how nonlinear ultrasonics (NLU) extends the functionality of the medical needle far beyond its present capability. The NLU actions were found to be localized to the proximity of the needle tip, the SonoLancet, but the effects extend to several millimeters from the physical needle boundary. The observed nonlinear phenomena, transient cavitation, fluid streams, translation of micro- and nanoparticles and atomization, were quantitatively characterized. In the fine-needle biopsy application, the SonoLancet contributed to obtaining tissue cores with an increase in tissue yield by 3-6x in different tissue types compared to conventional needle biopsy technique using the same 21G needle. In conclusion, the SonoLancet could be of interest to several other medical applications, including drug or gene delivery, cell modulation, and minimally invasive surgical procedures.Peer reviewe

A novel adaptive mechanical-wetting lens for visible and near infrared imaging

We demonstrate an adaptive mechanical-wetting lens with a concentric reservoir to reduce image aberrations and overcome the gravity effect. This lens adopts liquid pressure to change the interface between two immiscible liquids which, in turn, changes the focal length of the resultant liquid lens. Good optical performance, high resolution, and a wide dynamic range of both positive and negative optical power are achieved. Since no PDMS is employed, such lenses can extend their working range to infrared region by choosing proper liquids

Near axisymmetric partial wetting using interface-localized liquid dielectrophoresis

The wetting of solid surfaces can be modified by altering the surface free energy balance between the solid, liquid, and vapour phases. Liquid dielectrophoresis (L-DEP) can produce wetting on normally non-wetting surfaces, without modification of the surface topography or chemistry. L-DEP is a bulk force acting on the dipoles of a dielectric liquid and is not normally considered to be a localized effect acting at the interface between the liquid and a solid or other fluid. However, if this force is induced by a non-uniform electric field across a solid -liquid interface, it can be used to enhance and control the wetting of a dielectric liquid. Recently, it was reported theoretically and experimentally that this approach can cause a droplet of oil to spread along parallel interdigitated electrodes thus forming a stripe of liquid. Here we show that by using spiral shaped electrodes actuated with four 90º successive phase shifted signals, a near axisymmetric spreading of droplets can be achieved. Experimental observations show that the induced wetting can achieve film formation, an effect not possible with electrowetting. We show that the spreading is reversible thus enabling a wide range of partial wetting droplet states to be achieved in a controllable manner. Furthermore, we find that the cosine of the contact angle has a quadratic dependence on applied voltage during spreading and deduce a scaling law for the dependence of the strength of the effect on the electrode size

Shape Memory Alloy for Adaptive Optics

Shape memory alloys (SMAs) are smart materials that, upon deformation, return to their original shape when heated above a temperature threshold. This special behaviour makes SMAs very attractive as effcient strain/force actuators with very large strains (up to 8%). For this reason, SMAs have been widely used in several applications, from surgery to space. In thesis work, we considered the use of SMAs to build a deformable lens, i.e. a lens with tunable focal length. Usually, optical systems are realized by using optical components such as lenses and mirrors that have fixed optical power (focal length) and that sometimes can be moved, as it occurs in zoom systems. In such cases the movements are achieved manually or by using motors that are slow and subjected to mechanical wear. More recently, optical elements with variable focal length, hence called "adaptive" have been developed, allowing for a great simplification in optical design. Among all the available materials used for actuation, SMAs possess a higher strain with respect to the common actuation materials used to build deformable lenses (such as the piezoelectric). Moreover, they don't need high actuation voltages and, by using SMA wires, allow to realize very small actuators, and consequently very compact devices. Herein, we demonstrate that SMA wires can be efficiently employed for developing lenses with variable focal length with small dimensions, easily embeddable in wearable optics

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