Novel Devices Laboratory -

Click on the image below to download NDL Electrowetting Reference Guide.

NDL has also developed a means for creating a simple electrowetting kit. This kit can be assembled for a cost of less than $30. Kit development was achieved through an NSF-sponsored Research Experience for Teachers (RET) program in 2006. Complete instructions on kit assembly/operation can be downloaded here.

Electrowetting Microprism Arrays

Beam steering technology has been a limiting factor for laser radar, agile imaging, 3D displays and numerous other applications. This is because current approaches cannot yet provide large-area, thin, transmissive and wide-angle beam deflection. Researchers continue to make substantial progress in the area of liquid crystal beam steering. For example, Kent State scientists are developing a new continuous optical phased array tech-nique. Regarding micro-electro-mechanical (MEMs) technology, several solutions are well-developed for beam steering. However, MEMs only allows for the selection of a few discrete angles, and MEMs is also reflective-only, which causes a large system footprint. What is still lacking is a continuous beam steering architecture that is thin, wide angle (>+/-30°), and can efficiently transmit light of multiple wavelengths and polarizations. Although not switchable, prismatic plates like those long used in lighthouse Fresnel lenses provide most of the desired features for a wide-angle EMPs

beam-steering element. One might therefore argue that the ultimate platform would use highly refractive material and simply alter its geometry. Electrowetting microprisms operate by modifying the physical geometry between two immiscible liquids. In an electrowetting microprism, low index water (n~1.3) and high index oil (n>1.6) are confined by at least two elec-trowetting sidewalls (see Figure above). These sidewalls are connected to two distinct voltages (VL, VR) that independently control the water contact angle at each sidewall (θL, θR). As long as the voltages are selected such that θL+θR=180°, the meniscus is held flat and a variable prism is created. Click here for video. At NDL, we are now able to stabilize the liquids with four electrowetting sidewalls. When such microprisms are formed in large arrays, two operating modes are possible. The first is two-dimensional refractive steering. For this, all four sidewalls are biased with voltage, and beam steering is enabled over all angles within a cone. The second mode is discrete and one-dimensional phased-array steering. Here, each individual prism is a single period in a sawtooth phase profile with fixed pitch but variable amplitude.

NDL has also recently reported electrowetting manipulation of any optical film. A square channel was constructed with four sidewall electrodes, coated with a hydrophobic dielectric, and filled with saline and oil. In a first experiment a dielectric mirror film was suspended between the oil/saline meniscus. Electrowetting at each sidewall produced a saline contact angle change of 35°<θ<170°. This change in contact angle tilted the mirror and +/-105° of laser beam deflection was achieved. A second experiment utilized a Mylar film imprinted with a diffraction grating (625 lines/mm). Electrowetting tilting of the grating was shown to alter the diffraction of the laser beam. Shown at right are images of these devices in operation. Using a simplified version of the fabrication techniques used to make microprism arrays, we have recently demonstrated switchable microlens arrays. They are controlled by a single electrode and can be inverted from convex to concave, much like a Varioptic or Philips liquid lens. However, in arrayed format, such lenses have myriad applications, including stereoscopic displays, adaptive optic technologies, laser array collimators and portable wave-front sensors.

Electrofluidic Membranes

New multidimensional separation platforms must be discovered if the fullest potential is to be achieved for ‘human-on-chip’, lower cost drug development, and rapid detection for counter-terrorism. The separation peak capacity for a multidimensional system is predicted as the product of peak capacities of individual orthogonal dimensions. However, even a well-designed multidimensional system typically involves a complex integration of several individual systems and ancillary pumps/valves. There is a need for new transport/separation platform that is completely self-contained, nano-scale, and electronically reconfigurable into multiple dimensions of separation.

The goal of this NSF-supported project is to create a new platform with levels of separations multidimensionality and programmability that have not existed before. The rationale is that electrofluidic carbon nanofibers can implement reconfigurable multiphase fluid pumping, and can provide separation based on size, mass, surface energy, surface chemistry, ionic charge, and 2D location. This is a collaborative project with the University of Tennessee http://web.utk.edu/~prack/ and Oakridge National Laboratory Molecular-Scale Engineering and Nanoscale Technologies (MENT) group http://www.ornl.gov/sci/ment/.

A video of some of our recent results in this project can be found here. These new results are unpublished and more information will be released in 2009.

Electrowetting Displays and Electronic Paper

In some ways, the flat panel display market is extremely mature. Wide-screen plasma and LCD TVs are now commonplace, and e-book technologies (e.g., the Amazon Kindle) are readily available as well. However, peo-ple who work outside displays may be surprised to learn that liquid crystal displays are typically less than 10 percent optically efficient, and the electrophoretic ink used in e-books is only about 40 percent reflective. There is therefore very good reason to pursue alternate display technologies.

Of the many new display technologies under investigation, arrayed electrowetting devices are particularly compelling. The first electrowetting display technology to capture researchers’ attention was the dye-colored oil film approach discovered by Hayes and Feenstra at Philips (now at the Philips spin-off, EWDs

LiquaVista). This approach uses water covering a film of oil. The oil forms a film beneath the water because the water contact angle is very large (θY ~160 to 180°, so the contact angle for the oil is about 20° to 0°). When voltage is applied, this water electrowets the hydrophobic dielectric causing the oil to “de-wet” the surface. This reduces the viewable area of the oil from 100 to 20 percent. LiquaVista uses a reflective material beneath the display pixel that en-ables an active-matrix video display with reflectivity of greater than 50 to 60 percent.

At Cincinnati, we have further developed this oil film approach (see figure at left, numerous videos can be found here). We have also investigated alternate strategies for higher brightness. Consider, for example, a backlit display like those used in laptops, TVs and cell phones. We have demonstrated the patterning of a reflector underneath the area of the oil in the electrowetted state. This approach recycles light into the backlight until the light can only exit through the optically clear portion of the pixel. As a result, more than 80 percent transmission can be achieved, resulting in higher brightness or lower power consumption. We have further developed fabrication processes that can be implemented in the range of 100° to 120° C. Ac-cordingly, we do much of our fabrication on low-cost and flexible plastic substrates (PET, PEN, etc.). At Cincinnati’s electrowetting program, we work in collaboration with several companies and with the Industrial Technology Re-search Institute (ITRI) in Taiwan. ITRI has recently scaled the electrowetting display fabrication process to greater than 1700 cm2 on an active matrix backplane, using standard LCD manufacturing equipment.

Now that the traditional colored-oil film approach is well on its way to commercialization, we are focusing our research on alternate approaches for reflective displays. In 2008, we reported on arrayed electrowetting microwells, a new approach based on moving colored fluid in front of or behind a white perforated substrate. If the perforated area is small (less

than 5 to 10 percent viewable), then white reflectance on the order of paper (80 to 90 percent) could be achieved in displays. The operation works as follows. At no voltage, colored oil fills a di-verging cavity such as an inverted pyramid, cone, or corner-cube. This provides brilliant coloration to a viewer at the front of the display. When voltage is applied, water electro-wets the capillary; and the colored oil is largely hidden from view. This process is fully reversible and has also been demonstrated with colored water behind the substrate (with colored water, the colored/white response to voltage is simply reversed).

Electrofluidic Display

EFD

Shown at right is our new ‘electrofluidic’ display structure which reduces the visible area of the colored fluid by 2-3X more than that of an electrowetting display. The electrofluidic architecture is further unique from electrowetting displays in driving principles, device structure, potential for bistability, reduced parallax for multi-layer subtractive color pixels, in tight pixel confinement for rollable displays, and in use of water-dispersed pigments instead of oil soluble dyes. We chose the ‘electrofluidic’ nomenclature because the mechanism involves charge induced movement of liquids through microfluidic cavities. The basic electrofluidic structure contains several important geometrical features. First there is a reservoir, which will hold an aqueous pigment dispersion in less than 5-10% of the visible area. Secondly, there is a surface channel of 80-95% of the visible area, and which can receive the pigment dispersion from the reservoir when a suitable stimulus is applied. Third, there is a duct surrounding the device which enables counter-flow of a non-polar fluid (oil or gas) as the pigment dispersion leaves the reservoir. It is important to note, that all of these features are inexpensively formed by a single photolithographic or microreplication step. Turning attention to the figure, several additional coatings and a top substrate are added. First, the surface channel is bound by two electrowetting plates consisting of an electrode and hydrophobic dielectric. The top electrowetting plate utilizes a transparent In2O3:SnO2 electrode (ITO) such that the surface channel is viewable by the naked eye. The bottom electrowetting plate utilizes a highly reflective electrode such as Aluminum. With the device structure described, we now begin a general discussion of device operation. With no applied voltage, a net Young-Laplace pressure causes the pigment dispersion to occupy the cavity that imparts a larger radius of curvature on the pigment dispersion. Therefore at equilibrium, the pigment dispersion occupies the reservoir and is largely hidden from view. This is analogous to connecting two soap bubbles by a straw; the larger bubble has a larger radius of curvature, a lower Young-Laplace Pressure, and will therefore consume the smaller bubble. Next, as shown in the figure a voltage is applied between the two electrowetting plates and the pigment dispersion. This induces an electromechanical pressure that exceeds the net Young-Laplace pressure and the pigment dispersion is pulled into the surface channel. If the volume of the pigment dispersion is slightly greater than the volume of the surface channel, then the pigment will be simultaneously viewable in both the reservoir and the surface channel, and nearly the entire device area will exhibit the coloration of the pigment. If the voltage is removed the pigment dispersion rapidly (1’s to 10’s ms) recoils into the reservoir. Thus a switchable device is created that can hide the pigment, or reveal the pigment with visual brilliance that is similar to pigment printed on paper. Videos of this device in operation can be found on the videos page of this website.

Light Wave Coupling Electrowetting Displays

In 2005, we even demonstrated an emissive electrowetting display format. We created this display by doping the oil with RGB fluorescent dyes and using the substrate as a waveguide for 400 nm pump light. Static image prototypes are shown at right (click on the image for a larger version). Pixelated electrowetting versions of the technology were reported in Applied Physics Letters in 2005. These pixelated devices were fabricated on an optical waveguide substrate. The LWC device structure contains a polar water component and a non-polar oil component that compete for placement on a hydrophobic surface under the influence of an applied electric field. The oil film contains organic lumophores which fluoresce intense red (608 nm), green (503 nm), and blue (433 nm) light with ~90% quantum efficiency when excited by violet light. Violet InGaN light emitting diodes (LEDs) couple ~405 nm excitation light into the waveguide substrate. Electrowetting of the water layer displaces the fluorescent oil film such that it is either optically coupled to, or decoupled from, the underlying waveguide. The output luminance can be modulated from >100 cd/m2 to <5 cd/m2 as a DC voltage ranging from 0 V to –24 V is applied to the water layer. Maximum luminance of 15×30 arrays of the devices may exceed ~500 cd/m2 by simply increasing the output of the InGaN LEDs.

Electrowetting Textiles

Currently the industrial paradigm for all textiles is that wettability is determined at the time of manufacturing thus precluding end-user control. Academe has developed several possibilities for end user-control of wetting, including optically reconfigured spiropyrans, thermocapillarity, and redox-active surfactants. However, there remains a large gap between these significant laboratory results and the simplicity/robustness required for commercially viable textiles and liquids. Our central hypothesis is that electrowetting technology will allow creation of a spectrum of textiles with a range of reversible wetting properties in their interactions with diverse commercial liquids.

Electrowetting Retroreflectors

Corner cube and spherical retroreflectors are ubiquitous in range-finding applications, since they reflect light back to the illumination source with unmatched efficiency. Two forms of large-area retroreflectors dominate: glass beads or a truncated corner of a cube. The corner-cube approach yields several-fold higher retroreflective efficiency. A light ray incident into a corner-cube reflects off the mirrored facets and emerges parallel to the direction of incidence. As a result, an observer positioned next to the illumination source perceives a surface that is more than 50 times as bright as an optically scattering back-ground. Over the past decade, several forms of switchable retroreflectors have been developed. Naked-eye applications have not been strongly pursued because previous approaches are either difficult to scale to the array sizes needed for visualization at a distance, or limited to very-narrow-spectrum infrared. We have recently demon-strated an electrowetting retroreflector with the following features: low-loss and wide-spectrum, as limited only by the absorption spectrum of water; scalability to any size supported by microreplication/molding; and high contrast switching (>10:1) over a +/-30° field of view. The operation principles for these devices are fundamentally simple. Using our liquid self-assembly processes, we form Retroreflectors

an electrowetting lenslet inside each corner cube. At no voltage, the lenslet is concave. A concave len-slet breaks the optical symmetry of the corner cube and causes it to scatter the light (semi-diffuse, large beam divergence). With as little as 3V, the water/meniscus can be electrowetted to a flat geometry and the lenslet disappears. This restores the retroreflecting nature of the corner cube, and laser light, a headlight, or a flashlight is returned to the viewer with brightness that dominates over the surrounding background. These results could prove useful for a variety of applications, including flashing safety markings (which could have personal, road or structural uses), surveying and range finding, free-space communications, active decora-tive films, active barcodes, and military friend-foe-ID. We are working on scaling down the microreplication to the 10-µm size range that can be achieved commercially. This is important for increasing switching speed, but also for creating the thin and flexible form factor that is already available for conspicuity tape.

'Other Stuff'

Like any lab, we are constantly inventing and demonstrating new device technologies. Check back periodily for updates on existing and new NDL devices!