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Link to NDL lab tour and overview
Global Academic Leader in Reflective Displays
We are one of very few labs to publish on and contribute to the science and technology of nearly all types of displays (transmissive, emissive, 3D, reflective, projection, transparent, flexible, rollable, nature/biological). Our earliest exposure to displays was during our directors undergraduate degree (total of 18 months of full-time internship at 3M Precision Optics, projection TVs), and after his Ph.D. has launched two start-up companies in displays. Recently, we have led the authoring of the authoritative review for e-Paper technology (reflective displays click here for article). NDL continues research in displays and in large-area optical shutter technology.
Global Academic Leader in Applied Electrowetting
Berges modern electrowetting system consists of an aqueous solution that is electrically insulated from a planar electrode by a hydrophobic dielectric (see Figure below) In equilibrium, the aqueous solution exhibits a large Youngs contact angle (θY), ranging from about 160 degrees to 180 degrees in an oil environment. When voltage (V) is applied, the contact angle projection can be reduced by more than 100 degrees to the electrowetted contact angle (θV). Click here for video. As Jones de-scribed in 2005, an electromechanical force physically governs this electrowetting effect. For a quick reference guide, click on the image below.
Also, one must be very careful when choosing dielectrics and voltages for electrowetting operation. The following excel spreadsheet will help you ensure that you do not exceed the electric fields for any layer in your dielectric stack (click here).
Wearable Non-Invasive Sweat Biomarker Sensor (NDL Invention)
Sweat for medical diagnostics has been largely overlooked in comparison to blood, urine, breath, and saliva. Research now reveals that numerous water-soluble biomarkers for health monitoring or diagnosis can be found in sweat at comparable concentrations as in blood. Therefore, medical science is potentially on the verge of a transformative shift in fundamental approaches for non-invasive physiological sensing. Of particular impact would be wearable biomarker sensors providing a fast readout of concentration(s). However, this will require new electrical sensing technology, as current sweat biomarker biosensing is capable of only inorganic ion sensing (hydration, infant cystic fibrosis). We are now moving toward molecular biosensing of biomarker concentrations in the 1-10 pM range using a wireless patch containing all the electronics and microfluidics for electrical sweat stimiulation, with computing being provided through communiation with an RFID or bluetooth smartphone (see image above). For this project we have also created the worlds first sweat simulator (artificial skin, see SEM image at left) allowing rapid in-vitro testing and sensor development.
Electrowetting Microprisms (NDL Invention)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. What is still lacking is a continuous beam steering architecture that is thin, wide angle (>+/-30 degrees), 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 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) Click here for video. At NDL, we are now able to stabilize the liquids with four electrowetting sidewalls and make arrays of 1000's of prisms, each prism only ~150 um in width.
NDL has also recently reported electrowetting manipulation of any optical film(see image to left). We can suspend diffraction gratings, mirrors, or any other type of optical film between the oil and water phases, and tilt the film continuously in any 2D direction through control by four electrodes.
Click on the video below of UC's SmartLight, it's a Revolution in Interior Lighting Ready to Shine!
Programmable Lab-on-Chip by Laplace Barriers(NDL Invention)The overall objective of this program is to create an adaptable on-demand microchannel platform which can be electronically configured, maintain its programmed configuration even when electrical power is removed, and thereby allow a single module to access to the vast library of lab-on-chip functions. Our recent results demonstrate that by balancing in-plane and out-of-plane electrowetting and Young-Laplace forces, emergent control in microchannel formation and fluid transport can be realized. Our rationale is that the lab-on-chip community will be able to leverage the enormous infrastructure for liquid crystal display manufacturing and 4-bit computer interfacing, to provide the world with a highly affordable and agile lab-on-chip product (see image below). The ability to create channels on demand and hold the channels even without application of voltage is provided by Laplace Barriers. Our newest Laplace Barrier platform consists of partial-posts, and eliminates the disadvantages of full-posts or ridges, while providing ~60-80% open channel area for rapid electrowetting fluid transport (>5 cm/s).
Reconfigurable Liquid Metal Electronics (NDL Invention)The term reconfigurable electronics covers a wide variety of technologies, ranging from field-programmable gate arrays to radio-frequency micro-electro-mechanical systems. These conventional approaches use electronic switches to dynamically alter the interconnections between a static layout of electrical wires. What if instead, we could dynamically reconfigure, erase, or write, the electrical circuit wiring itself? Such capability would be of significant value for simple electronic switches, tunable antennas, adaptive microwave reflectivity, and switchable metamaterials, to name a few applications. For example, reconfiguring antenna circuits can dramatically change the resonant wavelength, as commercially proven with mechanical switches, transistors, or diodes. Recently, there has been growing interest in use of microfluidics to reconfigure simple circuits and antennas including integration into soft/conformal substrates. However, many of these prior approaches switch slowly, require bulky external control systems, and cannot reconfigure the actual wires comprising the circuit. We have developed an approach for reconfigurable circuits, based on competitive liquid metal shaping with Laplace and vacuum pressures. With <10 pounds per square inch (psi) vacuum applied to two plastic films, one film having a network of microreplicated trenches, Laplace pressure drives liquid metals such as eutectic GaIn or Hg into the trenches. Upon release of vacuum, the liquid metal pattern is erased, as it rapidly dewets into droplets that compact to 10-100x less area than when in the trenches (see figure above). We have demonstrated simple erasable resistive networks, and a switchable 4.5 GHz dipole antenna. Advantages of this approach include: ultra-simple and scalable fabrication; ultra-simple operation with few peripheral controls or interconnects; vacuum actuation, or even as simple as fingertip pressure. Collectively, this work provides both greater simplicity, and in some aspects improved performance for applications such as large flexible sheets of switchable metamaterials, ultra-low-cost tunable antennas, and tunable transmittance or reflectance of large-area electromagnetic shielding.
Electrowetting Displays (Global Academic Leader)
The first electrowetting display technology to capture researchers attention was the dye-colored oil film approach discovered by Hayes and Feenstra at Philips. 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 degrees, so the contact angle for the oil is about 20 degrees to 0 degrees). 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. 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. NDL has also created an emissive version (see Lightwave Coupling section of this research page). At Cincinnati's electrowetting program, we have worked in collaboration with several companies and with the Industrial Technology Re-search Institute (ITRI) in Taiwan. ITRI has scaled the electrowetting display fabrication process to greater than 1700 cm2 on an active matrix backplane, using standard LCD manufacturing equipment.
Electrofluidic Displays (NDL Invention)
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.
Electrofluidic Imaging Film (NDL Invention)
Many consumers are not aware that electronic displays significantly compromise the potential capability for portable electronic devices and electronic signage. For example, transmissive LCD and emissive OLED displays themselves are thin, but they require significant power causing the battery to often become the single heaviest component in a mobile device. These same displays that provide brilliant color indoors, collapse in color and contrast in sunlight down to levels far below even common newsprint. Furthermore, LCD and OLED have an unavoidable compromise between portability and screen size. Electronic paper (e-Paper) technology like E-Ink can in theory resolve all three shortcomings by: (a) holding an image without electrical power; (b) using reflected sunlight to create the screen image; (c) more easily allowing rolling/folding compaction of the screen size. However, e-Paper is far from universal in usage, because there is a lack of a single technology that can provide both color reflection greater than R~20% and image updates fast enough for at least crude video. Furthermore, even in monochrome operation, no e-Paper technology has yet reproduced the quality of magazine print with R=3% black / R=76% white. We have recently created a completely redesigned approach for e-Paper, we refer to as an electrofluidic imaging film. Unique from the dozen or more existing e-Paper technologies, an ink is electromechanically transported to the viewable front, or hidden in the back, of a highly engineered porous film. This film is a first for all types of electrowetting-style displays by allowing non-aligned lamination fabrication. Furthermore, the film is able to efficiently split merged fluid and therefore eliminate the need for ink microencapsulation or pixel borders needed in other displays that also move a colorant. Cumulatively, these advances provide an electrofluidic imaging film that has fast switching (<15 ms), potential for exceeding magazine quality reflectance (R>76%), and simpler manufacturing by non-aligned lamination.
Light Wave Coupling (NDL Invention)
The Bi-Primary Color System (NDL Invention)In 2011 the we reported on a new bi-primary color system for displays which doubles the reflectance and color saturation compared to the RGB and RGBW color systems employed by Qualcomm, and E-Ink, respectively. The unique bi-primary color-system cooperatively displays two complimentary primary colors (bi-primary) inside each sub-pixel (for example, red is the visible spectrum compliment of cyan). Each RGB primary is matched up with its CMY compliment in three sub-pixels. As shown in the simplified device diagram below, pigments particles can be stored in reservoirs or be spread over the pixel (overlap). If only one particle type is spread over a pixel, it creates that color for the pixel, if none are spread the color is white, if both are spread and mixed it creates black (K). Bi-primary is best suited for colorant-transposition technologies (those that physically move particles, pigments, or dyes). This is an advantage, because colorant transposition technologies exhibit some of the highest reflectance and color fractions (CFs). The maximum reflection and CF performance for bi-primary is best understood by examining top-view pixel illustrations and example calculations for display of W and R colors. Firstly, for W, bi-primary can remove all color from the pixel for a max theoretical reflection of 100%. Bi-primary therefore provides double the white reflectance of RGBW color- filtering, which has a theoretical maximum of 50%. Next example, consider reflectance for displaying R, the three side-by-side subpixels would display the RMY pigments for a reflectance of 55% (33% from R, and 67% from each M and Y). The color fraction (CF) is also doubled compared to RGBW. R contributes 100% to the CF and M&Y 25% each. The CF for M&Y is 25% because each is comprised of half R (50% CF) but also a non-red color (B, G) which reduces their CF by a factor of two. The net CF is 50% which is again double the CF for RGBW. In summary, biprimary is the ONLY single-layer display technology that can double both the brightness and color-fulness of e-Paper compared to the only other available single-layer approach of RGB or RGBW color filtering.
Electrowetting Retroreflectors (NDL Invention)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 +/-30degrees of view. The operation principles for these devices are fundamentally simple. Using our liquid self-assembly processes, we form 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 have now taken this project all the way to a complete system level integration (see image below) and are working toward commercial use of the technology.
Dielectrowetting Optical Shutter (NDL Invention)
Highly optically efficient shutters remain a significant challenge, despite the large number of technologies that have been developed for transmissive and reflective displays. Some applications, such as smart windows or switchable architectural/privacy glass, pose even greater challenges such as the need for very low cost, low haze, default to a clear state with power-failure, and the desire for optical transparencies that are beyond the <50%, typical of existing commercial products. For large area windows or architectural glass, unlike pixelated (matrix) displays, one aspect that is not highly constrained is the use of higher voltage or frequency, so long as the power consumption is adequately low (high voltage/frequency inverters are low cost, e.g. compact fluorescent bulbs). Recently, ~200-300 Vpp and ~10 kHz superspreading of fluids has been demonstrated by dielectrowetting (McHale and Brown). Similar dielectrophoretic modulation has also been recently used for tunable-focus microlenses (Hseih), color displays (S.T. Wu),. We have now demonstrated for the 1st time, dielectrowetting superspreading and deterministic dewetting of an opaque ink fluid to create an ultra-simple and large-area (>10 cm2) optical shutter which operates at very low power (~10s µW/cm2 at 0.1-10 Hz). The ink is wetted from numerous sessile droplets into a continuous and optically opaque ~15 µm thick film (T<1.5% ~420-680 nm). When voltage is removed, the fluid splitting features then rapidly and deterministically initiate breakup of the ink back into small sessile droplets, resulting in a very high 80% optical transmission with very low haze. The applications for this device include those desirable of large sizes, high optical clarity, clear appearance with power failure, independence of polarization or angle, very high contrast ratio (average ~400:1 for several portions of the visible spectrum), fast response, and simple low cost fabrication, all of which are valuable for applications in smart windows, switcable architectural glass, or transparent or reflective digital signage.
The Fundamental Limits of Electrowetting Materials
The main limitation of electrowetting is the saturation of the contact angle to around 50-70 degrees above a certain voltage threshold (see image at right). Numerous theories have been proposed to explain this phenomenon, without unequivocally elucidating it. Furthermore, study of electrowetting fluids has so far been restricted mainly to aqueous solutions. Although water has a high surface tension and easily accepts ionic content, it lacks robust environmental range, and tends to lead to corrosion of electrodes. Thus, in order to further explore and extend electrowetting performance, we are intensely investigating the three electrowetting (dielectric, polar phase, and non-polar phase) materials and their interactions. We have experimentally demonstrated that when using DC voltage, electrowetting contact angle saturation is invariant with electric field, contact line profile, interfacial tension, choice of non-polar insulating fluid, and type of polar conductive fluid or ionic content. While charge injection is not the main cause of saturation, it affects the electrowetting system reliability. Using good quality hydrophobic and dielectric materials and appropriate ionic content (large size and low concentration) prevents charging from occurring and limits or even eliminates the risk of dielectric failure. Now that we know electrowetting saturation at ~70 degrees cannot be resolved by materials improvements, we are focusing attention on materials reliability. We are working on improving the dielectric materials through the study of the relationships between their structure and their properties. These findings should advance the development of practical materials with enhanced reliability and performance for real applications, and thus enabling further commercialization.