Investigation of oil‐motion non‐uniformity in a reflective display based on electrowetting

Abstract

Abstract— A novel reflective display based on electrowetting technology has gained much attention because of its readability in sun light. This technology can provide high‐efficiency control of the display pixel reflectivity. In the on‐state, the oil droplet is expected to move to the same corner regularly. To realize uniform motion of the oil, it is possible to provide a symmetry‐breaking mechanism in the pixel. However, depending on the uniformity of the processing and the size of the built‐in symmetry breaking, it is possible that the oil actually moves to a different position. The visibility of defects, by the representation of oil‐motion non‐uniformity, depending on different notch pattern, notch size, and defect type, has been investigated in the perception experiments. Results indicate that the influence of the defects is more obvious for the more preferred pattern. The larger the notch sizes, the easier the defects can be observed. The best notch pattern is pointed out. When designing an electrowetting display, these results need to be considered to eliminate the influence of oil‐motion non‐uniformity on the image quality by making the droplet as small as possible or by adding optical layers such as diffusers films.

Oil Motion Control by an Extra Pinning Structure in Electro-Fluidic Display

“…Electrowetting has shown its power in manipulating micro-fluidics, which inspired applications like microfluidic chips [ 1 , 2 ] as well as electro-fluidic displays (EFD) [ 3 ]. To EFD, an oil/water two-phase microfluidic system, the oil motion control is the key for its optical performance, like gray scale, switching response, maximum aperture ratio, oil contraction direction, and so on [ 4 , 5 ].Therefore, the dynamic of oil/water interfacial movement attracts great attention in this field [ 6 , 7 , 8 , 9 , 10 , 11 ]. For a typical EFD pixel ( Figure 1 ), which is sub-millimeterin size, the dominant driving forces for oil motion are recognized as interfacial tension and electrostatic force [ 12 ].…”

“…The most common used method for controlling oil motion in EFD is by introducing a “notch” in the pixelated electrode layer (a designed area without electrode). The artificially designed difference in the electric field distribution shows reasonable guidance in the final location of oil contraction for pixel switching-on [ 4 ]. However, the in-smooth electrode edge brought by “notch” may generate localized high Maxwell stress, which increasesthe risk of dielectric breakdown and oil motion disturbance [ 7 , 11 , 14 , 15 ].…”

Photolithography Fabricated Spacer Arrays Offering Mechanical Strengthening and Oil Motion Control in Electrowetting Displays

“…After the switching-on process with a bias voltage, the colored oil in EWD display is repelled to be an oil droplet at the side or the corner of a pixel with the decreasing θ (Equation (1)). As a result, oil could be driven to move to various directions because of the high symmetry of square pixels, leading to random oil motion states and thus non-uniform optical view [14]. Here, colored oil motion is used to describe the movement of oil/water interface.…”

“…Consequently, various methods have been reported to improve the oil motion controllability. For example, pixel shapes with reduced symmetry have been designed to guide oil motion via capillary force difference, including rectangle, triangle, and trapezoid pixels [14,15]. Meanwhile, an extra obstacle in a pixel also could decrease the symmetry of square pixels.…”

“…Moreover, asymmetric electric field in space or in time could also induce a guided oil motion in pixel. Certain notch area on the electrode has been designed to control oil motion via asymmetric electric field distribution [14]. In general, this oil motion controllability improves with increasing notch size.…”