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Function-oriented Knowledge Base \ Electronic paper \ Improve image quality of paper-like display

An initializing alternating drive voltage prevents flexible display particle coagulation

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Paper-like displays have small weight, high contrast, and brightness in natural light, and are capable of changing an image with only a small energy consumption. In a display medium with particles in a layer of a contrasting liquid, the liquid penetrates into gaps between the coloring particles attracted to the display surface and reduces the color density provided by the coloring particles. The reduced color density results in insufficient contrast. Dry solid electrophoretic displays are free of the problems related to liquid. A completely solid display medium contains solid conductive coloring toner (e.g., black) particles and insulating coloring toner (e.g., white) particles in an atmosphere of an indifferent gas. However, after several changes of the image, substantial adhesion and coagulation occur in the display medium that encloses the particles in the space between the two electrodes, being repeatedly displayed on the image in the form of dot-like defects. In addition, if the medium is comprised of an element that maintains a gap between the electrode substrates and separates the space between the electrodes into a plurality of cells, the particles gradually adhere to the gap element. As a result, the defects in the image lower the display contrast. In order to obtain a higher display contrast, it is necessary to prevent particle coagulation in the flexible display.
To prevent particle coagulation in a flexible display, applying an initializing, alternating drive voltage to the electrodes prior to the switching voltage is proposed. During the operation of a display, a coloring particle can vary the electrification amount or size due to frictional electrification. Such variations influence the mobility of the particles under the effect of an electric field between the electrodes. In addition, the mobilities of the particles differ for one and the same electric field, depending upon the state of adhesion of the particles to a substrate or the contact state between adjacent particles. Correspondingly, if an electric field is applied between the electrodes, the mobile particles move, while those that cannot move adhere to the substrate or an adjacent particle. The particles that cannot easily move form coagulation with repeating changes in the display. The initializing, alternating drive voltage is applied before or after the display drive voltage. When the initializing, alternating drive voltage is applied between the electrodes, an electric field with a rapidly changing direction is formed. Under the influence of the alternating field, the mobile particles move reciprocally between the electrodes. While moving, the mobile particles collide with those that cannot easily move. The latter overcome their adhesion, dissociate from the electrode or adjacent particles, and become capable of moving. As a result, particle coagulation is prevented. In addition, even after particle coagulation is formed, the particles that do not coagulate are mutually mobile and collide repeatedly against the coagulation, thereby dissociating the coagulation. Therefore, an initializing, alternating drive voltage prevents particle coagulation in a flexible display.
Additional information
The frequency of the initializing voltage ranges from 20 Hz to 20 kHz. Frequencies below 20Hz correspond to the voltage rapidly switching the image color and even facilitate coagulation accumulation. At frequencies over 20 kHz, the movement of the particles lags behind the changes in the field and the influence of collisions on the coagulation dissociation is eliminated. The time the initializing alternating voltage is applied ranges from several periods to 0.5 second. The magnitude of the initializing alternating voltage is 150-350 V. The resulting contrast is determined by the ratio between the densities of black (1.6) and white (0.3), which are sustained for 30-40 thousands of display changes. In addition to the contrast, the speed of operation of the display is increased.
US Patent 6753844; Link >>
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