In recent years, development of rewritable recording medium (which is capable of being subjected to a number of times of recording/erasing cycle of highly visible image and does not require energy for maintaining a display state) has been advanced actively. Herein, such a rewritable display means that it inherits a characteristic of hard copy is referred to as “paper-like display”.
Necessary conditions of paper-like display are such that it is rewritable, requires no or less energy for maintaining a display state (memory characteristic), and is excellent in portability and display quality, and the like.
As a display method capable of being utilized for the paper-like display, there are those using particle movement-type display apparatuses. As one example of Harold D. Lees et al. (U.S. Pat. No. 3,612,758) or an electrophoretic display apparatus described in Japanese Laid-Open Patent Application (JP-A) No. 2003-195363 has been known.
The conventional electrophoretic display apparatus comprises a dispersion system, comprising an insulating liquid and colored electrophoretic particles dispersed in the insulating liquid, and a pair of electrodes disposed opposite to each other with the dispersion system therebetween. By applying a voltage to the dispersion system through the electrodes, the colored electrophoretic particles are adsorbed on the electrode of a polarity opposite to that of electric charges of the colored electrophoretic particles through Coulomb force utilizing an electrophoretic characteristic of the colored electrophoretic particles. Display is effected by utilizing a difference in color between the colored electrophoretic particles and a dyed insulating liquid. More specifically, in the case where the electrophoretic particles are adsorbed on the surface of a light-transmissive first electrode close to an observer, the color of the electrophoretic particles is observed and on the other hand, in the case where the electrophoretic particles are adsorbed on the surface of a second electrode distant from the observer, the color of the insulating liquid dyed so as to be different in optical characteristic from the electrophoretic particles is observed.
However, in such an electrophoretic display apparatus (hereinafter, referred to as a “vertical movement-type electrophoretic display apparatus), a color former such as a dye or ion has to be mixed in the insulating liquid but the presence thereof is liable to act as an unstable factor in an electrophoretic operation since the color former causes new giving and receiving of electric charges. For this reason, the use of the color former has lowered the performance, the life, and stability of the display apparatus in some cases.
In order to solve such problems, a horizontal movement-type display apparatus in which a pair of first and second electrodes are disposed on the same substrate and colored electrophoretic particles are moved horizontally when viewed from the observer has been proposed. In the display apparatus, by utilizing the electrophoretic characteristic, a voltage is applied to the colored electrophoretic particles are moved in parallel to the substrate surface between the first electrode surface and the second electrode surface (JP-A Nos. Hei 9-211499, 2001-249366, 2002-162650, and 2003-270674, and U.S. Patent Application Publication No. US2001/0030 639A1).
In the horizontal movement-type electrophoretic display apparatus, the insulating liquid is transparent in many cases and when viewed from the observer, the first electrode and the second electrode assume different colors one of which is identical to the color of the electrophoretic particle. For example, when the color of the first electrode is black, the color of the second electrode is white, and the color of the electrophoretic particles is black, in the case where the electrophoretic particles are distributed on the first electrode, the second electrode is exposed to assume white and in the case where the electrophoretic particles are distributed on the second electrode, the color of the electrophoretic particles, i.e., black is displayed.
However, in the conventional horizontal movement-type electrophoretic display apparatus, the electrophoretic particles are required to be moved from one electrode to the other electrode thereby to cover the other electrode surface, so that a movement distance is generally longer than that in the vertical movement-type electrophoretic display apparatus. For this reason, the horizontal movement-type electrophoretic display apparatus takes a long time from start of drive to stabilization in gradation display.
In addition, in the case where the first electrode and the second electrode are disposed on the same substrate as described above, there arises a difference in local electric field in ambient space, so that a difference in movement of the electrophoretic particles is caused to occur depending on place. More specifically, in the case where an amount of charge of the electrophoretic particles is not changed, a quick migration portion by a strong electric field and a slow migration portion by a weak electric field are present. Particularly, a lowering in moving speed of the electrophoretic particles at the weak electric field portion causes a lowering in display speed and a lowering in gradation display stability.
USPTO Backgrounds
USPTO Backgrounds