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

Inverse electrorheological fluid reduces switching time of electrophoretic display

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Electrophoretic displays are bistable devices capable of storing images over a long period of time with low power consumption. However, such external factors as heat or gravity can induce an undesired movement of the display medium particles, even in an encapsulated display. The display bistabiltiy is thus disturbed. This reduces the imaging time. In addition, electrophoretic displays need considerable switching time for pixel addressing. Reducing the switching time is obtained (for example) by reducing the thickness of the electrophoretic layer or using structured addressing electrodes (in particular, active switching matrixes). The structures are substantially complex. It is necessary to reduce the switching time of an electrophoretic display, simultaneously improving the display bistability through passive matrix addressing.
To reduce the switching time, using a nonlinear increase in the velocity of pigment particles by adding an inverse electrorheological fluid to the electrophoretic suspension is proposed. A conventional electrorheological fluid increases the viscosity in a strong electric field. An inverse electrorheological fluid reduces the viscosity under the influence of a relatively strong electric field that exceeds a threshold. The suspending fluid contains a polymer exhibiting an inverse electrorheological effect (for example, polyisobutylene). When the concentration of the polymer molecules is higher than the overlap concentration, branches of adjacent polymer chains with anionic (for example, phosphonate) and cationic (for example, vinylpyridine) functional groups are joined into charge pairs. A continuous, physically cross-linked network is formed in the solution due to the charge pairs. The polymer forms a gel that provides resistance to the movement of the pigment particles. When the external addressing field is above a threshold (i.e., overcomes the ionic bond of the paired charge sites of the adjacent molecules) the ionic bond is broken. The network is temporarily disrupted. With a larger number of broken network bonds, the viscosity of the medium sharply decreases. The decreased viscosity results in an abrupt reduction of resistance to the movement of the charged pigment particles. Applying an additional voltage via a microcapsule causes the particle to move within the microcapsule. In an inverse electrorheological fluid, nonlinear growth of the particle velocity occurs in response to the applied voltage. A charged pigment particle is typically surrounded by a layer of counter ions from the adjacent suspending fluid layer, so the pigment particle is effectively uncharged. After the disruption of ionic pairs in branches of the adjacent polymer molecules (and gel liquefying) caused by the over-threshold electric field, the charge of the separated ions is temporarily compensated by the tearing out of the counter ions from the suspending fluid layer bordering the pigment particle. This increases the effective pigment particle charge. Correspondingly, the force of the external electric field exerted upon the particle is increased. Under the additional voltage, the particles accelerate and rapidly travel the distance between the electrodes. The switching time is thus shorter. At the same time, the display stability is improved with respect to coagulation and external factors. Therefore, the nonlinear dependence of the particle velocity upon the applied field in the inverse electrorheological fluid reduces the switching time of the electrophoretic display.
Additional information
The switching time may be reduced at least twice. When the medium contains several particle types, there may be several viscosity change threshold levels (one threshold level for each particle type). One of the voltages is alternating in order to improve the flexibility of controlling the particle movement in pixels.
US Patent 6693620; Link >>
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