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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Introduction 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.
Description 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.
