Function-oriented Knowledge Base \ Electronic paper \ Decrease power consumtion of paper-like display
Nanoparticles reduce switching time of encapsulated electrophoretic display
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Introduction Encapsulated electrophoretic displays exhibit
a high degree of contrast and stability against coagulation
of their electrophoretic pigment particles. Under the influence
of an electric field applied across the addressing electrodes,
the pigment particles move to the upper electrode and provide
for the coloring of a display pixel microcapsule into one
of the contrasting colors. Another color may be provided (for
example) by a colored dielectric liquid. As a rule, the particle
size ranges from 0.25 to 2 microns. The particles are substantially
large and thus inherit the bulk optical properties of the
material from which they are formed. The optical properties
of the particles in the display remain constant. With the
constant properties of the electrophoretic particles, the
appearance of the display is changed only by moving the electrophoretic
particles toward the upper or lower electrode within the suspending
fluid under the influence of the applied electric field. However,
the number of colors produced by conventional electrophoretic
displays is limited. In addition, the time necessary to switch
the colors is longer than that in other displays and amounts
to hundreds of milliseconds. It is necessary to reduce the
switching time of an encapsulated electrophoretic display,
while simultaneously increasing the number of produced colors.
Description To reduce the switching time of an encapsulated
electrophoretic display, using colored nanoparticles instead
of common microparticles is proposed. The nanoparticle diameters
range from 1 to 100 nanometers. Due to their nanosize, the
nanoparticles do not practically scatter incident light (or
rather, the light-scattering power is proportional to the
sixth power of the particle radius) until they are concentrated
into cluster particles the sizes of which are comparable to
the light wavelength. A nanoparticle-based display changes
its state (for example) from transparent to opaque depending
upon whether the nanoparticles are dispersed or aggregated.
The thickness of the layer may be no more than 1 micron, so
the time necessary for a nanoparticle to travel the distance
between the electrodes is much less than that in an encapsulated
display based upon electrophoretic particles of micron size.
Therefore, nanoparticles reduce the switching time.
Additional information The number of produced colors may be increased
by adding nanoparticles of another color with a different
electrophoretic mobility. Due to the mobility difference,
the lower mobility nanoparticles reach one of the electrodes
more slowly than more mobile nanoparticles. Thus, when the
particles are aggregated at the upper, transparent electrode,
coloring is provided by the more mobile particles, as they
reach the electrode earlier than the less mobile ones. When
the nanoparticles are aggregated at the bottom electrode,
the less mobile particles reach the electrode later than the
more mobile particles. They coat the more mobile particles
and make them obscure. The coloring is thus provided by the
less mobile particles. When in the third, dispersed state,
the particles expose the lower electrode for viewing. If the
electrode has a third color, this third color becomes the
color of the pixel. The preferred nanoparticle size is below
50 nm. If the nanoparticles consist of a semiconductor as
opposed to dielectric nanoparticles, the color of the clusters
of the semiconductor nanoparticles also depends upon the distance
between the nanoparticles within a cluster. For example, for
gold, the color of a cluster may change from blue to black
depending upon the interparticle distance in the cluster.
In this case, dispersions of gold nanoparticles are typically
ruby red.
