Particles in displays
Particles 2007
Particle-based device technologies
August 18, 2007
Ian Morrison
Cabot Corporation, Billerica MA
Particles 2007, Toronto
Particles in displays?
• Prints with pigments “reflect” light.
– The “readability” is high.
– But the image is static.
• Displays “emit” light.
– The images are dynamic.
– But the “readability” is poor.
Particles 2007, Toronto
Beyond “readability”
• Persistent images, lower power
• Liquid coating rather than vacuum coating
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Flexibility
Less expensive
Light weight
Fancy form factors
• Passive addressing?
Particles 2007, Toronto
The Electronic Book
• How do you make
an electronic book?
• Put together
electronic pages.
Particles 2007, Toronto
The Electronic Book
• How do you make
an electronic book?
• How do you make
an electronic page?
• Put together
electronic pages.
• Put together
electronic pixels.
Particles 2007, Toronto
The Electronic Book
• How do you make
an electronic book?
• How do you make
an electronic page?
• How do you make
an electronic pixel?
• Put together
electronic pages.
• Put together
electronic pixels.
• Make a dot that
switches colors.
Particles 2007, Toronto
The Electronic Book
• How do you make
an electronic book?
• How do you make
an electronic page?
• How do you make
an electronic pixel?
• How do you make a
dot that switches?
• Put together
electronic pages.
• Put together
electronic pixels.
• Make a dot that
switches colors.
• Encapsulate an ink
that switches.
Particles 2007, Toronto
To create a display – start with a print
and invent ways to make it change:
Particles 2007, Toronto
Liquid ink-based printing technology
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An electrostatic image is written on a charged photoconductive
surface with a scanning laser beam.
Where addressed, charged pigment moves to the developer surface.
The particles are transferred from the developer roll to paper and
dried.
Particles 2007, Toronto
Electrophoretic displays
Charged
pigment
particles.
Suspended
in a dyed oil.
Addressed
with electrodes.
Particles 2007, Toronto
Why ink pixels are needed
Sedimentation:
Electrohydrodynamics:
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Shutter mode
Charged
pigment
particles.
Suspended
in clear oil.
Electrophoresis in
shutter mode
Polymer Vision
International –
Reflective TFTs
Addressed
with electrodes.
Particles 2007, Toronto
Suspended particle displays*
Particles.
Suspended
in an oil.
Transparent
*Invented by E.H.
Land in 1934
Addressed
with electrodes,
AC or DC.
Transparent,
or reflective.
The same effect can be obtained with small, polarizabile particles chaining in an electric field.
Particles 2007, Toronto
The charging of particles in oil:
To make particles move,
not ions, requires minimal
free ions, therefore need
nonaqueous dispersions.
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If only charged particles
move, and no charges
were injected at the
electrodes, the image
forms capacitively.
No net current and image
stability!
Particles 2007, Toronto
Electric charges on carbon black in oil
Zeta Potential (mV)
-40
-30
-20
-10
0
0.0
0.5
1.0
1.5
2.0
OLOA 1200 in dodecane (% weight)
Particles 2007, Toronto
Imaging by particle rotation
The Gyricon display
Charged,
bichromal
balls.
Suspended
in an oil.
Encased in
a plastic
sheet.
Addressed
with electrodes.
That Xerox
built.
Particles 2007, Toronto
Gyricon Display
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Capable of gray levels by partial rotation.
If the charge is unbalanced, a Coulomb force
translates the ball.
The torque varies as the ball rotates.
Ground Electrode
Fluid Filled
Cavity
q+
q−
d
Bichromal
Ball
Switching Electrode
Red and white bichromal balls
Black and white bichromal balls
Particles 2007, Toronto
SiPix Corporation
The Microcup® technology
Structure
Bright white particles are suspended in a dyed solution enclosed in a Microcup®.
Particles 2007, Toronto
The search for lower voltage
and higher speeds
• Switching time goes as square of
thickness:
τ transit
2
d
Vμ
• The necessary thickness is determined by
the optical density.
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Imaging with oppositely charged pigments
Development
electrode
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Positively
charged pigment
Addressed
electrode
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Negatively charged
pigment
Hydrocarbon
oil
Particles 2007, Toronto
Typical charge control agents
• CCA1:
Soluble tail: Polyhydroxystearic acid
Head group: Quaternary ammonium methyl sulfate
• CCA2:
Soluble tail: C13 Hydrocarbon chains
Head group: Sodium sulfosuccinate
Particles 2007, Toronto
Charging of a surface modified pigment
60
CCA Titrations on "Basic" Pigment #1
Zeta Potential (mV)
40
CCA 1
20
0
0.0%
CCA 2
0.1%
0.2%
0.3%
0.4%
0.5%
0.6%
-20
-40
-60
Wt% CCA 2
Particles 2007, Toronto
Charge titration with “mixed” micelles
60
Zeta Potential (mV)
40
20
% CCA 2
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
-20
-40
Particles 2007, Toronto
Cabot’s modified pigments
Covalently attached
chargeable sites.
Particles 2007, Toronto
Titration of an “acidic” and a “basic” pigment
CCA 1
80
Zeta Potential (mV)
60
40
CCA 2
Note the
concentration for
oppositely
charged particles.
20
0
-20
"Basic" Pigment #1
-40
"Acidic" Pigment
-60
-80
Particles 2007, Toronto
Stability of oppositely charged particles
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Calculate the force between particles
as a function of distance.
Determine the distance between
particles that just equals the applied
electric field.
Calculate the energy of particles at
that distance.
Calculated how much closer the
particles can come before the energy
increases by kT.
Half of that distance is the minimum
steric barrier.
Particles 2007, Toronto
Requirements on steric barrier
For low electric
field, the
necessary
thickness
increases with
particle charge.
At high field,
the necessary
thickness
decreases with
particle charge.
Particles 2007, Toronto
E Ink Corporation
Dark State
Light State
- Sony Reader utilizing E Ink Imaging Film TM
- Photos courtesy of Sony Corporation
Particles 2007, Toronto
Scaling laws for an ideal system
Assume: the charged particles just
neutralize the applied voltage.
M is the mass of particles/area.
The charge and zeta potential of the
particles are:
DV0
q=
n0L2s
The switching time doesn’t depend on size:
The voltage is:
V0 =
3ζ M Ls
ρ a2
ζ =
q
4π Da
3ηL2s
t=
2Dζ V0
Bigger is better!
Particles 2007, Toronto
Particle Morphologies
Not:
But:
Not:
But:
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Particles 2007, Toronto
Passive addressing: Thresholds
• From the physics of colloids:
– “Inverse” electrorheological fluids
– Field dependence of zeta potential
– AC electric fields – time dependencies
– Particle-particle or particle-wall adhesion
– Structures in fluid (particles or wide variety of polymer gels)
Particles 2007, Toronto
Xeroxgraphy – Dry printing
Dry toner particles
exchange charge
(tribocharge) with
large, generally
magnetic carrier bead.
Photo courtesy Xerox
A developer bead coated with small toner particles
An image is formed when the toner
particles are pulled off and attach to
the electrostatic image on a
photoreceptor.
Particles 2007, Toronto
Bridgestone’s Quick Response Liquid
Powder® Display
• The “Liquid Powder” is a dry
dispersion of two types of particles.
• The particles are about 10 microns,
pigmented polymer beads, spherical,
smooth, and chemically treated.
• The powder flows freely. The different
colors have different sign charged.
• The image forms quickly with the
separation of dry powder in the
electric field.
Particles 2007, Toronto
Caveat emptor
• An “adequate” print resolution is about
1200 dots/inch.
• Or pixels of about 20 microns (for black
and white). Smaller for
cyan/magenta/yellow.
• Or about 10,000 pixels across a page!
• Well beyond current electronic displays!
• Progress will be limited by the electronic
addressing.
Particles 2007, Toronto
Particles in displays
• Dielectrophoresis - particle alignment or
chaining
• Segmented structures
• Particle rotation
• Electrophoresis to a viewing surface
– Particle and dye
– Dual particle
• Electrophoresis in shutter mode
• Passive addressing
• Dry powder flow
Particles 2007, Toronto
Particles 2007, Toronto