Download Electrophoretic liquid crystal displays: How far are we?

Electrophoretic liquid crystal displays: How far are we?
Susanne Klein
HP Laboratories
HPL-2013-23
Keyword(s):
Liquid crystal; electrophoresis; display
Abstract:
We are in the middle of another revolution in information processing and distribution. After the invention
of script and then print, pictures are taking over again. The number of gadgets with displays is larger than
the world population. On average every single person on earth owns at least one device with a display on it.
These displays are the portals to everything we do: communicating, reading, shopping, learning etc. But are
they up to the job? The most successful display technology to date is still the liquid crystal display. It is a
well established technology which can be scaled in size from small to large and, depending on the drive
electronics, has no significant inherent limits to resolution. However, poor light efficiency and data
congestion caused by high pixel densities are still technical challenges in need of good solutions. Could
electrophoresis in a liquid crystal host be the answer? The fundamental basics and technical challenges of
this interesting new approach will be discussed.
External Posting Date: March 21, 2013 [Fulltext]
Internal Posting Date: March 21, 2013 [Fulltext]
Additonal Publication Information: Published in Liquid Crystals Reviews.
 Copyright 2013 Liquid Crystals Reviews.
Approved for External Publication
Electrophoretic liquid crystal displays: How far are we?
Susanne Klein
HP Labs, Long Down Avenue, Bristol BS34 8QZ, UK
[email protected]
We are in the middle of another revolution in information processing and distribution. After the
invention of script and then print, pictures are taking over again. The number of gadgets with
displays is larger than the world population. On average every single person on earth owns at least
one device with a display on it. These displays are the portals to everything we do: communicating,
reading, shopping, learning etc. But are they up to the job? The most successful display technology
to date is still the liquid crystal display. It is a well established technology which can be scaled in size
from small to large and, depending on the drive electronics, has no significant inherent limits to
resolution. However, poor light efficiency and data congestion caused by high pixel densities are still
technical challenges in need of good solutions. Could electrophoresis in a liquid crystal host be the
answer? The fundamental basics and technical challenges of this interesting new approach will be
discussed.
Keywords: Liquid crystal, electrophoresis, display
1. Introduction
In the developed world, print on paper has largely become a thing of the past. The main platform for
information distribution has become electronic. The portal is a display. Most information is now
consumed on the move. The requirements for a display on a handheld device are stringent:
lightweight, flat, robust, readable in all lighting conditions and low power consumption. The two
most successful technologies for handheld displays are the backlit, active matrix liquid crystal display
(LCD) and for reading applications the electrophoretic display (EPD). The LCD has the advantage of
having a high (video) optical response speed, full colour, a slim profile and it is relatively cost
efficient since by now it is a mature technology, but like all emissive technologies which are
commercially available, like OLED or Plasma, it is hard to read when used outdoors or in other high
brightness environments. Reflective displays, like electrophoretic displays, can be read in all kinds of
ambient light, from dim to bright sunlight. They do not rely on an inbuilt light source but use the
ambient light to display the content, similar to paper. Electrophoretic displays are the most paper
like displays, but compared to LCDs they are slow to write. Could the combination of electrophoresis
and liquid crystal be the answer to the quest for a flat-panel display readable in all conditions?
Maybe, but there are still major technological challenges to overcome which are addressed in this
paper.
2. Classic liquid crystal display
The classic liquid crystal display found in many handheld devices like mobile phones, laptops etc.,
but also used for computer displays and televisions, is based on the ability of a birefringent material,
the liquid crystal, to change the polarization of light. A standard liquid crystal display consists of a
number of layers as can be seen in Figure 1.
Alignment layers
Spacer
Colour filters
Substrates
Polarisers
Liquid Crystal
Electrode
Figure 1: The layout of a commercially available liquid crystal display. The polarizers in combination with the
liquid crystal form the light switch. The alignment layers guarantee a uniform orientation of the liquid crystal
and therefore reliable switching. Colour is generated by opening or closing the light valves in front of the
colour filters. Not shown are the pixel borders which would surround the unit shown in the figure.
Its active part is the combination of the polarizers and the liquid crystal which form a light valve.
Light passing the first polariser is polarized, i.e. only one vibration mode is allowed through.
Depending on the orientation of the liquid crystal molecules which react to an applied electric field,
this mode stays untouched or is changed. The second polarizer or analyser blocks or transmits the
light depending on its state of polarization as can be seen in both parts of Figure 2.
Colour is generated by opening or closing the light valve on top of Red, Green, and Blue (RGB) colour
filters. The unit depicted in Figure 1 is called a pixel. The pixel size is dependent upon the application.
The higher the resolution (i.e. the higher the number of pixels), the smaller the pixel size. There is no
real physical constraint on the size of the pixel. The major constraint is the data flow and, coupled
with it, the refresh rate. The more the product of the number of pixels multiplied by the refresh rate,
the more data that has to be channelled to each line in the display. It is easy to visualize how the bit
rate increases considerably for a high resolution display with a 100 Hz refresh rate which is typical of
liquid crystal displays available today.
Analyzer
Polarizer
Orientation of the
liquid crystal director
Unpolarized light
2a
Analyzer
Polarizer
Orientation of the
liquid crystal director
Unpolarized light
2b
Figure 2: Operational principal of a liquid crystal light valve: whether light is transmitted or not depends on the
orientation of the liquid crystal molecules and the resulting birefringence. The first polarizer transmits only
one out of many possible polarizations which are always perpendicular to the beam direction. In Figure 2a the
liquid crystal molecules are oriented in such a way that incident light experiences a birefringent material and
its polarization direction is rotated until it is parallel to the transmission direction of the second polarizer. In
Figure 2b the liquid crystal is re-oriented by an electric field. It acts now as a single refractive index material
and leaves the light polarization untouched. The untouched polarization is blocked by the second polarizer. No
light is transmitted.
Even though the liquid crystal display is extremely successful, one of its major drawbacks is light
efficiency. Only roughly 12% of the light incident onto the display is transmitted when the display
appears white; that is, when all light valves are open. Since polarizers not only polarize but also
absorb, only 40% of the incident light passes the first polarizer. After the RGB colour filters about
15% of the incident light remains, depending on the bandwidth of the filters. The second polarizer
then absorbs a further 20% of the transmitted light which produces the figure of 12% of the total
transmission. The consequence is that images are only visible when the display is illuminated by a
very strong backlight. It is not possible to use this kind of display in reflective mode. The intensity of
ambient light is too low to show more than a dark grey image (see Figure 3). On the other hand, light
transmitted is so weak that liquid crystal displays perform poorly in outdoor conditions. On a sunny
day, a full colour liquid crystal display will appear dark to the observer.
Figure 3: A classic liquid crystal display in its white state. The display is not lit by a backlight but viewed in
reflection.
A major step to improve the light efficiency is the removal of polarizers and colour filters. In this
case, the transmitted intensity would increase from 12% to at least 70% (taking pixel borders and
losses at the interfaces into account) which, in the reflection mode, amounts to ~ 49%. Compared to
white paper with ~ 40% reflectance [1] it would lead to a paper-like display which could be used with
ambient light as a light source.
3. Electrophoresis
When two substances come into contact, the electrical charges arrange themselves at the interface
to minimize free energy, the internal energy of the system. For colloidal particles in solution, the
result is a so-called double-layer which consists of charges on the particles and balancing charges in
the surrounding solution. A schematic diagram of this double layer is shown in Figure 4 which
follows the Stern model [2]. For a more extensive explanation and different models for the doublelayer see [2]. The thickness of the charge ‘cloud’ around the particles is described by the so-called
Debye length
[3] given by:
 1 
 r  0 k BT
N
e z n
i 1
(1)
2 2
i i
Where:
 r  0 is the dielectric permittivity of the dispersion medium,
k B is the Boltzman constant
T is the absolute temperature in Kelvin
N is the number of different ionic species in the system
e is the elementary charge
zi is the charge number or valance of the ith ionic species
ni is the mean concentration of charges of species i.
Figure 4: The re-arrangement of charges at the interface between a particle and its surrounding solution leads
to a charge cloud or double-layer at the interface. In the Stern model this cloud is approximated initially by a
tightly bound layer of charges, the so called Stern layer, followed by a diffuse layer. Both the Stern layer and
the diffuse layer are needed to compensate for the charge at the particle’s surface. When the particle is set in
motion the diffuse layer will shear off at the Stern plane. The result is a charged particle.
For solutions with high ionic strength and thus high charge concentration, such as water, the Debye
length is on the scale of nanometres. For non-ionic fluids such as dodecane, having a very low charge
concentrations, this length can be on the order of micrometres because the mean charge
concentration decreases rapidly for low  r  0 .
The diffuse layer will shear off at the Stern plane when the particle is set in motion. When
movement is caused by an electric field, the positively charged particle will drift towards the
negative electrode and the negative ions from the diffuse layer will drift towards the positive
electrode. The combination of particle movement and ion movement is called electrophoresis. The
particle velocity v is proportional to the applied field strength E given by:


v  E
(2)
Where µ is referred to as the electrophoretic mobility of the particle in
. The equation
describes experimental observations that the particle movement is a function of field direction and
field strength. In the case of a positively charged particle (like in Figure 5) the particle motion is in
the field direction, in the case of a negatively charged particle its motion it against the field direction.
The particle velocity is independent of the sample thickness but directly proportional to the
electrophoretic mobility. In its simplest form electrophoretic mobility µ is given by the Smoluchowski
formula [4] as:

 r 0


(3)
Where:
 r  0 is the dielectric permittivity of the dispersion medium,
 is the viscosity of the dispersion medium and
 is the zeta potential defined as the potential at the shear plane, that is the plane where the liquid
velocity relative to the particle is zero.
Figure 5: Under the influence of an electric field the diffuse layer will shear off the Stern layer and move in the
opposite direction of the particle.
The Smoluchowski formula holds true for particles where the Debye length is small compared to the
particle dimensions, that is, either for suspensions with a high ionic strength, for example water, or
for large particles. In case of a sphere, this can be written as a  1 where a is the radius of the
sphere. In the case of very small particles or thick double-layers, that is for a  1 , the
electrophoretic mobility is given by the Hückel formula [5]:

2 r  0

3
(4)
The implication of this formula is that if the same particle was transferred from a dispersion with
high ionic strength to one with low ionic strength and yet the same field was applied, it could only
reach two-thirds of the velocity reached in the polar environment.
These are the two extreme cases for single particles or highly dilute systems. As always reality is
more complicated – especially when it comes to display applications. To reach any optical contrast,
concentrated suspensions are necessary. Hydrodynamic and electrostatic interactions between
particles cannot be neglected anymore. For detailed information of how the complex behaviour of
clusters of particles in an electric field deviates from single particles see [5-8].
4. Electrophoretic displays
A print on paper is colour, either dye or pigment, on a diffuse reflector, felted cellulose. Its
appearance can vary from matt to glossy depending on the surface treatment. Displays based on
electrophoretic technology generate images and text very close to print in appearance and are often
referred to as ‘E-paper’. The technical challenges facing electrophoretic displays and how are they
are tackled in commercially available products are described in the following paragraphs.
4a Charge:
Too little or too much charge will affect motion in an electrophoretic system.
In an organic medium with a low dielectric constant, of around 2, the number of ionic species per
unit volume is low. Not many charges are available per particle. Hsu et al [9] report that in dodecane
they find as few as 290 +/- 30 electrons on PMMA particles with a radius of 780 nm instead of 104 on
the same particle in water. Nevertheless, zeta potentials were also measured and found to be as
high as those in aqueous dispersion media. When the particles are charged they will move. However,
the charging mechanism in non-polar media is complex [10, 11]. Even with added ‘charge directors’,
such as surfactants in many cases, not every particle is charged. Uncharged particles will not react to
any applied field. These particles will deteriorate the contrast of the display.
Considering polar media, the high ionic strength causes another problem. The double-layer around
the particles is compressed to such an extent that some types of particles are no longer charge
stabilized. When two particles come together, the ion cloud around them should inhibit aggregation
by electrostatic repulsion, but this is only the case when the double layer is thicker than the reach of
the attractive van der Waals force [12]. A measure of how close two charges can come together
before Coulombic interaction takes over, or more precisely where the Coulombic interactions are
balanced by the thermal energy k BT , is given by the Bjerrum length λB as follows:
B 
e2
4 r  0 k BT
(5)
where e is the elementary charge. For water at 22oC with  r  80 the Bjerrum length is only 0.7 nm,
but for dodecane at the same temperature and  r  2 it is 28.3 nm. The Van der Waals energy
between two spheres of radius R1 and R2 with a surface separation l  R1 , R2 is
VVdW  
A R1 R2
6 ( R1  R2 )l
(6)
A is the so-called Hamaker constant and is given for the simple case of two particles made from the
same material interacting across a medium by [13] the following equation:
2
2
2 2
  p m 
3h e (n p  nm )
3


A  k BT

     16 2 (n 2  n 2 ) 3 / 2
4
m 
p
m
 p
(7)
 p is the dielectric constant of the particle,
 m the dielectric constant of the medium
h is Planck’s constant
 e the plasma frequency for a free electron gas. For water and dodecane it is 3  1015 s 1 and it is
assumed to be the same for the material of the particles.
np is the refractive index of the particle
nm is the refractive index of the material
Let us consider two PMMA particles both with a radius of 100 nm. How big is the Van der Waals
energy in units of k BT when their separation is the Bjerrum length? A dielectric constant of 2.6 for
PMMA and a refractive index of 1.5 are assumed. The refractive index of water is 1.333 and that of
dodecane 1.421. So, for these particles in water the attractive Van der Waals energy is about 29
times that of the thermal energy. i.e. the particles will aggregate very quickly. However, in dodecane
it is smaller than the thermal energy, approximately
, which means that the particles stay
dispersed when they are charged. For a different material, for example titanium dioxide, with
and
[14], the situation is even more dramatic: in water the Van der Waals
energy is ~775 times bigger than the thermal energy when the particles are separated by the
Bjerrum distance whilst in dodecane it is still 17 times the thermal energy. Titanium dioxide cannot
be charge-stabilized either in water or in dodecane. These particles have to be surface treated to
avoid irreversible aggregation. Any surface treatment will change the interface between dispersion
medium and particle and therefore the charging features.
4b Shielding of electrodes:
Ion clouds and particles arriving at opposite electrodes will have an effect on the effective field
strength in the system. On insulated electrodes the charges arriving at the electrodes will
accumulate and shield the electric field. With each arriving particle or ion, the electric field will
become weaker and weaker until all movement stalls. Why not have ‘naked’ electrodes? They would
provide a charge sink and source at the same time. Unfortunately uncovered electrodes are not an
option for display applications. Since displays are switched ten thousands of times, the
electrochemistry at the electrodes during charge exchange causes the electrodes to deteriorate
quickly leading to a non-functional device.
4c Particle drift:
Applying an electric field to the electrophoretic suspension not only triggers electrophoresis but also
another electrokinetic effect which can lead to unwanted drift of particles: Dielectrophoresis.
Dielectrophoresis does not require charged particles. It relies on induced dipoles and a non-uniform
electric field. When the dipole induced in the particles is stronger than those in the surrounding
solvent the asymmetric field forces will accelerate the particles more than the solvent which leads to
particles shifting towards regions of higher field strengths. When the induced dipole is stronger in
the solvent, particles will accumulate in regions of lower field strength. The electric field applied
across a particle suspension will not stay completely uniform. When the first ions reach the
electrodes field gradients form and the particles will start drifting. In an anisotropic host fluid, like a
liquid crystal, field gradients develop even without any particles present because of anisotropic
dielectric properties of the solvent. Electrophoresis and dielectrophoresis occur in parallel. Both
trigger particle movement and solvent flow.
Both lead to particles accumulation. But
dielectrophoresis is hard to control. When it has set in in a display cell it is almost irreversible and
leads to quick deterioration of switching performance. For more information about this
electrokinetic effect see, for example, [15-17].
How are these problems dealt with in commercially available displays?
The most successful e-readers are based on a technology disclosed in a series of patents in 1997 and
1998 [18]. To avoid aggregation beyond a certain floc size, drift caused by dielectrophoresis and
contact with the electrodes, the electrophoretic dispersion is encapsulated. The size of the capsules
is about 40 µm, smaller than a pixel but much bigger than the pigments suspended in the dielectric
fluid as can be seen in Figure 6. Figure 7 shows one of the most popular current commercial e-reader
devices, the Amazon Kindle e-reader which has a 6” (diagonal dimension) 800 x 600 pixel resolution
e-ink display.
Figure 6: In Figure 6a the electrophoretic suspension consists of dyed dielectric fluid and particles in a
contrasting colour. All particles carry the same charge. When a field is applied, the particles either move
towards the observer and become visible or away from the observer and are obscured by the dispersion
medium. The disadvantage of this simple system is that a ‘true white’, for example, cannot be achieved since
the dyed dispersion medium will always fill the voids in-between the particles. Figure 6b shows a schematic
drawing of a dual particle approach which should achieve a better true white than that in Figure 6a. Two
groups of particles are charged oppositely. When a field is applied the two groups move in opposite directions,
one towards the observer and the other away from the observer. The group closest to the observer will
obscure the one underneath. The purity of the two states is affected by a ‘cage effect’. Particles of the
opposite species can be surrounded by a cage of their opponents and cannot move into the opposite direction
but are dragged along by the main group which ultimately leads to contrast reduction.
Figure 7: A commercial embodiment of the electrophoretic technology described above. The Amazon Kindle ereader was introduced in 2007.
5. Liquid Crystal
Liquid crystal is an interesting material. It has an additional phase: In between the melted, isotropic,
phase and the crystalline phase sits the mesomorphic (Latin ‘of intermediate form’) or liquid
crystalline phase. The liquid crystalline phase is an ordered but liquid state. At rest, it possesses an
orientational order, i.e. the molecules tend to point in the same direction. However, it lacks the
positional order of a crystal, i.e. the centres of mass of the molecules are not on periodic lattice
points, so that their distribution is isotropic as can be seen in Figure 8. There are many different
classes of liquid crystals. The two main ones are called thermotropic and lyotropic. A liquid crystal is
called a thermotropic liquid crystal when the appearance of the mesomorphic phase is a function of
temperature. Thermotropic liquid crystals are used in all display applications. A lyotropic liquid
crystal reaches liquid crystalline phases when the concentration of the solute is changed. These two
groups contain sub-groups according to the molecule shape or the order observed within the
mesomorphic phase. When the molecule shape can be approximated by a rod, the liquid crystal is
called calamatic, for a disc it is discotic and for a brick or lath-like ones it is sandic. Within the
mesomorphic phase different forms of order can be found. The simplest one is called nematic with a
uniaxial long-range orientational order [19-23]. We will concentrate here on the most relevant case
for electrophoresis in liquid crystals: the thermotropic, calamitic, nematic phase.
In the nematic phase, one molecular axis, in many cases the long axis, tends to point along one
direction which is called the director and is represented by the unit vector . Electrical, mechanical
and optical properties are defined with respect to . To quantify the amount of order in the liquid
crystalline phase, the order parameter S is defined as a measure of how much fluctuation we find
around the preferred direction (see Figure 9). The angular brackets in the equation below denote
Figure 8: Three of the phases a thermotropic liquid crystalline substance can reach when cooled from the
isotropic phase into the crystalline phase. In the isotropic phase neither the direction of the molecules nor
their centres of mass have any order. All properties of the liquid are isotropic, i.e. independent of direction. In
the nematic phase the molecules tend to align along a preferred direction, but their centres of mass are still
unordered. This makes the electric, mechanical and optical properties directional, but the substance is still in
liquid form. The material is now called anisotropic since its properties are directional. In the final state, the
centres of mass sit on lattice points. The material is now an anisotropic solid.
either an average over the positions of many molecules at one moment in time or a temporal
average for one molecule.
S
3
1
cos 2  
2
2
(8)
The order parameter S is a function of temperature. For the isotropic phase
and for the
crystalline phase
. When the crystalline phase is heated and melts, the order parameter starts
decreasing slowly until it reaches the nematic/isotropic transition temperature where it drops from
a finite value to zero.
Figure 9: The director given by an arrow represents the preferred direction in which the long axes of the
molecules point. Single molecules will deviate from the director by an angle  at any point in time or space.
A nematic liquid crystal is birefringent, that is, it has two refractive indices. is the refractive index
for light polarized parallel to the director and
for light polarized perpendicularly to the director.
For optically positive liquid crystals
for optically negative liquid crystals
.
In the isotropic phase birefringence disappears and there is only one refractive index the refractive
index
.
Similar definitions apply to the dielectric constants. is defined for the electric field parallel to the
director and for the perpendicular case. The isotropic phase is characterized by a single dielectric
constant
.
The viscosity of a liquid crystal is rather complex. We will introduce four basic viscosities, three
translation viscosities, the so called Miesowicz viscosities [24], and one rotational viscosity (for more
information about the shear behaviour of liquid crystals see [23]). Imagine the following set up: an
aligned liquid crystal is sandwiched between two glass substrates which are then sheared. As a
consequence the liquid crystal will start flowing. The Miesowicz viscosities are defined as follows as
can be seen in Figure 10 where:
1: The director is perpendicular to the flow and parallel to the velocity gradient.
2: The director is parallel to the flow and perpendicular to the velocity gradient.
3: The director is perpendicular to the flow and perpendicular to the velocity gradient.
Figure 10: For 1 the director is oriented perpendicular to the substrates, for 2 parallel to the substrate and
parallel to their directions of motion, for 3 the director is parallel to the substrates but perpendicular to their
directions of motion.
The Miesowicz viscosities are easily described but hard to determine experimentally [25, 26].
For the classic liquid crystal display the most important viscosity is the rotational viscosity Figure
This is the viscosity experienced when the molecules rotate round an axis perpendicular to the
director. The switching speed of a display is roughly proportional to
where d is the distance
between the two substrates.
It follows that each of these parameters has significant influence on electrophoretic movement of
the particles in a liquid crystal. The possibility of switching the dielectric constant and the refractive
index will influence double-layer and Hamaker constant and with that the charging behaviour and
aggregation behaviour will change as well. The different viscosities will affect the speed of the
particles and whether they move along the director or perpendicular to it.
Figure 11: The rotational viscosity () is defined for a rotation perpendicular to the director. It is caused
mostly by an electric or magnetic field and is important in determining the switching speed of the liquid
crystal.
6. Colloidal Particles in Liquid Crystals and display applications
On one hand immersing particles into a liquid crystal allows more ways to influence the
electrophoretic movement of the system on the other hand the more complex solvent could be a
complete show stopper. When a particle is immersed into a liquid crystal, the liquid crystal will
interact immediately with the particle.
For two decades colloidal particles have been of interest for the liquid crystal research community.
[27-40] give an incomplete snapshot of the different research strands (for the latest collection of
topics see [41]). As soon as particles were mixed into the liquid crystal the potential for display
applications was detected. Kreuzer and Eidenschink [42-44] suggested the ‘filled nematic display’
where the suspended nanoparticles form a network with liquid crystal pockets. The display can be
switched between clear and scattering by changing the orientation of the directors within the
pockets and index match liquid crystal and particles. Kawasumi et al. suggest in [45] that dispersed
clay platelets follow the director when the liquid crystal is switched by an electric field and can be
switched reversibly from scattering to clear. Both states remain after the field is switched off. The
display is bistable. And finally there are the electrophoretic displays with liquid crystal as the solvent.
This special solvent can provide two features which are hard or impossible to achieve when an
isotropic solvent is used zero field stability and a field threshold. Zero field stability means that there
is no significant change in the optical state once the driving force is removed – this is often simplified
and referred to as 'bistability'. For example all commercially available liquid crystal displays are
monostable. When the driving electric field is removed the liquid crystal molecules re-arrange in
such a way to follow the alignment layer on the substrates. No input energy is needed. In the driven
state the system is lifted out of a potential well and, as long as the field is applied, kept in an
‘excited’ state. As soon as the field is removed, the system falls back into the potential well. In a
simple bistable configuration there are two potential wells. By applying a field, the system is lifted
from one well to another. In both wells the system is stable and does not need any holding field.
How is this bistability achieved for electrophoretic systems? From direct observation [27, 46] it is
known that defects form near the surface of particles. These defects can lock the particles in the
liquid crystal matrix and/or lock the particles together, preventing flow and thus causing bistability.
However, in practice, firstly these defects (figure 12) have only be observed for particles much bigger
than the liquid crystal molecules and secondly too many defects will gel the system completely and
the system will not react to any applied field anymore. From indirect observation, that is gelling and
bistability, we know that nanoparticles are causing defects but we have no means to show whether
they follow patterns as in figure 12.
Figure 12: Defects observed in nematic hosts around particles at least three orders of magnitude larger than
the longest dimension of the liquid crystal molecule: a) satellite, b) Saturn-ring and c) surface ring. The grey
lines represent to local orientation of the director.
A threshold means that the system reacts only to field strengths above a certain value and not to the
integral of sub-threshold stimuli, for example particles would arrive from the bottom at the top
substrate when 5V DC are applied, but not after 5 pulses of 1V DC. A threshold does not come
automatically with bistability nor vice versa. Some commercially available liquid crystal display
modes exhibit a threshold, (e.g. for row and column passive matrixing of Super Twisted Nematic
(STN) ) but are not bistable, and many system can exhibit multistability, but had no threshold, i.e.
they reacted to the smallest field applied and stayed in the new state after the field was removed.
Recently [53, 55, 56] Lavrentovich demonstrated unusual electrophoretic effects which depend on
the solvent being a liquid crystal: A quadratic dependence of v on E for spherical particles with
motion orthogonal to the field direction, levitation and motion of uncharged particles. These effects
are generated by a certain type of topological defect forming around spheres suspended in a liquid
crystal. They can be easily observed when the sphere diameter is greater than ~ 3µm. For display
applications particles with diameters between 1 to 300 nm are used. As mentioned above whether
the same topological defects form around these spheres, an order of magnitude smaller the ones
used in [53, 55, 56], is unclear. They can no longer be observed by optical microscopy and are not
accessible to other methods, such as X-ray scattering for example. Another difference is particle
concentration. In [53, 55, 56] particle concentrations are low, often only about 1wt% to avoid
uncontrolled aggregation. At these low concentrations the particles behave like single particles and
the defects do not interact unless the particles are pushed together. To achieve any optical effect in
display applications particle concentrations of 10wt% and higher are required. From
disproportionally increased viscosity and bistability we deduce that a network of defects holds the
particles in place. At these concentrations we have never observed levitation but often fast
aggregation and substantial irreversible adhesion to the substrates especially to rubbed polyimide, a
high energy surface.
We have seen sideways drift that is particle motion at an angle to the field direction but credited it
to dielectrophoresis when the particles move along field gradients. This assumption was supported
by the observation that the more particles had already drifted in one direction and therefore
increased the field gradient the more followed.
A major problem we encountered was immobile particles. We identified two main reasons for that
behaviour: uncharged particles and caging. Uncharged particles occur since even a midpolar solvent
like liquid crystals can only support an certain amount of charge. The amount of charge per particle
will vary between 0 and a maximum amount defined by the polarity of the medium. The particle
sizes in the case of display applications do not allow the observation of single particles. All immobile
particles observed are therefore aggregates and we cannot distinguish between lack of charge or
neutralization of charge because of caging, especially not in the case of a two particle system. As
explained before, in a two particle system contrast is achieved by having two oppositely charged
species of different colour. Ideally in a field the two species would move into opposite directions, but
a member of one species can be caught in a cage of opposite charge and can become neutralized.
This effect is similar to the trapping of charges in inverse micells described for example in [11, 57].
Nevertheless two types of displays are reported in the literature where electrophoresis in liquid
crystal hosts is used: the bistable electrophoretically controlled nematic display [47, 48] and the
mobile fine particle display [49-52].
6a Bistable electrophoretically controlled nematic display
In this technology, the electrophoretic movement of particles is not used to generate an optical
effect directly. Rather, this movement is used to stabilize the liquid crystal in two optically distinct
states. As an example, the Hybrid Aligned Nematic (HAN) and homeotropic states are shown in
figure 13.
Figure 13: Principle of the bistable electrophoretically controlled nematic display. In figure 13a the cell is in a
HAN state: homeotropic floc and homeotropic alignment layer are on the same side. The bent director profile
makes the liquid crystal layer birefringent and light is transmitted between crossed polarizers. In figure 13b the
cell is in the homeotropic state. The homeotropic floc covers the planare alignment layer and the together
with the homeotropic alignment layer on the other side aligns the director homeotropic across the whole cell.
No light is transmitted between crossed polarizers.
The optical effect can be either achieved by the classic light valve technology (figure 14) as described
before or by dichroic dyes mixed in the liquid crystal (figure 15). The particles used for this kind of
display are very small, between 1 and 50nm in diameter, and should be as close to the refractive
index of the liquid crystal as possible to suppress scattering form index mismatch. About 2wt% are
sufficient to generate the desired effect. The particles can be moved electrophoretically from one
substrate to the opposite but can also be left floating in the middle of the cell. This feature could be
related to particle levitation as described in [53] but the situation here could be more complicated.
There are experimental observations which suggest that the ‘particle’ is not a single sphere but a floc
with a porous surface. The spheres are held together by attractive forces but also by a defect
structure inside the floc. On the surface of the aggregate the liquid crystal has a strong tendency to
align homeotropically which is exploited in the kind of display here. By moving the particles from one
substrate to the other we are moving an alignment layer. In one configuration we have a so called
HAN cell that allows light through between crossed polarizers. In the other state the whole cell is
homeotropically aligned and all light is blocked by the second polarizer. Both states are stable when
the field is taken off. Depending on whether the liquid crystal has a positive Δε or a negative Δε,
different configurations can be stabilized by this method, basically any two states where
Figure 14: Prototype of the bistable electrophoretically controlled nematic display in reflection. The substrates
are plastic. The liquid crystal layer is between parallel polarizers. The HAN state is the dark state.
in one states the two substrates have the same alignment and in the other state the two substrates
have different alignments. This mode also exhibits a threshold. The threshold is probably caused by
the energy required to overcome flow/ viscosity terms describing the liquid crystal streaming
through the pores in the floc and/or terms caused by the defect network in and around the floc.
Small angle X-ray scattering experiments would be a means to study the deformation or possible
disintegration of the particle floc in an applied electric field.
Figure 15: Prototype of the bistable electrophoretically controlled nematic display in reflection. The liquid
crystal is dyed with a blue dichroic dye and the display is viewed in reflection on a white diffuser. No polarizers
are used. The HAN state is the dark state, i.e. the dichroic dye is tilted in such a way by the liquid crystal that
the absorbing transition moment is exposed to the incident and reflected light.
6b Mobile Fine Particle Display
The mobile fine particle display (MFPD) is similar in concept to the classic electrophoretic display.
Optical distinct states are generated by moving particles in and out of the field of view that can
happen by either an up and down or a sideways motion. Electrode design is of great importance
since liquid crystals belong to the group of midpolar solvents and can therefore accommodate a
substantial number of charges. The different electrode designs will not be considered here, instead
we will concentrate on the basics. At first glance the mobile fine particle display and the bistable
electrophoretically controlled nematic display (BECD) seem very similar but the requirements on the
suspension for the MFPD are quite different. Whereas in the case of the BECD a floc of particles is
moved around in the case of the MFPD a compressed floc is formed and re-dispersed again,
especially in the so called curtain mode when the particles are moved sideways (Figure 16).
Figure 16: The curtain mode. In 16a the particles are dispersed across the whole field of view. The observer
sees the colour of the suspended pigments. In b the particles are electrophoretically moved underneath the
top electrode which is covered by a mask. The observer sees now either the colour of the dye doped liquid
crystal or the colour of a bottom reflector.
The curtain mode allows stacking of cyan, magenta and yellow layers to provide full colour, but self shielding of the electrodes and field gradients can be a real challenge. A more forgiving set up is the
so called up and down mode (as already shown in figure 6) where the particles are moved towards
and away from the observer. The suspension always contains a scatterer, often a titanium dioxide
particle with a diameter of about 300 nm, and an absorber, which is either a dye or another particle.
To achieve sufficient optical contrast the scattering pigment has to form a compressed layer of
about 2 to 3 µm at the substrate closest to the observer. This has consequences for the cell thickness
and for the particle concentrations in suspension. Calculations and experiments have shown that
cells with cell gaps of less than 20 µm are not feasible since the concentration of the absorber has to
be so high that either, in the case of a dye, the liquid crystal phase is lost due to chemical
‘contamination’ or, in case of a pigment, the total particle concentration reaches values which lead
to a situation where a particle network with liquid crystal pockets fills the cell gap and not a liquid
crystal with suspended particles. In an electric field the network will deform but the two particle
species will not move past each other. Cells gaps of 100 µm and more require containment
structures. The liquid crystal is not kept between the substrate by capillary forces anymore. For
research purposes and for the ease of cell assembly cell gaps of 50 µm were chosen. Mixtures with
an acceptable contrast consisted of about 10wt% scatterer and of absorber at about 2wt%. The
particle content is still 5 to 6 times higher than in the BECD. We experimented with different types of
liquid crystals to determine whether different Δε would influence the electrophoretic mobility of the
particles, but we could not observe any dependency on Δε. To optimize our chances to find the right
surface treatment for the particles and therefore to achieve a stable suspension we decided to use
the commercially available mixture ZLI 2293 from Merck for all further experiments and for the
prototypes. The particles have to be stabilized sufficiently to hinder permanent aggregation. Even
then defect formation around the particle resulted in a dramatic increase of the viscosity of the
suspension. The cells could not be filled by capillary action anymore. A drop of the suspension was
applied to the bottom substrate and then covered with the top substrate. The cell gap was
controlled with Mylar film and the cell was held together with UV curable clue. To discourage
particle sticking at the substrates the insulating dielectric film on top of the electrodes was covered
with a low surface energy homeotropic treatment. Because of the high particle concentration the
surface treatment of the substrates does not influence the bulk behaviour of the suspension. It is
determined by the interaction between liquid crystal and particles and hence their surface
treatment. We could not see any difference in switching behaviour or optical appearance whether
the substrates were covered with a planar or a homeotropic alignment layer, but we observed
increased adhesion of the particles to the substrates when planar alignment layers were used. Figure
17 shows three types of up and down display prototypes. Figure 17a clearly shows problems with
aggregation. All three samples are disconnected from any power supply.
Figure 17: Three versions of the up and down display. a) fully pixelated on plastic. Dyed liquid crystal with
white scatterers. b) Gray scale sample of a dual particle system. The white particle is titanium dioxide, the
black particle copper oxide. c) Both pigments are absorbers. Magenta and Cyan are toner pigments provided
by Clariant.
The prototypes are bistable and show a relatively soft threshold. They can be switched with a DC
biased AC voltage as well, but the increased viscosity of the suspensions led to high voltages. The
lowest voltage where particle movement was observed under the microscope was 30V DC across 50
µm, but optical switching, that is a colour change, was only observed at 80V DC and above.
Electrophoretic mobilities of
m2/Vs were measured. To observe the change of optical contrast
as a function of applied field we used low frequency AC voltages with frequencies between 0.5 and
10 Hz. In some cases strong electroconvection set in and the result was irreversible aggregation.
Uneven cell gaps favoured dielectrophoresis which again had aggregation as a consequence. When
pigments with some conductivity, for example carbon black, were used aggregation which bridged
the cell gap, often triggered by electroconvection, led to a short in the cell. When samples were
opened after several switching cycles, sticking of particles to the substrates was observed. This
sticking appears to occur within the first 5 to 10 switching cycles when a rapid deterioration of the
contrast and then a plateauing was observed. The contrast was always affected by immobile
particles. Even though liquid crystals are midpolar solvents not all particles seem to charge. Neither
with dyes nor with absorbing pigments could a true white be reached. In the case of dyes the
particles will always be surrounded by dyed liquid crystal. Absorbing pigments could never be
completely removed from the scattering layer because of the caging effect. It came out that one of
our best mixtures, a white titanium dioxide particle in a green dyed liquid crystal was indeed a dual
particle system. The green dyed had crystalized into a needle network which filled the cell gap. The
white scatterers were moved through the gaps when a field was applied.
7. Scientific, engineering challenges and conclusions
The major scientific challenge for display applications of electrophoresis in liquid crystal is the design
of the right surface treatment for the particles. The surface treatment should mimic the liquid
crystalline solvent to reduce defect formation on one hand but on the other should not supress it
completely since defects are needed to make the system bistable. The surface treatment should
provide enough steric hindrance to prevent aggregation when the particles are compressed either in
the hidden state when driven in curtain mode or when close to the substrates. Again the right
molecule size is important. Molecules too big would lead to increased viscosity, molecules too small
to aggregation. The ideal situation would be the chemical design of the liquid crystalline solvent in
combination with the design of the surface treated particle. This is only possible when all
components are known, i.e. engineering for commercially available liquid crystal mixtures of
undisclosed composition is not possible.
What about the engineering problem of encapsulation to avoid contact with electrodes and particle
drift? Confinement structures have to be designed with upmost care. Liquid crystals interact with
any surface they come into contact with and align to it in one way or the other. Low energy surfaces,
like air for example, align the liquid crystal homeotropic. High energy surfaces, like rubbed polyimide
will align the liquid crystal planar. When this happens in a sphere with 40 µm diameter, the liquid
crystal will try to align across the whole volume but with different alignment directions coming from
different sides as defect formation cannot be avoided. Any containment in this size range has to be
designed carefully to avoid the introduction of additional defect lines and therefore causes an
increase in viscosity of the suspension.
The additional features of mesomorphic solvents seem to bring electrophoretic displays closer to a
full colour e-paper display. Prototypes built so far are promising, but highlight the immense
difficulties which need to be overcome to reach this goal. A central problem is the chemical design of
the combination of liquid crystal host and matching particle. The particle surface does not only need
to be treated with some molecules which provide steric hindrance and inhibit irreversible
aggregation, the same surface treatment must allow charging and must be compatible with the
liquid crystalline host. Ideally it should be a liquid crystal itself and make the particle ‘invisible’ for
the surrounding nematic matrix. This is not an easy task. It took twenty years between the first
patents for the electrophoretic e-reader and the first commercial product and it will probably take
the same time for a liquid crystalline technology to become more mainstream.
Acknowledgement
I would like to thank my work colleagues Fraser Dickin, John Rudin and Steve Simske for many
helpful discussions. The majority of the work on electrophoretic displays in liquid crystals within HP
was performed by the late David Sikharulidze.
References:
[1] Schramm, M.T. and Meyer, G.W.: Computer Graphic Simulation of Light Reflection from Paper,
412, IS&T’s 1998 PICS Conference
[2] Hunter, R. J.: Foundations of Colloidal Science, Oxford: Oxford University Press, 2001, Chapters 7
and 8.
[3] Delgado, A.V., Arroyso, F. J.: Electrokietic Phenomena and Their Experimental Determintaion : An
Overview, in Delgado, A.V. editor. Interfacial Electrokinetics and Electrophoresis. Surfactant Science
Series, Vol. 106, Marcel Dekker, 2002
[4] Hiroyuki Ohshima: Electrophoresis of Charged Particles and Drops, in Interfacial Electrokinetics
and Electrophoresis, Surfactant Science Series Volume 106, New Yokr (NY): Marcel Dekker, 2002
[5] Levine, S., Neale,G.H.: J Colloid Interf Sci, 1974, 47, 520-529
[6] Kozak, M.W., Davis, E.J: J Colloid Interf Sci, 1989,127, 497-510
[7] Kozak, M.W., Davis, E.J: J Colloid Interf Sci, 1989,129, 166-174
[8] Hiroyuki Ohshima: J Colloid Interf Sci, 1997, 188, 481-485
[9] Hsu, M.F., Dufresne, E.R., Weitz, D.A., Langmuir, 2005, 21, 4881-4887
[10] Morrison, I.D., Colloid Surface A, 1993, 71, 1-37
[11] Roberts, G.S., Sanchez, R., Kemp, R., Wood, T., Bartkett, P., Langmuir, 2008, 24, 6530-6541
[12] Parsegian, V.A. : Van der Waals Forces, New York (NY): Cambridge University Press, 2006
[13] Israelachvili, J. : Intermolecular& Surface Forces, London: Academic Press, 1991, p.184
[14] Bergström, L., Adv Colloid Interfac, 1997, 70, 125-169
[15] Pohl, H.A., J Appl Phys, 1915, 22, 869-871
[16] Jones, T.B.: Basic Theory of Dielectricphoresis and Electrorotation, IEEE Engineering in medicine
and Biology Magazine, November/December 2003
[17] Pethig, R. , Biomicrofluidics, 2012, 4, 022811-1 to 35
[18] World Patents: WO9819208A2; WO9960554A1; WO9841899A2; WO9841898A2
[19] de Gennes, P.G., Prost, J.: The Physics of Liquid Crystals, Oxford: Oxford University Press, 1993
[20] Bahadur, B. (Ed.): Liquid crystals, Applications and Uses, Singapor : World Scientific , 1990
[21] Collings, P.J., Hird, M.: Introduction to Liquid Crystals, Chemistry and Physics, London: Taylor &
Francis, 1997
[22] Dierking. I.: Textures of Liquid Crystals, Weinheim: Wiley-VCH, 2003
[23] Larson, R.G.: The Structure and Rheology of Complex Fluids, Oxford: Oxford University Press,
1999
[24] Miesowicz, M., Nature, 1946, 158, 27
[25] Tsvetkov, V.A., Beresnev, G.A, Instrum Exp Tech, 1977, 20, 1497-1499
[26] Janik, J., Moscicki, J.K., Czuprynski, K., Dabrowski, R., Phys Rev E, 1998, 58, 3251-3258
[27] Poulin, P., Stark, H., Lubensky, T.C., Weitz, D.A., Science, 1997, 275, 1770-1773
[28] Ruhwandl, R.W., Terentjev, E.M., Phys Rev E, 1997, 56, 5561-5565
[29] Ruhwandl, R.W., Terentjev, E.M., Pys Rev E, 1997, 55, 2958-2961
[30] Raghunathan, V.A., Richetti, P., Rous, D., Nallet, F., Sood, A.K., Langmuir, 2000, 16, 4720-4725
[31] Loudet, J.-C., Barois, P., Poulin, P., Nature, 2000, 407, 611-613
[32] Loudet, J.-C., Richard, H., Sigaud, G., Poulin, P., Langmuir, 2000, 16, 6724-6730
[33] Loudet, J.-C., Poulin, P., Phys Rev Lett, 2001, 87,165503-1 to -4
[34] Cleaver, J., Poon W.C.K., J Phys-Condens Mat, 2004,16, S1901-S1909
[35] Pizzey, C., Klein, S., Leach, E., Van Duijneveldt, J.S., Richardson, R.M., J Phys-Condens Mat, 2004,
16, 2479-2495
[36] Van Duijneveldt, J.S., Klein, S., Leach, E., Pizzey, C., Richardson, R.M., J Phys-Condens Mat, 2005,
17, 2255-2267
[37] Qi, H., Hegmann, T., J Mater Chem, 2006, 16, 4197-4205
[38] Connolly, J., van Duijneveldt, J.S., Klein, S., Pizzey, C., Richardson, R.M., J Phys-Condens Mat,
2007, 19, 156103-156120
[39] Hegmann, T.,Qi, H., Marx, V.M, J Inorg Organomet P, 2007, 17, 483-508
[40] Shivakumar, U., Mirzaei, J., Feng, X., Sharma, A., Moreira, P., Hegmann, T., Liq Cryst, 2011, 38,
1498-1514
[41] ‘New frontiers in anisotropic fluid-particle composites’, themed issue of Phil Trans R Soc
A 2012, to be published.
[42] Eidenschink, R., De Jeu, W.H., Electron Lett, 1991, 27, 1195-1196
[43] Kreuzer, M., Tschudi, T., De Jeu, W.H., Eidenschink, R., Appl Phys Lett, 1993, 62, 1712-1714
[44] Kreuzer, M., Eidenschink, R.: Filled Nematic, in Complex Geometrics, Ed. G.P. Crawford, S.
Zumer, Taylor&Francis 1995, 307pp.
[45] Kawasumi, M., Hasegawa, Naoki, Usuki, A., Okada, A., Appl Clay Sci, 1999, 15, 93-108
[46] Tkalec, U., Ravnik, M., Copar, S., Zumer, S., Musevic, I., Science, 2011, 333, 62-65
[47] Sikharulidze, D, Appl Phys Lett, 2005, 86, 033507-1 to 3
[48] patents GB2424716B, US7791706B2, US7264851B2, US6970211B2
[49] Yasua Toko et al.: A new type of display for electric paper using fine particles dispersed in the
nematic liquid crystal cell, NIP 17: International Conference on Digital Printing Technologies, Florida,
2001, 748-751
[50] Taiju Takahashi et al.: Behavior of fine particles dependent on cell structures in mobile fine
particle display cells, NIP22: International Conference on Digital Printing Technologies, Colorado,
2006, 485-488
[51] Masahiro Ogawa et a.: Behavior of fine particles dependent on cell structures in mobile fine
particle display (MFPD) cells, SID Symposium Digest of Technical Papers, 2003, 34, 584-587
[52] Patents: GB2407646B, GB2425611B, GB2445375, US7633581B2, US7796199B2
[53] Pishnyak, O., Shiyanovskii, S. V., Lavrentovich, O.D., Phys Rev Lett, 2011, 106, 047801-1 to4
[54] Klein, S., Richardson, R.M.: The optical response of polarizer-free liquid crystal systems: dye
molecules versus nano-particles, HP Technical Report, HPL-2006-167
[55] Lavrentovich, O.D., Lazo, I., Pishnyak, O.P., Nature, 2010,467, 947-950
[56] Lazlo, I., Lavrentovich, O.D., Phil. Trans. R. Soc. A, 2013, 371, 20120255
[57] Neyts, K. et al., J. Phys.: Condens. Matter, 2010, 22, 494108 (10pp)