Keyword(s): Abstract:

Electrophoretic liquid crystal displays: How far are we?

Susanne Klein

HP LaboratoriesHPL-2013-23

Liquid crystal; electrophoresis; display

We are in the middle of another revolution in information processing and distribution. After the inventionof script and then print, pictures are taking over again. The number of gadgets with displays is larger thanthe 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 arethey up to the job? The most successful display technology to date is still the liquid crystal display. It is awell established technology which can be scaled in size from small to large and, depending on the driveelectronics, has no significant inherent limits to resolution. However, poor light efficiency and datacongestion caused by high pixel densities are still technical challenges in need of good solutions. Couldelectrophoresis in a liquid crystal host be the answer? The fundamental basics and technical challenges ofthis interesting new approach will be discussed.

External Posting Date: March 21, 2013 [Fulltext] Approved for External PublicationInternal Posting Date: March 21, 2013 [Fulltext]Additonal Publication Information: Published in Liquid Crystals Reviews.

Copyright 2013 Liquid Crystals Reviews.

Electrophoretic liquid crystal displays: How far are we? Susanne Klein

HP Labs, Long Down Avenue, Bristol BS34 8QZ, UK

Susanne.Klein@hp.com

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.

Polarisers

Substrates

Colour filters

Alignment layers

Spacer

Liquid CrystalElectrode

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.

Unpolarized light

Polarizer

Analyzer

Orientation of the liquid crystal director

2a

Polarizer

Analyzer

Unpolarized light

Orientation of the liquid crystal director

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 double-

layer see [2]. The thickness of the charge ‘cloud’ around the particles is described by the so-called

Debye length [3] given by:

N

i

ii

Br

nze

Tk

1

22

01 (1)

Where:

0 r is the dielectric permittivity of the dispersion medium,

Bk 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 0 r .

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:

Ev (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:

0r

(3)

Where:

0 r 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 1a where a is the radius of the

sphere. In the case of very small particles or thick double-layers, that is for 1a , the

electrophoretic mobility is given by the Hückel formula [5]:

3

2 0r

(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 TkB , is given by the Bjerrum length λB as follows:

Tk

e

Br

B

0

2

4

(5)

where e is the elementary charge. For water at 22oC with 80r the Bjerrum length is only 0.7 nm,

but for dodecane at the same temperature and 2r it is 28.3 nm. The Van der Waals energy

between two spheres of radius R1 and R2 with a surface separation 21, RRl is

lRR

RRAVVdW

)(6 21

21

(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:

2/322

2222

)(

)(

216

3

4

3

mp

mpe

mp

mp

Bnn

nnhTkA

(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 115103 s 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 TkB 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 e-

reader 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.

2

1cos

2

3 2 S

(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.

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