5.2.1.1 Color from Cholesteric Liquid Crystals

An absorbing layer can be placed behind these textures to further enhance the contrast of these images, with the Focal conic texture showing the color of the absorbing layer and the Planar texture showing the additive sum of the color of the liquid crystal Bragg reflection and absorber color. It is desired to place the absorbing layer as close as possible to the liquid crystal layer to limit any reduction in contrast from haze of the back substrate Figure 5.2.

The color reflected by the liquid crystal can be tuned by changing the pitch of the helix. The pitch can be modified by changing the concentration or the helical twisting power (HTP) of the chiral dopant according to the equation P_o = c. HTP. The HTP is a parameter dependent on the chiral dopant, which imparts chirality and hence handedness to the doped liquid crystal. This can be achieved either in situ or ex‐situ. For example, the HTP can be temporarily changed using UV light to allow image creation, as in Figure 5.3 [2].

Recent work on color tuning application of an electrical field was shown using a new state of cholesteric liquid crystals, the Oblique heliconical structure [10]. Electrothermal color tuning has also been used for the same purpose [11]. A good review of the many methods developed to change the peak wavelength, including strain, humidity, and pH, is discussed elsewhere [12].

5.2.1.2 Optimizing Cholesteric Liquid Crystals for ePaper Applications

Although in ref. [11, 13] temperature is being used to tune the color; for most ePaper applications, the color needs to remain stable over a large temperature range (from at least 0 to 65 °C). Much work has been done to achieve this, including doping with left‐ and right‐hand chiral dopants [14] and using dopants showing an opposite change in HTP with temperature in the same mixture [15].

The peak wavelength λ_o has a significant dependence on the incident angle, blue shifting with increasing angles θ, [16] according to 〈n_e – n_o〉P_o cos θ. To reduce this undesirable dependence, an imperfect Planar texture is desired. This can be done by designing the surfaces in contact with the liquid crystal to form an imperfect Planar texture [17]. In surface stabilized cholesteric displays, the cell surfaces are treated with weak homogeneous or homeotropic alignment layers to stabilize the imperfect Planar helix. Under a microscope, the perfect Planar texture is reflective with very few defects, while the imperfect Planar texture shows multiple domains of bright spots about 5–50 μm in diameter [18]. An example of this reduction in the change in peak wavelength shift as the viewing angle can be very significant, as shown in Figure 5.4. The measurement was done in an integrating sphere to simulate diffuse illumination, mimicking reflective media usage's lighting conditions.

5.2.1.3 Optical and Mechanical Effects of Polymer Networks

In Polymer Stabilized Cholesteric Textures (PSCT) displays, a small amount of polymer is used to improve the bistability or performance of the system. Still, it also disrupts the Planar texture as depicted in Figure 5.5 while improving the viewing angle [8]. PSCTs are typically achieved using 0.5–10% of the polymer as long as the system remains in the liquid crystal phase. Higher polymer concentrations can generate structures where the polymer surrounds the liquid crystal‐like in microemulsions and Polymer Dispersed Liquid Crystals (PDLC) [19]. A PDLC made with ChLC is shown in Figure 5.6. In these systems, the viewing angle and reflectance depend on many factors, including the droplet size, uniformity, shape, encapsulation material, polymer wall thickness [20]. For example, tiny droplets will make the Focal conic texture backscatter too much light, lowering the contrast [21]. In addition to the change in reflectivity, the peak wavelength can also be affected by the encapsulation with the peak wavelength either red or blue shifting from its value in a surface stabilized system; the system in Figure 5.7 shows a redshift. With the proper choice of materials and encapsulating conditions,,. This change in reflectivity and wavelength can be controlled, as discussed later.

The extensive work on encapsulated ChLC paved the way to one of the most exciting applications of ChLC ePapers; flexible displays have tremendous advantages such as the possibility of continuous processing and the design of flexible, rugged, and lightweight devices. Encapsulating the ChLC in polymeric networks provides mechanical robustness and prevents liquid flow, allowing for a significant number of applications. The design of the polymeric networks from a structural perspective enables the improvement of mechanical properties such as adhesion and impact resistance. Figure 5.8 shows two different polymeric structures after their flexible substrates are removed. The failure mode indicates a cohesive fracture on the left since there is a remaining polymer on the substrate surface ( also present on the other substrate not shown here). The right‐hand side figure indicates an adhesive failure as the polymeric network layer detaches from the substrate surface [22].

Figure 5.9 is only one example of many flexible display prototypes developed over the years [23–25]. Different levels of resolution, drapability, color gamut, and ruggedness have been achieved. However, few of them are commercially available to writing this chapter.

Sometimes it is desirable to significantly broaden the reflection bandwidth, making the reflected color more achromatic, a feature ideal for black and white ePaper. This is achieved by using polymer networks to disrupt the Planar structure to a poly‐domain Planar Texture [26, 27]. One can also stack a yellow and blue cell on top of each other, attach a black absorber to allow color mixing to create a white Planar texture and a black Focal conic texture. It is also possible to create pitch gradients in the Planar state by using controlled photopolymerization of mixtures monomer and cholesteric liquid crystals [28–30]. The bandwidth has also been broadened using electrical fields [31].

To achieve even higher reflectivity, one can stack cells with left‐ and right‐handed cholesteric cells on top of each other to increase the reflectivity of the resultant cell to over 80%. It is preferred to use the larger domain size as the bottom layer in an encapsulated system.

In some cases, purer color is desired than can be achieved by controlling Δn, alignment layers, and encapsulation techniques. To compensate for the broadened bandgap and loss of color purity, reference [32] used color filters to absorb unwanted reflections at each of the color layers of a stacked RGB system to increase the gamut by 120%, as shown in Figure 5.10. Some groups use dyes in the liquid crystal layer for the same purpose.

5.3.1.1 Driving Schemes

A representative electro‐optic response curve of a bistable cholesteric pixel is shown in Figure 5.12; there are two curves, one where measurements are taken with the pixel is reset to Planar texture and the other with the pixel reset the Focal conic texture. Each of the points on the curve is taken after the pixel has relaxed to its stable state, typically about 500 ms. With the pixel initially reset in the Planar texture before voltage pulse application, there is a threshold voltage, V1, before the pixel starts dimming. The region between V1 and V2 is where some domains remain in the Planar texture and others switch to the Focal conic texture. Between V2 and V3, the pixel is in the Focal conic texture, increasing reflectance after that as some domains unwind and switch to Planar until V4, where all the domains switch to Planar. With the reset pixel initially switched to the Focal conic texture, the curve stays dark until there is sufficient voltage > V3 to get some of the domains to Planar until V4, where they all switch to Planar. There is a difference at which the initially Planar and Focal conic pixels start grayscale and all switch to Planar due to the ease at of the Planar texture switching to homeotropic texture. This curve shifts to higher values as the voltage pulse width is decreased, eventually making it difficult to switch a Planar pixel to Focal conic [17, 24].

This curve allows for direct and grayscale addressing. A pixel can be driven to either a bright state by applying a voltage > V4 or a dark state by a voltage < V3. The grayscale values between V1 and V2 are reproducible and stable.

The threshold voltage V1 allows for Bistable cholesterics to be driven very simply using simple multiplexing without active matrix backplanes. This ability and ease to multiplex cholesteric displays make it very attractive to use flexible substrate, as the flexible active‐matrix backplanes are not readily available. In displays using the conventional drive scheme, Figure 5.13 shows one way that this is implemented [24, 35], where V_na = 0, V_a=(V₃ + V₄)/2, ΔV = V₄ − V₃. There is no cross talk between pixels as long as (1/2)ΔV < V1. Grayscale is achieved by driving the columns between (1/2)ΔV and −(1/2)ΔV.

To speed up the slow addressing time of the conventional drive scheme, the dynamic drive scheme takes advantage of the fast homeotropic‐transient Planar transition and the hysteresis between the Focal conic and the Planar transition [36]. This drive scheme can provide an update rate of about one page per second. The cumulative drive scheme was developed to demonstrate video for smaller resolution displays and eliminate the black line observed when updating the previous two schemes. This scheme works on the cumulative effect of repeatedly driving the same pixel multiple times, with each pulse moving the domains to the desired state [37, 38]. In addition to multiplexing, active‐matrix backplanes are also used to drive the Cholesterics.

5.5.4.1 Writing Mechanism

The effect of flow on the ChLC textures has been the subject of analysis over the years. One of the first reports tracked the transmission and reflection of a layer of ChLC exposed to different shear rates [52]. The objective of the experiments was to explain the effect of shear forces on the Planar textures of the ChLC mixtures., At low shear rates, the reflectance of the ChLC layer increased and shifted toward blue, concluding that the helical structure of the ChLC behaves as a rigid unit tilting with little distortion. Intermediate shear rates seemed to create a complex stratified distribution that may include a Focal conic layer in the middle and Planar layers in contact with the shearing surfaces. Higher shear drives the system to a homeotropic state. In all cases, the system relaxed to a Planar texture after a short time.

More recently, researchers have proposed computational simulation of the microfluidic flow of cholesteric liquid crystals to study the rheology of ChLC with different anchoring conditions in the context of a Poiseuille flow. Their hydrodynamic model was solved numerically, showing other texture transitions at different pressure gradients for a ChLC with homeotropic anchoring conditions going from finger patterns at low pressures to the formation of Planar structures at larger pressure gradients [53]. To fundamentally understand the effect of flow on a polymer dispersed ChLC writing tablet, one needs to consider the complexity of the rheology of anisotropic material, such as ChLC, in confined heterogeneous geometries, such as polymeric networks (Figure 5.20). The current state of the art clarifies that it is an intricate task and an incredible opportunity for the scientific community.

As a qualitative depiction of the cycle of writing and erasing of a writing tablet, Figure 5.23 shows how an unwritten surface appears black as the weakly scattering focal conic texture allows light to go through the layers to be absorbed by the black background. If a pointed object is used to deflect the front surface of the construction, force is transferred to the polymer dispersed ChLC changing the volume and inducing localized flow on the initially Focal conic domains forcing them to reorient to a Planar texture that follows the trajectory of the writing. The net flow of ChLC is constrained by the polymeric network providing the final shape and size of the written line that remains after the pressure is released. Notice that up to this point, there is no need for an electric field thanks to the bistable properties of the polymer‐stabilized ChLC. After the writing is finished, an electric pulse can reposition all ChLC domains back to the Focal conic texture.

Human writing is a complex process that involves several neurophysiological and biomechanical variables [54]. However, for all practical purposes, one could assume that the effect of writing on a surface can be described by the writing speed and the force applied to the surface through the writing object. Previous measurements show that writing speeds and forces vary largely depending on the user, age, and writing style. However, it can be assumed that the human writing speed distribution is centered around 75 mm/s and force around 150 gF [55, 56]. These approximations will be used in the chapter to describe the general effect of human writing on the writing properties of ChLC writing tablets.

One way to characterize the writing tablet properties is to draw the line at a given weight and speed using a non‐compressible stylus tip of a known radius. Its width can be measured manually or digitally. The effect of writing weight and speed on the intensity profile of written lines is shown in Figure 5.24. All measurements correspond to one writing tablet using the same instrument. Notice that the width and shape of the written mark changes substantially with speed at a fixed weight. In the case of fixed speed and varying weight, the shape remained similar while the intensity decreased significantly when lighter weights were applied on the writing tablet. One can infer that slower writing speeds promote larger outward flows as there is more time for liquid crystal molecules to be influenced by substrate deflection.

Interestingly, line widths cover a significant range from less than 1 mm to about 4 mm; note that the writing tip is a hard‐sphere of 3 mm diameter, which means the effect is not necessarily related to the writing object size but to the flow exerted by it. The message here is that the size of the mechanically induced Planar texture is essential. The patterns and features inside the line can be of significant interest on the fundamentals of how a ChLC material responds to different forces when confined inside polymeric networks.

As hinted before, it is possible to obtain different ChLC embedded in morphologies with different features. Consequently, it is expected that the line response to weight (writing force) and speed would be different for different systems, as exhibited in Figure 5.25, where each trend represents a different polymer morphology. It becomes evident that it is possible to obtain highly sensitive systems that respond quickly to speed or weight changes while others remain flattered over the same range of speeds and weights.

Another critical property of lines created by pressure on a ChLC writing tablet is reflectance. Generally, higher reflectance is achieved with systems that show wider line widths. However, as seen in Figure 5.26, it is also possible to obtain thinner lines with the same or higher reflectance as thicker ones. The factors affecting the reflectance of writing tablets are not entirely understood given the complexity of the morphology, alignment between the ChLC and polymer walls, interactions between ChLC and/or polymer with the surface of conductive polymer, scattering effects, etc. Writing reflectance measurement can be made through diffuse illumination at an 8° viewing cone, with specular component included (SCI) under a D65 illuminant for a 10 ° (CIE 1964) observer. The measurement is made, so the aperture of the equipment covers an area that is fully written on.

Consequently, any reference to reflectance should be considered as per unit area. The comparison between the reflectance of the written line (Planar) versus the reflectance of the background (Focal conic) is a measure of the contrast of the device. The background reflectance is a combination of the optical properties of the Focal conic texture of the ChLC, polymeric network, and back substrate. As seen in Figure 5.26, contrast‐enhancing is usually achieved by increasing the reflectance of the written line without increasing the reflectance of the background.

One interesting finding is that the effect of the cell gap does not follow a monotonic relationship with the reflectance of the written line. Unlike pure ChLC, writing tablets may exhibit different regimes [57]. As seen in Figure 5.27, thinner cell gaps exhibit higher reflectivity up to 4 μm, where the behavior changes perhaps now dominated by the well‐known dependence on the number of pitches that can fit in the cell gap [58]. One plausible hypothesis is that thinner cell gaps maximize the alignment of ChLC molecules in the Planar texture to the substrate surface concerning the lateral polymeric walls. Similarly, one can imagine that thinner cell gaps promote the formation of ChLC pools in interconnected droplets like the structures presented in Figure 5.20; it is expected that said structures result in better and larger Planar domains.

5.5.4.2 Electro‐Optic Response of Writing Tablets

Like measuring the curves with a pure electrical response, the display needs to be reset to a known initial state before measuring its voltage response. An erase and reset waveform at the drive frequency (typically around 3 Hz) that would switch the display to the weakly scattering Focal conic state are determined to obtain these curves. Typically, this waveform has two bipolar pulses, one to switch the writer to Planar and another to reset the display to the dark state. The erase pulse amplitude is set to the maximum voltage step of the measurement. The best reset amplitude is the voltage provides the darkest focal conic state. A typical result is obtained u the schematic shown in Figure 5.28 for every voltage step, a specific result is obtained as shown in Figure 5.29.

Note that, in some cases, the reflectivity of the written line is higher or lower than the reflectivity obtained by switching the display to the bright state. Looking at Figure 5.29, the reflectivity of the written line before voltage V1 is higher than the reflectivity after voltage V4. At voltages less than V1, the voltage is not high enough to switch the display to homeotropic and enable the transition to Planar. Therefore the reflectance of the written line dominates. But at voltages higher than V4, the voltage is high enough to switch the display to Planar.

5.5.4.3 Exact Erase

The writing tablets described above are single‐pixel devices; that is, the same electrodes are shared over the whole writing area. Therefore, the whole tablet is erased. It is possible to erase a portion of the tablet to erase minor mistakes, as shown in Figure 5.30. This type of erasing takes advantage of the pressure from the stylus that locally changes the cell gap from the nominal d₁ to a reduced d_2, as shown in Figure 5.31. This reduction in cell gap and the flow of the liquid changes the electro‐optic response in that area of pressure Figure 5.32 indicates that there is a selective erase voltage, V_EE, where the reflectivity of the written image, R_W, is maximized and reflectivity of the erased image, R_E, is minimized. Beyond that range, it is possible to fully erase the display using, for example, voltage V_FE without the need for eraser pressure [59].

5.5.4.4 Writing Reliability

Unlike paper, writing tablets have to survive numerous writings during the life of the device. It would be unacceptable, for example, if the writing were illegible or altered due to previous writing. To develop a reliability test, the writing profiles of 10 individuals writing on cholesteric writing tablets were captured using a digitizer behind the tablet [60, 61].

From this study (Figure 5.33), the average writing speed is 65 ± 50 mm/s, and the average writing force is 205 ± 35 g‐force. Further analysis of the data shows that,, individuals only write faster than the above averages 55% of the time and only about 17% of the time at speeds greater than 115 mm/s. They also only write with forces above average about 12% of the time. The average maxima of the writing speeds and forces were also found. The average maximum writing speed is 320 ± 149 mm/s, about 5 times higher than the average writing speed above. The average maximum writing force is 390 ± 920 g‐force, which is about 2 times higher than the average writing force above.

With these results, a writing test and pattern were developed (Figure 5.34a). The pattern was chosen to have some repetition to accelerate wear. The writing test is done using a robot that normally holds a stylus with a 150‐gramg force attached and writing at a nominal velocity of 63 ± 33 mm/s. Using these parameters and even higher forces, writing tablets show remarkable robustness, especially at typical writing forces, as shown in Figure 5.35.