Cetyltrimethylammonium bromide (CTAB) was employed as an ionic dopant dispersed into a nematic liquid crystal characterized by its negative dielectric anisotropy. The electrohydrodynamic (EHD) effect in liquid crystal cells impregnated with various contents of CTAB was studied by dielectric spectroscopy and the corresponding electro-optical responses of the cells driven by an AC electric field were investigated. Compared with the undoped counterpart, the frequency range of the EHD regime after doping with 0.2-wt% CTAB into the liquid crystal was widened, and distinct optical textures, including dynamic scattering, stripes, and grids were unambiguously observed. The CTAB addition reduced the threshold voltage required for switching the planar state to other optical states generated by the EHD effect.
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Attempts continue to reveal the new world of liquid crystals (LCs) thanks to the ease of incorporating guest materials through non-synthetic approaches and of developing their innovative properties. Indeed, the added materials may change the dielectric and electrical properties of the pristine LC, and this can be observed by measuring the dielectric constant and estimating the mobile ion concentration that are directly affected by the electric field applied to the LC cell. The ion concentration in a LC material could be increased owing to the pollution of the external environment , dissociation from LC molecules over time , irradiation to ultraviolet light , or injection from the substance of alignment layers . The presence of ions in LC could raise the needed driving voltage for the LC cell and slow down the response process. To tackle this problem, previous studies focused on reducing the impact of ion effects on the electro-optical characteristics and various strategies have been suggested along the line of doping inorganic nanoparticles to suppress the ion effect [5–9]. It is worth noting that, on the other hand, the effect of ionic additives on electro-optical properties and the resulting significant impact on the formation of additional optical states and textural transitions have been observed, which may help in employing these materials in other applications such as smart windows  and optical components . The addition of a small amount of ionic surfactant to positive nematic LC (with dielectric anisotropy Δε > 0) enables a field-on twisted-nematic structure to transform from the field-off planar structure after applying a DC electric field and substantially enhances the contrast ratio and the number of grayscale levels in comparison with the typical or commonly adopted twisted–homeotropic transition . With AC electric field, new optical states arise as a result of the response of the added ions diffusing through the LC bulk and moving in the form of cross-sectional circles, thereby disturbing the alignment of LC molecules and even causing the LC mass flow, which is called the electrohydrodynamic (EHD) effect .
In the LC display industry, the ion effect has always been regarded as a notorious effect. However, recent works have pointed out that the use of ion effects can positively alter optical properties. Consequently, new types of optoelectronic elements and devices have been proposed. The addition of ionic dopants in negative nematic LCs (nLCs)—namely, nematic LCs possessing negative dielectric anisotropy (Δε < 0)—with initially vertical alignment was targeted to induce the EHD effect, generating the dynamic scattering (DS) state under excitation of low-frequency AC electric field in order to obtain an opaque (precisely, severe hazy) state . The haze of the opaque state was measured for cholesteric LC, polymer-dispersed LC, and ion-doped LC samples, referring to the ability of ion-doped LC to effectively scatter the incident light. In another work, the concentration of ion impurities was promoted after the UV exposure of polymer-stabilized cholesteric LC, deforming the polymer network under an applied DC electric field and causing a widened bandwidth of the Bragg reflection band .
The contribution of ions as a dopant to generating EHD flow and a related study of the optimal conditions for developing the DS state can yield an important application for photonic devices and thus are of scientific and technical interest. In this work, the EHD effect was implemented by doping a minute concentration of cationic surfactant into a nLC confined in planar-alignment cells. So far, incorporating a cationic surfactant—tetrabutylammonium bromide (TBAB), dodecyltrimethylammonium bromide (DTAB), or cetyltrimethylammonium bromide (CTAB)—as an ionic additive into a negative achiral or chiral nematic LC or SmA LC has been proven promising for facilitating EHD instabilities [13,15–17]. Here, a nLC was chosen to allow electrical generation of various unconventional LC patterns, especially the DS state, via the EHD effect whereas CTAB, a typical, popular, and economic cationic surfactant in LC-related research fields, was selected as the ionic dopant to enhance the EHD effect and modify the dielectric and ionic properties of the LC host. The phase transition temperatures and the complex dielectric spectra of undoped and doped nLC cells were measured and compared. The ion concertation and diffusivity as well as their influences on the electric conductivity are presented with respect to the surfactant concentration and the ambient temperature. The electro-optical behavior of a nLC doped with cationic surfactant at a particular concentration was explored in order to observe the generated textures or optical states. The diffusion of ions inside a nLC bulk offered a new route toward studying the driving voltage needed for enabled switching from the initial high-transmission optical state (planar state) to lower-transmission optical states of stripes, grids, and dynamic scattering.
A nLC material of Δε = −5.4 branded with DV-10001 exhibiting clearing temperature TC = 111 °C was purchased from Daily Polymer Co., Taiwan. It is a multi-component LC mixture consisting of seven neutral and six polar compounds. A cationic surfactant, cetyltrimethylammonium bromide (CTAB), obtained from ACROS Co., Taiwan, was doped as an additive at several concentrations ranging from 0 to 1.5 wt% into DV-10001 weighed by a microbalance (Mettler Toledo AX26) with an accuracy of 10−6 g. The used sandwich-like empty glass cells (Mesostate Co., Ltd., Taiwan) have transparent, electrically conductive indium–tin-oxide (ITO) coatings on internal substrate surfaces and covered with planar-alignment layers (SE-2170, Nissan Chemical). Each cell possesses a specifically designed overlapped electrode area of 0.25 cm2 and a cell gap of 15 μm. The DV-10001/CTAB mixtures were heated to 120 °C and vigorously stirred for 30 minutes and then injected into the ITO cells via the capillary action. The optical texture was observed using an Olympus BX51 polarizing optical microscope (POM), which was attached to a digital camera (Olympus XC30) for exploring the optimally miscible CTAB concentration. The CTAB contribution to varying the phase transition temperature and the dielectric spectroscopic parameters were analyzed with an Agilent E4980A precision LCR meter in the frequency range of 20–2 × 106 Hz or with a HIOKI 3522-50 LCR HiTester for frequencies lower than 20 Hz, connecting in series to the GPIB interface for LabVIEW computer control. The temperature of a nLC cell was regulated by a temperature controller (Linkam T95-PE) in the temperature range of 30–120 °C. The electro-optical responses of a specific DV-10001/CTAB mixture were observed via acquiring the transmission spectrum (400–800 nm) with a fiber-optic spectrometer (Ocean Optics HR2000+, NIR512) in addition to a halogen lamp (Ocean Optics HL-2000-CAL). The experiment of voltage-dependent spectral study was carried out under the application of AC square-wave electrical signal supplied from a function generator (Tektronix AFG-3022B) and precisely controlled by a power amplifier (Trek Model 603). The same setup of electro-optical responses was used for studying the transmittance at a single wavelength (632.8 nm) vs. the applied signal voltage (V–T%) and vs. the frequency (f–T%) after placing the nLC cell between a He–Ne laser source and a calibrated silicon photodetector.
3. Results and discussion
3.1 Solubility of CTAB in nematic DV-10001
The CTAB cationic surfactant was homogeneously dispersed in the nLC DV-10001, with the dopant concentrations of 0, 0.5, 1, and 1.5 wt%. The prepared mixtures were investigated under a POM by adjusting the rubbing direction at 45° relative to either of the transmission axes of crossed polarizers and at an adjusted temperature of 30 °C by a temperature controller (Figs. 1(a)–(d)). Aggregates or immiscible domains of CTAB were found in the optical texture of DV-10001 inclusive of doped CTAB at concentrations of 0.5, 1, 1.5 wt% besides the regular rod-shaped spacers inside the LC cells. Accordingly, the CTAB concentrations of 0.1, 0.2, 0.3, and 0.4 wt% (Figs. 1(e)–(h)) were narrowed down for searching into the optimal CTAB concentration that can preserve a clear optical texture. The micrographs as depicted in Fig. 1 indicate that, when the CTAB concentration c ≤ 0.2 wt%, the optical texture showed no agglomeration.
3.2 Influence of CTAB doping in the phase transition temperature of DV-10001
In this section temperature-dependent dielectric spectroscopy as a simple and fast method was utilized to detect the phase transition behavior of DV-10001/CTAB . Figure 2(a) shows the change in the real part of the dielectric constant (ε’) with temperature as a variable at 300 kHz where ε’ is frequency-independent. The ε’ values of four nLC samples were measured with a small probe voltage (≤ 0.5 Vrms) and the effective dielectric constant can be expressed by
3.3 Dielectric parameters of DV-10001/CTAB
The dielectric spectra of the real and imaginary parts of the dielectric constant, ε’ and ε”, and the dielectric loss tangent, tan (δ) = ε”/ε’, of pure DV-10001 were characterized by the relaxation frequency fR at the peak of the tan (δ) curve and the two critical frequencies fL and fH at the intersections of the ε’ and ε’’curves which divided the frequency into three regions  as illustrated in Fig. 3. In region I, where f < fL, the ε” value decreases as the frequency decreases and the polarity oscillation of the external AC electric field gradually slows down, causing ion accumulations on the ITO electrodes and thereby forming electric double layers to offset the external electric field applied to the LC cell. In region II, fL < f < fH, the free ions diffuse in the LC cell, and the dielectric spectrum is affected by space-charge polarization and can be expressed as a function of frequency [20–22]
The dielectric spectra of ε’ and ε” were measured for DV-10001/CTAB samples in the frequency range of 1 Hz–100 kHz for CTAB concentrations of 0, 0.1, 0.2, 0.5, 1, and 1.5 wt% at 30 °C (Figs. 4(a) and (b)). The ε’ curve relaxed and the ε” curve reached to the peak at a frequency (fR), which blue-shifted after the CTAB doping. In general, the dielectric relaxation from dipolar reorientation of LC molecules of a positive (Δε >0) or a negative (Δε >0) LC material is obtained in the frequency range of MHz . The complete dielectric relaxation behavior observed in the investigated frequency range (i.e., 1 Hz–100 kHz) of DV-10001/CTAB samples is thus attributable to space-charge polarization originating from the transport of mobile ions. For our detailed study, eight samples were prepared by doping the nLC host with CTAB within the observed miscible range, 0–0.2 wt%, (0, 0.001, 0.005, 0.01, 0.02, 0.05, 0.1, and 0.2 wt%), and the dielectric spectra of ε’ and ε’’ were acquired in a narrower frequency range, 20–105 Hz (Figs. 4(c) and (d)). The resultant complex dielectric spectra showing pronounced frequency variations of ε’ and ε” in a wider frequency range with increasing CTAB concentration can be explained by the promotion in ionic effect due to the addition of CTAB in DV-10001 that permits the space-charge-polarization-dominated dielectric relaxation to be induced at higher frequencies. The three characteristic frequencies—fL, fR, and fH—were deduced from Figs. 4(c) and (d) in order to analyze the CTAB influence on the DV-10001 dielectric properties.
Table 1 summarizes the three characteristic frequencies varying with the CTAB concentration c, which clarifies the response of the diffused CTAB ions through DV-10001 to the applied AC electric field causing a dissipation of the applied electric energy to a lossy medium (DV-10001/CTAB) and, in turn, rendering the frequency range of region II (fL < f < fH) to be wider and to move to higher frequencies. In order to explore the effect of the CTAB dopant, the ion density or the concentration of mobile charges, n, and the diffusion coefficient D were calculated from Eqs. (2) and (3) on the basis of the measured ε’ and ε’’ data as shown in Figs. 4(c) and (d) within the frequency range of fL < f < fH. Figure 5 reveals the two calculated material parameters. It can be observed that at 30 °C raising c from 0 wt% to 0.2 wt% led to the increase in ion density from (1.38 ± 0.02) × 1013 cm−3 to (1.84 ± 0.03) × 1013 cm−3 and that, at c > 0.02 wt%, the ion density exhibited a steady behavior with its value ranging between 1.84 × 1013 cm−3 and 1.91 × 1013 cm−3. On the other hand, the diffusion coefficient increased monotonically from (3.52 ± 0.5) ×10−6 cm2·s−1 at c = 0 wt% dramatically to (663.1 ± 30) × 10−6 cm2·s−1 (which is about 200 times higher) at c = 0.2 wt%.
In addition to the above discussed ionic behaviors in the frequency range between fL and fH, we further focused on LC-orientation-contributed dielectric permittivity at frequencies higher than fH (f > fH) to investigate the effect of CTAB doping on the dielectric anisotropy (Δε) of the nLC DV-10001. Here, pure DV-10001 or 0.2 wt% CTAB-doped DV-10001 was injected into another homeotropically aligned cell with a cell gap of 4 μm and a planar-aligned cell with a cell gap of 6 μm to measure its parallel (ε||) and perpendicular (ε⊥) components of dielectric permittivity, respectively. It should be noted that varying the cell gap does not affect the exact value of dielectric permittivity from LC orientation but would lead to a shift of frequency range for the space-charge-polarization-dominated dielectric relaxation due to the change in distance between substrates for ion transport. Figure 6 displays the real-part dielectric spectra in the frequency range of 100 Hz−100 kHz for pure DV-10001 and DV-10001 doped with 0.2-wt% CTAB in homeotropic-aligned cells (ε’||(f) curves in Fig. 6(a)) and planar-aligned cells (ε’⊥(f) curves in Fig. 6(b)) at 25°C. The frequency-independent dielectric permittivity at frequencies higher than 50 kHz in each ε’(f) curve in Fig. 6 indicates that the ionic effect is suppressed and the values of ε’ in ε’||(f) and ε’⊥(f) curves solely represent ε|| and ε⊥ of LC molecules, respectively. Consequently, in the case of T = 25°C, it is found that ε|| of CTAB-doped cell is lower than that of the undoped counterpart (Fig. 6(a)) but values of ε⊥ of the two samples are nearly identical (Fig. 6(b)). This connotes that the absolute value of Δε of DV-10001, calculated in accordance with the formula of Δε = ε|| − ε⊥, was increased after doping with CTAB. In contrast, when the frequency was changed from 50 kHz to the commonly inspected frequency of f = 1 kHz, values of ε|| and ε⊥ of 0.2 wt% CTAB-doped DV-10001 were significantly increased due to the severe ionic effect; thus, the value of Δε much higher than that of undoped counterpart at f = 1 kHz would be attributable to both the ion transport and LC orientation. As acquired from the measured dielectric spectra at different temperatures, summarized in Table 2 are the values of ε||, ε⊥, and Δε of undoped and 0.2 wt% CTAB-doped DV-10001 dominated by both the ion transport and LC orientation at the frequency of f = 1 kHz and contributed solely by LC orientation at f = 50 kHz in the temperature range between 20 and 30°C.
3.4 Temperature dependence of DV-10001/CTAB’s dielectric parameters
The temperature dependences of DC conductivity, diffusion coefficient and ion density for DV-10001 samples doped with CTAB at 0-, 0.001-, 0.01-, and 0.1-wt% concentrations were studied by controlling the ambient temperature of the LC cell between 30 °C and 80 °C within the nematic phase range. In order to deeply understand the transportation behavior of mobile ions in a LC layer in correlation with temperature variation, measured ε’ and ε” data at each temperature were fitted to Eqs. (2) and (3) to calculate the ion concentration n and the diffusion coefficient D. The frequency spectrum of the AC electrical conductivity σAC can be obtained from the ε” experimental data at different temperatures, 30–80 °C, utilizing the following equation within the frequency range of fL < f < fH:26]; second, a kind of kinetic energy could be gained by the ions when the temperature rises. As a consequence, the ion impurities inside the LC can easily diffuse against the lowered fluid resistance. Generally speaking, σDC is proportional to the ion density n and the ion mobility μ, according to the equation:
Therefore, the increase in σDC with rising temperature is understandable because of the observed thermal dependence of the ion concentration and the diffusion coefficient (Fig. 7). In addition, the DC conductivity exhibited an obvious dependence on the CTAB concentration as well. For example, σDC of the nLC increased from 6.18 × 10−10 S·cm−1 to 9.34 × 10−8 S·cm−1 at 30 °C as c increased from 0 to 0.1 wt%. In regard to the diffusion coefficient, it can be written as a function of temperature, in accordance with the Arrhenius formula. Accordingly, the data presented in Figs. 7(a) and (c) were fitted to the following form [27,28]:
3.5 Electro-optical characterizations of DV-10001/CTAB
By measurement of transmission spectra and observation of optical textures with a POM, we explored the EHD effect originating from moving ions in response to an AC electric field in pure DV-10001 and DV-10001 doped with 0.2-wt% CTAB at room temperature. The corresponding optical textures were acquired from a neat DV-10001 cell at voltages of 0, 10, 20, 30, and 40 V and a fixed signal frequency of 20 Hz (Fig. 8(a)). The textural observation started with the planar (P) state giving optical transmission of ∼85% at null voltage. By increasing the applied voltage to 10–30 Vrms, the optical transmission dropped to ∼40% and lower. Stripes (St) were observed as dark and bright periodic bands in the LC texture generated from the ion flow because of the AC electric field, causing a disturbance for the LC director. When the voltage arrived at 40 Vrms, optical transmission was reduced to less than 5% and the dynamic scattering (DS) state was noticed, where the stripe structure was severely disturbed and disrupted, and many smaller domains were formed. Strong light scattering occurring in the DS state was caused by the mismatch of the refractive index between the domains. Comparatively, the 0.2-wt% CTAB doping effect in DV-10001 was studied at 20 Hz as well (Fig. 8(b)). The optical texture transformed from the P state into the St state at 50 Vrms, and a grid state (Gr) was generated when the voltage was increased to 60 Vrms, where the transmittance fell to about 35%. It becomes clear that the threshold voltage Vth to induce EHD flow is higher for the doped nLC than for the neat counterpart at 20 Hz. Figure 8(c) discloses the results of the same DV-10001/CTAB sample driven by voltages at 1 kHz. It is clear that the addition of CTAB shifted the voltage-responsive EHD frequencies toward higher values.
It is believed that Vth needed to prompt LC instabilities is dependent on the applied frequency f, and to observe the effect, a helium–neon laser operating at λ = 632.8 nm was exploited to measure the voltage-to-transmission curves (V–T% plots) for two LC samples, pure DV-10001 and DV-10001 doped with 0.2-wt% CTAB, through the frequency range of fL < f < fH. The pure DV-10001 sample was investigated under signal frequencies, 20, 500, 1000, and 2000Hz, and a positive correlation between Vth and f was found; that is, the higher the driving frequency, the higher the threshold voltage for the EHD occurrence (Fig. 9(a)). Reasonably, as the frequency of the electric field continued to increase, free ions gradually failed to follow the speedier polarity alternation of the applied field, and the distance that the free ions can move convectively within half a cycle of the applied signal decreased, resulting in a weakened efficacy to arouse significant LC flow. Under this condition, more kinetic energy would be needed to produce the EHD effect. However, the correlation between f and Vth for the doped cell was somewhat more complicated. One can see from Fig. 9(b) that Vth of the CTAB-doped sample first decreased drastically and then increased with increasing frequency. It is worth mentioning that to compare the V-dependent transmission data in Fig. 9 and Fig. 8 would not be of much significance because of the two different measuring systems having distinct light sources and different distances from the samples for photodetection.
Since the phenomenon observed from DV-10001/CTAB is dissimilar to pure DV-10001, the dielectric spectra in correspondence with the frequency–voltage diagrams of the two types of samples are presented side by side to facilitate comparison (Fig. 10). The selected operating frequency of 20 Hz to observe the electro-optical properties can be found in region II between fL and fH (fL < f < fH) in the pure DV-10001 dielectric spectra in Fig. 10(a), but for the DV-10001/CTAB, 20 Hz sat in region I (< fL) instead, due to the blueshift in the relaxation frequency stemming from the CTAB doping (Fig. 10(b)). The shift in the dielectric spectrum can be understood from the Einstein relation6 and 7(c)). With a higher ion mobility in a doped cell, the ions can follow or respond readily to the field polarity change at a higher switching rate, causing changes of the three characteristic frequencies, fL, fR, and fH, to higher values. A further extension by plugging Eq. (7) into the mathematical relationship between σDC and μ (see Eq. (6) as discussed in the preceding section) allows one to comprehend the similar behaviors of σDC (Fig. 7(a)) and D (Fig. 7(c)). After all, the small variation in ion density n on the order of magnitude of 1013 cm−3 enables it to be regarded constant when considering σDC = nq2D/kBT .
For Fig. 10(b) in association with the DV-10001/CTAB cell, the frequency range can be divided into four intervals, f < fL, fL < f < fR, fR < f < fH, and f > fH, with two middle internals actually partitioned from region II. At f < fL, the moving ions had sufficient time to travel to and accumulate on the substrate surfaces so as to establish a counteracting field, wherefore a larger voltage would be required to agitate the EHD effect. When the operating frequency was situated between fL and fR (fL < f < fR), the ions were capable of circulating through the entire cell thickness to easily generate EHD flow, so Vth reduced. In the frequency range of fR < f < fH, the synchronous following of ions to the field polarity switching became harder and Vth started to rise with increasing f. When the excitation frequency f > fH, the oscillation of the field polarity was faster than the ions diffusion speed, so a higher voltage was required to drive the ions to move. As such, we selected 1 kHz as the fixed frequency—within the frequency range, fL < f < fR—for the DV-10001/CTAB sample for investigation of the transmission and textural properties as displayed in Fig. 8(c). As the applied voltage gradually increased, the optical texture switched from the P state to the St state at 10 Vrms. When the voltage increased to 15 Vrms, the St texture disappeared, and the optical texture exhibited a transition state. When the voltage progressively rose to 20 Vrms, the sample entered a DS state and the transmittance became lower than 10% by virtue of considerable light scattering. It was observed that, for a driving frequency f lying in the optimal range, fL < f < fR, Vth for textural transition triggered by EHD flow was as low as ∼10 Vrms as depicted in Fig. 10(b). It can be summarized that, after doping the nLC DV-10001 with 0.2 wt% of CTAB, the EHD frequency range has been dramatically broadened from 1 Hz–5 kHz to 1 Hz–100 kHz in the voltage range between 0 and 100 Vrms and the CTAB addition reduced the threshold voltage required for switching the planar state to other optical states generated by the EHD effect (Fig. 10).
4. Concluding remarks
A typical cationic surfactant, CTAB, was dispersed in a dielectrically negative nematic LC, DV-10001, for a dielectric and electro-optical study. The diffusion of CTAB ions in the DV-10001 bulk caused a noticeable blueshift in the complex dielectric spectrum at the doping concentration of 0.2 wt%. The mobile ion concentration and the diffusion coefficient increased from 1.38 × 1013 cm−3 to 1.84 × 1013 cm−3 and from 3.52 × 10−6 cm2·s−1 to 6.63 × 10−4 cm2·s−1 (Fig. 5), respectively, with increasing CTAB concentration from 0 to 0.2 wt%. The electro-optical measurements showed that doping 0.2 wt% of CTAB into DV-10001 broadened the driving frequency range for the EHD effect from 1–5000 Hz to 1–105 Hz at applied voltages ≤ 100 Vrms. It is concluded that the selection of the operating frequency within fL < f < fR enabled to perform the EHD manipulation and bring about the DS state with low deriving voltage (< 25 Vrms). So far, the EHD-induced DS texture has been extensively explored for the designs of various optical and electro-optic devices. From the point of view of practical applications, one can take advantage of the established optimal frequency range (fL < f < fR) unambiguously revealed in this study as a useful guidelene for material optimization for the purpose of operating DS state in an energy-efficient manner at a desired voltage condition. This work concentrating on and accounting for the dielectric and electro-optical characteristics of CTAB-doped LC may pave a new way to the development of tunable and switchable gratings, shutters, light vales, smart windows, and other optoelectronic components.
Ministry of Science and Technology, Taiwan (107-2112-M-009-012-MY3, 110-2112-M-A49-023).
The authors thank Mr. Guan-Fu Sung for his valuable suggestions and Mr. Yueh-Hung Lin for his help with dielectric measurements for this work.
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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