Electrically tunable wettability of liquid crystal/ polymer composite films

: An electrically tunable wettability in a liquid crystal/ polymer composite film is demonstrated, in which liquid crystal molecules are anchored among polymer grains. The tunable wettability of the composite films originates from the reorientation of the anchored liquid-crystal molecules, which is switched by an in-plane electric field with squared pulses of voltages. These liquid crystal/polymer composite films with electrically tunable wettability have potential applications in polarizer-free displays, ink-jet printing, microfluidic devices, and lab-on-a-chip.

In this paper, we demonstrate an electrically tunable wettability in the LC/polymer composite film by controlling the orientation of LC molecules anchored among the polymer grains. The contact angle of a water droplet on the LC/polymer composite films at room temperature can be switched in the range between 65° and 80° by an in-plane electric field. To prove concept, an adaptive-focus liquid lens using this electrically tunable wettability is demonstrated.

Sample preparation and operating principle
Figures 1(a) and 1(b) depict the device structure and operating principles of the electrically tunable wettability system. The device consists of an LC/polymer composite film on an IPS glass substrate overcoated with a patterned ITO (indium tin oxide) electrode. The ITO electrode on the glass substrate was etched with interdigitated chevron patterns. The angle of the zigzag ITO stripes is 160 o . The width and gap of the electrode stripes are 4 μ m and 10 μ m, respectively. The zigzag ITO electrode stripes generate in-plane electric fields.
To fabricate the LC/polymer composite film [14][15][16], we mixed a nematic LC mixture E7 (Merck) and a liquid crystalline monomer (4-(3-Acryloyloxypropyloxy)-benzoic acid 2methyl-1, 4-phenylene ester) at 60:40 wt % ratios. The LC/monomer mixture shows a nematic phase between −20 o C and 80 o C. We prepared an empty cell with a gap of 12 μ m which consists of a top glass substrate (without ITO) and a bottom substrate with IPS electrodes. The top substrate was overcoated with a thin polyimide layer and then mechanically buffed at +10 o with respect to the y-axis. As shown in Fig. 1(b), the bottom electrode stripes are along the yaxis. The cell filled with the LC/monomer mixture was exposed to a unpolarized UV light with a homogeneous intensity I = 10 mW/cm 2 for ~30 min at 70 °C. The UV incidence was on the side of the top substrate. As a result, the patterned electrodes on the bottom substrate did not affect the phase separation process. After phase separation and photo-polymerization, the top glass substrate was peeled off by a thermal release process, leading to the formation of a solidified and uniaxial LC/polymer composite film of 12-μm thickness, as depicted in Fig.  1(a). The uniaxial polymer topologies induced by the uniaxial phase separation are because the LC molecules and LC monomers are aligned by the rubbed PI layers before photopolymerization and the polymer grains of the LC/polymer composite film aggregates and elongates along the rubbing direction after phase separation. The mechanism is analogous to the nano-corrugated surface topologies published by Ibn-Ehlhaj et. al [17]. When we peel off the top glass substrate and leave the LC/polymer composite film on the other substrate during fabrication process, the LC molecules stay on the both surfaces, the surface on the top substrate and the surface on the LC/polymer composite film, and then leave the anisotropic topologies with valleys and polymer network structures. To see the LC/polymer composite film surface more clearly, we magnify a potion of Fig. 1(a) as shown in Fig. 1(b). At V=0, the LC directors are aligned almost along y-direction, parallel to the rubbing direction. Under applied a.c. voltage (f = 1 kHz), the LC directors are reoriented by the electric fields.

Experiment and results
The LC/polymer films were imaged with an atomic force microscope (AFM) (Dimension 3100, Digital Instruments) at tapping mode at room temperature in air. Silicon nitride cantilevers (Nanosensors) with a resonant frequency of 260 kHz were used. The cantilevers were excited just below its resonant frequency. The apical radius of the cantilevers is about 15 nm. The AFM images of liquid crystal/polymer films with different LC concentrations are shown in Figs. 2(a)-2(d). As can be seen, the surface of the LC/polymer films shows elongated aggregation of polymer grains along the rubbing direction (indicated by arrows). The root-mean-square (RMS) roughness of the films surfaces is 1.5 nm at 10 wt% LC, 1.0 nm at 20 wt% LC, 1.5 nm at 30 wt% LC, and 5.0 nm at 40 wt% LC, respectively. To verify the LC directors anchored between polymer grains can be switched by the inplane electric fields, we observed the LC/polymer films on the top of an IPS glass substrate ( Fig. 1(a)) by using a polarizing optical microscope. The polarizer (P) and analyzer (A) are orthogonal to each other and the rubbing direction of the bottom IPS substrate is parallel to the transmission axis of the polarizer. As shown in Fig. 3, at V=0 V rms the LC directors and polymer grains tend to align along the direction of the transmission axis of the polarizer. However, the LC/polymer film still has a small light leakage because the LC directors and polymer grains are not perfectly aligned along the same direction. At 200 V rms at f = 1 kHz, the transmission of the LC/polymer film increases because of the birefringence effect induced by the LC reorientation. To observe the dynamic wettability of the LC/polymer films under in-plane electric fields, we carried out contact angle measurements using a CCD camera, as shown in Figs. 4(a) and 4(b). The CCD camera was looking at the water contact angle along the y-axis as Fig. 1 shows (or P-direction in Fig. 3). A 3 μl drop of de-ionized water was deposited on the LC/polymer films. The droplet covers over 500 electrode stripes. We applied 200 V rms squared pulses (f = 1 kHz) to the LC/polymer film (with 60 wt% LC) for 600 ms.  In addition, the water contact angle on the LC/polymer film is electrically tunable. In Fig.  6, the difference between the maximal and minimal contact angles increases when we increase the amplitude of squared pulses of an in-plane electrical field with a fixed time duration at f = 1 kHz from 100 V rms to 200 V rms at 60 wt% LC. The threshold voltage is around 100 V rms . The  To prove that the contact angle change results from the reorientation of the LC directors anchored among the polymer grains, we carried out the water contact experiments on pure polymer films without LC molecules. The water contact angle was found to be 57.63 o . We also measured the water contact angle on the top of IPS glass substrate without LC/polymer film and it was found to be 54.45 o . The measured water contact angle on the pure polymer films without LC molecules does not change with squared pulses of an in-plane electric field with a fixed time duration at f = 1 kHz, as shown in Fig. 5. Without LC, the surfaces, pure polymer film and IPS glass substrate, are more hydrophilic. Such surfaces are not suitable for electrostatic actuation. However, the water contact angle on the LC/polymer film with 60 wt% LC changes periodically with a periodically applied voltage even though the macroscopic surface is more hydrophilic. So we conclude that the orientation change of the LCs anchored between polymer grains in responding to the electric fields leads to the observed dynamic change of surface wettability. The electrostatic force can be neglected for two reasons: 1) the LC/polymer film is not operated in high-frequency-limit region (>10 kHz) in which dielectrophoretic (DEP) effect has influence on water droplet [18], and 2) the LC/polymer film is thick (about 12 μm), therefore, the electric field near the water-film surface is relatively weak to stretch the droplet using DEP, but it is strong enough to change LC orientations.
One application of LC/polymer film is the dynamic liquid lens, as Fig. 7 shows. A water droplet of ~6 μl was put on the top of the clear LC/polymer film and electrodes. The distance between the image and the substrate is 2 mm. The focal length changes periodically with the applied pulsed voltages. The measured focal length of the liquid lens is 4.2 mm at 0 V rms and 5.3 mm at 200 V rms . In Fig. 7, the contact angle change is along the direction of the in-plane electric fields. In addition, the images inside the water droplet area have different magnification with applied electric field. This is because the contact angle change results in a change on the water droplet curvature which, in turn, leads to a focal length change.

Discussion
It is known that several factors can affect surface wettability, such as chemical properties of the materials, roughness, and chemical heterogeneity [19][20][21]. The effect of the roughness can be described by the Wenzel's relationship [22]: where R w is the surface roughness factor, θ 2 is the contact angle in the smooth surface and θ 1 is the contact angle in a rough surface. Equation (1) describes the amplification of surface properties due to the surface roughness. The contact angle θ 2 satisfies Young's equation [23]: where γ represents the surface tension (i.e., energy per unit surface) of the interface, and S, L, and V indicate the phases of the solid, liquid and vapor. When a surface has the chemical heterogeneity and surface porosity, the contact angle (θ ) of modified Cassie's equation can be expressed as [24]: where f 3 and f 4 are the fractions of the surface having inherent contact angles θ 3 and θ 4 . Here, f 3 and f 4 should satisfy: f 3 + f 4 =1. However, the water contact angle on the LC/polymer composite film depends not only on the morphologies (both polymers and liquid crystals) but also on the orientation of LC directors which can be controlled by the applied electric fields. Thus, the contact angle (θ' ) as a function of applied voltage (V rms ) in the LC/polymer composite film should be: where f lc and f p are the fractions of LC and polymer grains for the inherent contact angles θ lc and θ P , respectively. From Eq. (5), when the voltage is higher than the threshold voltage, . Therefore, the surface wettability of the LC/polymer composite film is electrically switchable.

Conclusion
We have demonstrated electrically tunable wettability of an LC/polymer composite film, in which LC molecules are anchored among polymer grains. The LC reorientation among polymer grains at the LC/polymer composite film surfaces, which are switched by the electric field, is responsible for the observed tunable wettability of the LC/polymer composite film. The major advantages of LC/polymer composite film are threefold: 1) small Joule's heating which is a common issue for DEP or electrowetting in water-based devices, 2) small hysterisis, and 3) low power consumption. The mechanism of DEP is to apply an inhomogeneous electric field to a dielectric liquid to generate a pulling force which changes the contact angle of the droplet. On the contrary, the wettability change of a LC/polymer composite film is because the molecular orientation anchored the polymer grains of the surface. The electrically switchable wettability of LC/polymer composite films has potential applications in liquid lens, windshield, polarizer-free displays, ink-jet printing, microfluidic devices, and lab-on-a-chip.