Droplet manipulation on a liquid crystal and polymer composite film

A droplet manipulation on a switchable surface using a liquid crystal and polymer composite film (LCPCF) based on phase separation is developed recently. The wettability of LCPCF is electrically tunable because of the orientation of liquid crystal directors anchored among the polymer grains. A droplet on LCPCF can be manipulated owning to the wettability gradient induced by spatially orientation of LC directors. We discuss the droplet manipulation on LCPCF and demonstrate several applications of LCPCF, such as polarizer-free displays, and human semen sensing.


INTRODUCTION
Electrical droplet manipulation on a surface is important in biosensors and photonics devices. Conventionally, the switchable surfaces utilize changes in molecular conformation of a self-assembled monolayer under external stimuli [1,2]. However, the droplet manipulation is confined because the weak chemical gradient cannot overcome the hysteresis of the surface. Recently, we have developed a switchable surface, liquid crystal and polymer composite film (LCPCF). After the photo-induced phase separation between the LC and polymer, the LC molecules on the surface of LCPCF will be anchored among the polymer grains. By applied external electric field, the LC molecules will be reoriented and the surface free energy on LCPCF will be changed because of different surface free energy between the phenyl rings and the terminal group (cyano) of LC molecules. According to the modified Cassie's model and the measurement based on the Chibowski's film pressure model, the surface free energy of LCPCF is electrically switchable from 36 × 10 −3 J/m 2 to 51 × 10 −3 J/m 2 [3]. In this paper, we will introduce the operating principle of LCPCF first. Then, by applied gradient electric field on LCPCF to manipulate droplet between less hydrophilic region and hydrophilic region, we will demonstrate different applications based on droplet manipulation on LCPCF such as bistable polarizer-free displays and sperm testers [4,5].

OPERATING PRINCIPLES AND EXPERIMENTAL RESULTS
The structure consists of a LCPCF on a patterned indium tin oxide (ITO) glass substrate as shown in Fig. 1(a). The ITO electrodes on the glass substrate provide fringing electric field to LCPCF were etched with interdigitated chevron patterns, shown as the zigzag electrodes. The zigzag ITO strips have corner angles of 150 • . The width and the gap of the electrode strips are 4 and 14 µm, respectively. The surface of the LCPCF consists of polymer grains and liquid crystals anchored among the polymer grains. When the voltage is off, the liquid crystals are aligned along x-direction. When the voltage is on, the surface of LCPCF is more hydrophilic due to the reorientation of liquid crystal molecules by the fringing electric field. The mixtures of LCPCF consist of a nematic mixture E7 (Merck) and a liquid crystalline monomer (4-(3-acryloyloxypropyloxy)-benzoic acid 2-methyl-1, 4-phenylene ester) at 70 : 30 wt.% ratios.
The surface free energy of the LCPCF in the air (γ LCPCF-air ) with an unit of J/m 2 can be expressed as Eq. (1) according to the Chibowski's film pressure model [6].
where γ L-air is the surface free energy of the testing fluid in the air, ϕ is the average tilt angle of LC molecules with respect to x-axis in Fig. 1, and cosθ a and cos θ r are advancing angle and receding angle of the testing fluid respectively. The measuring results of contact angles for deionized (DI) water on LCPCF are shown in Fig. 1(b). The surface free energy of DI water is around 72.8 × 10 −3 J/m 2 . As a result, the surface free energy of the LCPCF can be calculated from Fig. 1(b) and it increases from 36 × 10 −3 J/m 2 to 51 × 10 −3 J/m 2 with the applied pulse voltages. According to Cassie's model, the surface free energy in the air depends on the roughness factor (R W ), surface free energies of polymer (γ p-air ) and LC (γ LC-air ), and the fraction of polymer (f p ) and LC (f LC ): From SEM image and AFM image, the LC domain can be estimated as ∼ 225 nm, the rootmeansquare (RMS) roughness is ∼ 15 nm, R w is ∼ 1.214, and f LC and f p are 0.32 and 0.68, respectively. γ p-air is around 30.9 J/m 2 obtained by measuring the advancing and receding angles on pure polymeric film. As a result, the surface free energy of the LC molecules on LCPCF can be calculated from 29 × 10 −3 J/m 2 to 66 × 10 −3 J/m 2 with the applied pulsed voltage.

Bistable Polarizer-free Displays
According to the measurement results of the surface free energy, we can apply gradient electric field to manipulate the droplet motion on LCPCF and realize the bistable polarizer-free displays. The structure of the displays consists of black matrix with aperture, colored droplet, LCPCF, regional patterned ITO glass substrate and reflective diffuser, as shown in Fig. 2(a). When we apply a voltage in the left interdigitated region, the left region of LCPCF is more hydrophilic, so the droplet experiences a net Young's force to move toward the left and aperture was then filled with color gradually, as depicted in Fig. 2(b). When we turn off the voltage, the droplet will stop moving and stay in the left region, as depicted in Fig. 2(c). By the same operating method, the droplet can be manipulated from left region to right region reversibly. To realize the bistable polarizer-free displays using droplet manipulation on LCPCF, we use red dye-doped ethylene glycol as the colored droplet and place a black matrix with an aperture with a diameter of 300 µm on the top of the droplet. The time-dependent percentage of the white area in Fig. 2(b) is shown in Fig. 2(d). The response time was around 400 ms from white to red and around 600 ms from red to white. We also measured the spectrum by a spectrometer (Ocean Optics USB 2000). The contrast ratio at wavelength of 630 nm is ∼ 2.8 : 1 in the transmissive mode and ∼ 8 : 1 in the reflective mode.

Sperm Tester
The sperm testing device can be realized based on the semen droplet manipulation on LCPCF. The patterned ITO glass substrates of sperm testers are the same as we used in bistable polarizer-free displays. A semen drop of 3 µL was laid on the LCPCF and then applied 150 Vrms square pulses (f = 1 kHz) to the left electrodes for a time duration of 500 ms. Among all samples, two motions of semen droplets were observed: back-and-forth stretch and collapse. Because of the high viscosity of semen drop, the net forces of the semen drop are inadequate to move the semen droplets forward even though the surface of LCPCF has surface free energy gradient. The mechanism of the stretch of the semen drop is illustrated in Figs. 3(a), 3(b), and 3(c). When we apply the electric field at the left electrode, a fluidic flow (or the flow of seminal plasma) inside the semen drop was then induced. All the sperms are then flushed by such a fluidic flow. However, the fertile sperms swim upstream against the fluidic flow because of the nature of the fertile sperms [7], as shown in Fig. 3(b). When we turn off the voltage, the wettability of LCPCF goes back and the fertile and infertile sperms dispersed inside the semen drop, as shown in Fig. 3(c). Therefore, we observe the back-and-forth stretch of a semen drop when the voltage is on and off periodically. The mechanism of the collapse of the semen drop is also illustrated in Figs. 3(d), 3(e), 3(f), and 3(g). When the semen drop has lots of infertile sperms, the infertile sperms are washed away by the fluidic flow and the infertile sperms are attracted and trapped on the surface of LCPCF. The surface of LCPCF is then be modified, as shown in Figs. 3(e) and 3(f). Therefore, the collapse of semen drop is observed, as shown in Fig. 3(g).  According to World Health Organization guideline, a standard sperm analysis is performed for measuring the parameters of spermatozoa [8,9] which includes concentration, motility, grade A, grade B and sperm morphology. By the experimental results, the semen drop stretches only when the semen concentration is larger than ∼ 100 million/mL, morphology is larger than ∼ 15%, motility is larger than ∼ 50%, grade A is larger than ∼ 30%, and grade B is less than ∼ 30%. The stretch of a semen drop is a result of better quality semen for human reproductions while the collapse of a semen drop shows the contrary.

CONCLUSIONS
In summary, the electrical droplet manipulation can be realized by an electrically and reversibly switchable surface of liquid crystal and polymer composite film (LCPCF). The surface free energy and wettability can be controlled by external electric field due to the orientation of liquid crystal molecules which were anchored among the polymer grains. By applying voltages on regional electrodes, the gradient wettability of LCPCF can manipulate the droplet on LCPCF. We demonstrated the applications such as bistable polarizer-free displays and sperm testers. Other applications of LCPCF are blood testers, concentrators and sun trackers for a concentrating photovoltaic system, and image stabilization systems.