Micromanipulation using the backflow effect in liquid crystals

The so-called backflow effect in liquid crystals (LCs) is the coupling between flow and directional change of LC molecules. In the past—for optical devices such as LC displays (LCDs)—this effect had the disadvantage of reducing contrast, which limited its application. But the advent of microfluidics has reactivated interest in LC flow. Indeed, LCs are proving to be an excellent matrix for microfluidic applications owing to our ability to control LC molecules by applying voltages to them.

Figure 1 illustrates the principle of LC flow. LCs are an anisotropic fluid composed of cylindrical organic molecules. Within a cell, LC molecules can be freely aligned by treating the substrate surface in a variety of ways. For example, coating the top and bottom electrodes with polymer layers and rubbing in parallel directions results in a state called the nematic π-cell. Applying voltage aligns the long axis of the LC molecules in the direction of the electric field, as shown in Figure 1(a). Switching the electric field off results in the relaxed state shown in Figure 1(b) and induces the flow represented by U_off. Conversely, switching the voltage on reverses the state and generates a counterdirectional flow (U_on).

Using the backflow effect for micromanipulation required solving two problems: generating net flow without cancelling U_on and U_off, and suppressing electrophoretic flow caused by ionic impurities.¹ The problem of net flow was solved by changing the duty ratio, i.e., the duration of the on state, per cycle, of the applied rectangular wave in the case of nematic LCs,² and by applying a sawtooth wave for ferroelectric LCs.³ The issue of electrophoretic flow was addressed by applying a high-frequency wave above 100Hz to prevent impurities from following the electric field.^2,3Figure 2 shows the movement of a thin polyethylene film in a nematic π-cell using these techniques.

In addition, we have succeeded in developing a two-dimensional (2D) micromanipulator (see Figure 3) using the twisted nematic (TN) cell, which is often used in LCDs. The principle of the 2D micromanipulator is illustrated in Figure 4. Alternating the states of Figure 4(a) and 4(b) generates the twisted flows U_on and U_off, respectively: shown in Figures 4(c) and 4(d). Decreasing the duty ratio of the applied rectangular wave keeps U_on and U_off from cancelling, and causes U_off to predominate. Microparticles (spherical polystyrene beads) can then be introduced into this TN cell. Because the microparticles are naturally charged, they move in the z direction in response to electrostatic force. This movement is reversed by the polarity inversion of the applied voltage, as shown in Figure 4(b). Accordingly, changing the polarity of the applied voltage enables 2D (xy) manipulation. For example, the microparticles in Figure 3 were negatively charged, so positive polarity drove the particles in the +x direction. Conversely, negative polarity drove the particles in the +y direction.

Our techniques exploiting LC flow are suitable for precisely manipulating hard-to-handle micro-objects such as microlenses and micromirrors. We expect that, in future, these methods will also be applied to biochips and μ-TAS (micrototal analysis systems).

This research was partially supported by the Grants-in-Aid for Scientific Research for Young Scientists (B) (17760124) and Priority Area ‘System Cell Engineering by Multi-scale Manipulation’ (18048042) from the Ministry of Education, Science, Sports, and Culture of Japan.