Takuya Ohzono (Leader) of Soft Mechanics Group and Jun-ichi Fukuda (Senior Researcher) of Soft Matter Modeling Group (Leader: Makoto Yoneya), the Nanosystem Research Institute (Director: Kiyoshi Yase) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Tamotsu Nomakuchi), have discovered that, when a liquid crystal is confined within microwrinkles, a novel periodic orientational structure of the liquid crystal is spontaneously formed.

When a liquid crystal is confined within microwrinkle grooves, a liquid crystal structure is formed that is periodic along the direction of the wrinkle grooves. It has been shown theoretically that both the effect of confinement and the anisotropic elasticity of the liquid crystal are important in the formation of this structure. This orientational structure of the liquid crystal contains a zigzag line defect and can trap single silica micro-particles at the kink points where the line defect changes direction. This structure, based on the self-organization of liquid crystals, can be applied to optical devices and to easy patterning and capturing of micro-objects such as colloidal particles.

Details of the results of this study will be published online in Nature Communications on February 29, 2012 (JST).

Figure 1 : Periodic orientational structure formed by the self-organization of a liquid crystal confined within microwrinkles

Understanding and developing processes of microstructure formation are important for functional materials and device technologies. In recent years, there has been a strong need to reduce the costs and the environmental loads of microstructure manufacturing. However, as the structures required become more complex and miniaturized, concern is growing about the limits of the microfabrication processes such as those used in semiconductor device technologies. In contrast, in nature, particularly in biological systems, spatial structures suitable for achieving a variety of functions are spontaneously formed and maintained. In recent years, there has been a strong need for research and development to apply the self-organization processes commonly observed in biological systems and the structures formed by these processes to the fabrication of microstructures.

A liquid crystal is a typical example of a soft material that forms self-organized structures and its optical properties are effectively used particularly in liquid crystal displays to contribute to today’s information society. However, there have been only few studies concerning microstructure formation processes of liquid crystals by self-organization.

AIST has been conducting research and development of functional materials using microwrinkles. Specifically, it has been developing and applying techniques for fine-patterning and manipulation of liquid shapes at a micrometer scale by using the grooves of the microwrinkles. Because the microwrinkles can be readily produced at low cost and allow easy control of their shape, their applications in various fields are expected.

In this study, the researchers investigated the orientational structure of a liquid crystal confined within a micro-space by using the micro-shaping technique for liquid that they have researched and developed.

This research and development project was supported by a Grant for Industrial Technology Research (FY2008–2012) from the New Energy and Industrial Technology Development Organization and by Grants-in-Aid for Young Scientists (B) from the Japan Society for the Promotion of Science.

A nematic liquid crystal is confined within microwrinkle grooves and spontaneously formed orientational structures in the liquid crystal are investigated. First, a hard thin film of polyimide is formed on the surface of silicone rubber and unidirectionally compressed. Microwrinkles with grooves unidirectionally oriented and spaced at several to several tens of micrometers (the spacing can be controlled by changing the thickness of the polyimide film) form spontaneously over the surface of the thin film. The microwrinkles are then brush-coated with a liquid crystal (Fig. 2, top center). When the contact angle of the liquid crystal on the polyimide surface is reduced to a certain value, the liquid crystal is confined within the grooves. It was already found that the liquid crystal molecules orient tangential to the surface and perpendicular to the groove direction on the microwrinkle surface (i.e. the polyimide surface). At the liquid crystal — air interface, the orientation of the molecules is perpendicular to the interface. Because of the orientations of the liquid crystal at the interfaces and the curved structure of the microwrinkle groove, the confined liquid crystal cannot have a uniform and stable orientational structure (Fig. 2, bottom) and is presumed to be substantially distorted inside.

Figure 2 : Experimental procedure for confining a liquid crystal within microwrinkle grooves (top) and orientations of the liquid crystal molecules at the interfaces (bottom)

Figures 3a and 3b are polarized microscope images of the liquid crystal confined within a microwrinkle groove. The liquid crystal confined within the wrinkle groove shows a periodic pattern despite the absence of periodicity in the microwrinkles along the groove direction, indicating the spontaneous formation of the periodic structure. A polarized microscope image obtained with a sensitive color plate shows that the liquid crystal has a periodic structure, in which the liquid crystal orientation deviates alternately from the direction perpendicular to the groove (Fig. 3c). It is also found that a zigzag line defect along the groove is formed, along with the periodic structure. The periodic structure is a general one independent of the size of the groove. The structure is formed in many grooves of microwrinkles simultaneously, making it easy to form a large-area periodic structure.

Inside the periodic structure, the liquid crystal molecules at the bottom surface of the grooves are oriented tangential to the surface and perpendicular to the groove direction. From the bottom surface towards the air — liquid crystal interface at the top, the orientation of the molecules gradually changes towards a direction perpendicular to the interface and gradually twists away from a direction perpendicular to the grooves (Fig. 3d). The directions of the twists are found in Fig. 3b. The sense of the twist determines the local orientation of the liquid crystal. Theoretical considerations with the aid of numerical calculations reveal that the twist elastic constant smaller than other elastic constants (which holds for many nematic liquid crystals) is responsible for the periodic structure accompanied by twist deformations. In other words, the smaller twist elastic constant reduces the energy required for deformation required for the above-mentioned distortions in the liquid crystal. The twist deformations drive the line defect away from the groove center. Because the groove width is finite, the line defect would reach the wall of the groove unless the line zigzags periodically. As a result, the clockwise and counterclockwise twists shown in Fig. 3 appear alternately by self-organization.

Figure 3 : Transmission polarized microscope images and illustration of the periodic orientational structure of the liquid crystal

Figures 3a and 3b are images of the same region observed with different sets of polarizing plates. P and A show the orientations of the polarizer and the analyzer, respectively. S shows the orientation of the sensitive color plate. The orientation of the liquid crystal can be determined from the color. Figure 3c shows the orientation distribution in different planes. In the illustration in the xy plane at the bottom, the deviations of the orientation away from the direction perpendicular to the groove are shown in an exaggerated manner. Figure 3d is a three-dimensional illustration. The nail symbols (T) indicate the projection of the molecules inside the groove on the plane on which the symbol is located and the head of the nail comes out of the plane.

Next, silica micro-particles (about 500 nm in diameter) are deposited into the nematic liquid crystal confined within microwrinkles. As in the case without the particles, a periodic orientational structure is formed. However, it is found that isolated silica particles are trapped at the kinks of the zigzag line defect (Fig. 4). Liquid crystal distortions are concentrated at the kinks of the line defect. The replacement of the regions with large distortions by silica particles reduces the total energy of the system, thus stabilizing it. This is the reason why particles are trapped. The periodicity of the kinks results in the periodic arrangement of particles. In other words, the periodic orientational structure of the liquid crystal serves as a template for the periodic arrangement of particles.

When the nematic liquid crystal was transformed to an isotropic liquid by raising the temperature, the periodic orientational structure disappears and, at the same time, the trapped particles are released and move freely (Fig. 4c). This indicates that the orientational structure of the liquid crystal indeed traps the particles.

Figure 4 : Silica micro-particles trapped in the zigzag line defect in the periodic orientational structure of the liquid crystal

Figure 4a is a three-dimensional illustration of how the silica particles are trapped in the line defect. Figure 4b is an optical microscope image of the particles and the liquid crystal. Figure 4c shows the particles released from the trapped state when the liquid crystal is transformed to an isotropic phase and the line defect disappears.

The researchers aim to control the self-organized periodic orientational structure of the liquid crystal by changing the groove geometry and by an external field, including electric and magnetic fields. They also aim at its applications to optical devices by precisely controlling the arrangement of the trapped particles. In addition, the researchers will investigate the possibility of new self-organized orientational structures of other types of liquid crystals in microwrinkles, and will conduct research towards the formation of more complex ordered structures.