1. Introduction

Nematic liquid crystals (NLCs) are anisotropic fluids, where rod- or disc-shaped molecules align along a particular direction, the director. The director field can be controlled by properly choosing bounding surfaces with particular anchoring conditions. However, various kinds of topological defects or singularities emerge in NLCs depending on their interfaces and their anchoring conditions [1, 2]. NLC in a capillary tube with homeotropic bounding conditions [3], NLC droplets in a polymer matrix [4], and colloidal microparticles in NLC [5] are some of these examples in their early studies. Particularly, colloidal microparticles located in a uniform director field of calamitic NLCs provide many characteristic topological defects and are one of the topical issues in the field of LC defect structures. This research has been intensively discussed in literature [6–11]. One of the important aspects in the colloidal microparticle studies is the interaction mediated by these defects. Microparticles interact with each other through distortions of the director field accompanied by topological defects. They self-assemble to form one- or two-dimensional lattices depending on the types of topological defect structures [8, 12–20].

Another important issue is the manipulation of microparticles or the director field around them by external fields. The optical trapping technique using laser tweezers is often employed for manipulating microparticles themselves [14, 15, 21, 22]. Reorientation of the director field can be achieved by external stimuli such as an electric field [17, 23], an optical field [24], a thermal field [25]. Such director reorientations sometimes lead to the transport of microparticles [10, 26] and even possible particle selection [27]. Defect structures are determined by the anchoring condition of interfaces, normal or tangential [5, 12]. This suggests that the reorientation of the director fields and the associated defect structure changes are possible by manipulating surface anchoring conditions. In most of the works on colloidal systems in NLCs, surface anchoring conditions were controlled by choosing the material of particles or by pretreatment before dissolving microparticles in LC [14, 15]. As far as we know, the work by Yamamoto et al. [28, 29] is the only one, in which a surface active layer was fabricated on colloidal surfaces. Actually, they succeeded in manipulating the defect structures around glycerol droplets [28], and water droplets [29] in NLC doped with amphiphilic azobenzene derivatives. The manipulation was based on trans-cis photoisomerization by ultraviolet (UV) light irradiation. Senyuk et al. [30] found that the diffusion properties of gold nanorods with azo molecules attached to their surfaces changed upon UV light irradiation. Although they suggested that the cis-trans isomerization altered the surface anchoring condition, leading to the director field change, they could not observe the director field and defect around the nanorods because of the small size of the rods. Here we report successful optical manipulation of defect structures around microparticles in an NLC. The surface of the microparticles is covered with a monolayer of azo-dendrimer molecules and it is active under UV light irradiation. We found that the response time of the defect structure depends on the light intensity and attains about 50 ms.

Yonetake et al. [31] synthesized the poly(propylene imine) liquid crystalline dendrimer. It was reported recently that the dendrimer molecules dissolved in NLCs were spontaneously adsorbed onto glass surfaces, resulting in a uniform homeotropic orientation and possible application for in-plane switching mode LC displays [32]. Recently, azo groups were introduced in the dendrimer (hereafter called azo-dendrimer) [33], providing active surfaces based on cis-trans isomerization upon UV light irradiation. At least three devices using this azo-dendrimer have been reported; (1) photo-induced dewetting and an associated morphology change [33], (2) a bistable device using an anchoring transition with azo-dendrimer acting as a command surface [34], and (3) reversible molecular reorientations in N, cholesteric, and smectic A droplets in glycerol by UV light irradiation [35]. Namely, the azo-dendrimers were dissolved in LC materials and mixed with glycerol, leading to the adsorption of the dendrimers at the droplet surfaces. This work motivated us to perform the present study. We may expect that the dendrimers dissolved in LCs are spontaneously adsorbed at the surfaces of microparticles embedded in the LCs and works as active surfaces. Here we report the static and dynamic behaviors of the changes of the director field and the associated defects based on photo-induced surface ordering transition.

2. Experimental

We introduced 4’-n-pentyl-4-cyanobiphenyl (5CB) mixed with 0.1 wt% of poly(propylene imine)-based-azo-dendrimers (Fig. 1(a)) and small amount (volume fraction of about 2x10⁻⁴) of silica microparticles (about 3 μm in diameter) into empty cells with homogeneously treated surfaces (EHC, Tokyo). The cell gap was 25 μm. Polarizers were crossed so that one of the polarizers was along the rubbing direction (director orientation). In some experiments, we used a wave-plate compensator inserted diagonal to the polarizers to determine the director orientation around the beads. We found that, in cells with LC/dendrimer mixture, the director was unidirectionally aligned along the rubbing direction with slightly perturbed birefringence compared to the cells with the pure LC. This is because the dendrimers are also adsorbed at the substrate surfaces, although the density is much lower there than that at the micro-sphere surfaces [34]. These dendrimers slightly disturb the planar orientation, although still the effect of rubbed polyimide surfaces governs the overall homogeneous planar orientation. By UV light irradiation, cis conformers bending along the rubbing direction promote planar orientation, resulting in a somewhat higher birefringence without disturbing the planar director orientation, as already reported [34]. However, perfect homeotropic orientation was observed in the area without an alignment layer (bare glass). Irradiation by UV light induced a reversible homeotropic-to-planar transition. This area was outside the range used for the following experiments. In contrast, azo-dendrimers were efficiently adsorbed at microparticle surfaces, as will be shown in the following. The full extended molecular conformation is estimated to be 7.8 nm [33]. Since we confirmed that the central core of the dendrimer is attached at silica surfaces, as shown in Fig. 1(b), by optical second-harmonic generation [34], the thickness of the dendrimer layer is about 4 nm. Under such conditions, azo-dendrimers work as a command surface [36], i.e., the azo-dendrimer surface commands the 5CB molecules to reorient, resulting in a director reorientation, upon trans-cis isomerization induced by UV-light irradiation, as shown in Fig. 1(b).

 

Fig. 1 (a) Azo-dendrimer molecule used. (b) Photoisomerization and associated NLC molecular orientations; Normal to the surface without UV light irradiation (left) and tangential to the surface during UV light irradiation (right).

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3. Results and discussions

It is well known that several kinds of topological defects emerge around a microparticle situated in an oriented NLC field [12]. In most of the reported cases hyperbolic hedgehog defects (Fig. 2(a)) and Saturn rings (Fig. 2(c)) were observed due to the director anchoring normal to the boundary (normal condition). In our system, we originally observed a hedgehog structure, as shown in Fig. 2(d). Upon UV irradiation, the optical texture changes to a dark cross (Fig. 2(e)) due to the structure shown in Fig. 2(b). This structure is often referred to as ‘boojums’ in the literature. This is caused by the change of the anchoring conditions at the particle surface by UV light irradiation: normal to tangential. The defects are always aligned with the particles on an axis parallel to the director alignment. After terminating the irradiation, a Saturn ring perpendicular to the rubbing direction (Fig. 2(f)) was observed when the anchoring changed from tangential to normal again. Then the successive light on and off gave repetitive changes between boojums and Saturn rings quite reproducibly. A possible explanation is that the Saturn ring is a stable configuration, while the hedgehog occurs as a metastable structure during preparation of the cell, in the flow field. Fukuda et al. [37] theoretically showed that the Saturn ring is energetically more stable compared with the hedgehog when the ratio of the nematic coherence length to the particle radius is large. We actually found that the hedgehog defects became more stable when we used larger particles, such as 10 μm in diameter. Detailed size-dependence experiments are necessary, and are now being carried out. To confirm the model structures, optical microscope observation was made by inserting a λ plate at 45 degrees with respect to the crossed polarizers. The results are shown in Figs. 2(g)–2(i). The birefringence colors (bluish and yellowish) in domains parallel and perpendicular to the axis of the λ plate are clearly different in (h) and (i). Note that the positional relationships of both colors in (h) and (i) are exchanged, suggesting that the director orientations near the particle surface in (h) and (i) are more or less perpendicular to each other. This observation is consistent with the model structures shown in (b) and (c). We also note that the birefringence colors are consistent with the models.

 

Fig. 2 Changes by UV light irradiation. Model structures of the director field and associated defect structures of (a) hedgehog, (b) boojum, and (c) Saturn ring. Photomicrographs of the defects under crossed polarizers, (d) before, (e) during, and (f) after UV light irradiation. Photomicrographs of the defect observed with a λ waveplate inserted diagonal to the crossed polarizers (slow axis from bottom left to top right). Three different textures with characteristic birefringence colors are observed in (g)-(i) corresponding to the model structures (a)-(c).

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It is also noted that the orientation change of the director field depends on the UV light intensity and is quite fast at the high intensity (140 μW/mm² max). Figure 3 shows three series of successive image frames in our video recording of the processes (a) from hedgehog to boojum upon UV irradiation, (b) from boojum to Saturn ring by UV termination, and (c) from Saturn ring to boojum upon second UV irradiation. It is clearly seen that the orientation changes are completed within at least two frames. To investigate the response time more quantitatively, we monitored the transmitted light intensity at a diagonal position showing a yellow color in the texture through a yellow color filter in the process from Saturn ring to boojum. The transmittance change is more or less exponential as shown in the inset of Fig. 4(a). The switching rate (inverse switching time) was determined from the exponential fit of the transmittance shown in Fig. 4(a). The switching rate increases, or the switching time becomes faster, with increasing UV light intensity, reaching about 50 ms for the highest illumination intensities applied. This is comparable with the electro-optic switching speed in NLC devices. The pronounced dependence of the switching speed upon UV irradiation intensity evidences that the limiting speed factor in the texture transformation is not the nematic director dynamics but the dynamics of the command layer. Namely, the UV-intensity-dependent response time is attributed to a shift of the equilibrium between the transitions cis-trans and trans-cis, i.e., different abundance ratios of trans and cis conformers depending on the UV light intensity. The speed of the cis-trans isomerization is practically instantaneous compared to the director response. The UV-intensity-dependent cis/trans ratio may lead to different order parameters particularly near the dendrimer layer [38–40], anchoring conditions as well as anchoring energies. This means that the surface condition is not only planar and homeotropic, but intermediate conditions may exist. Actually, Knobloch et al. [41] mentioned that, in low light intensity, switching is hindered due to geometrical constraints, and hence the LC molecules could be in an intermediate surface orientation. Furthermore, light intensity dependent response times were reported by Shishido et al. [42]. More detailed measurements with higher temporal resolution and theoretical consideration are necessary. We believe that the clear quadratic dependence shown in Fig. 4(b) provides a good hint for further discussion.

 

Fig. 3 Series of video frames in the processes (a) from hedgehog to boojum by UV light irradiation, (b) from boojum to Saturn ring by UV light termination, and (c) from Saturn ring to boojum by second UV light irradiation.

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Fig. 4 (a) Switching rate (inverse switching time) as a function of UV light intensity. The birefringence change is nearly exponential as shown in the inset, from which the switching time was estimated. (b) Squared switching rate as a function of UV light intensity.

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4. Conclusions

We demonstrate controlled changes in the director field and the topological structure of the associated defects, i.e., hedgehog, boojum, and Saturn ring transformations, around silica microparticles by UV light irradiation. The response time depends on the UV intensity and attains to be about 50 ms in our experimental condition, which is comparable with the electro-optic switching speed in NLC devices. The UV-intensity-dependent switching speed is attributed to some factors originated from different trans/cis ratio. It is noted that the sample preparation is very easy; there is no need of pretreatment of microparticles but it suffices to dissolve only a small amount of azo-dendrimer molecules in the NLC. Because of the simplicity of sample preparation, this technique can be applied to many other systems such as microrods [30], LC droplets [43–45], 1D and 2D colloidal systems [15], and controlled interaction of microparticles [46]. We have already confirmed the dynamic motion of microrods by UV light irradiation which will be reported in a separate paper.