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Contribution of Water Droplets to Defect Array Formation in Water-in-Cholesteric Liquid Crystal Emulsions Makoto Yada,*,† Jun Yamamoto,† and Hiroshi Yokoyama†,‡ Yokoyama Nano-structured Liquid Crystal Project, ERATO, Japan Science and Technology Corporation, 5-9-9 Tokodai, Tsukuba, Ibaraki 300-2635, Japan, and Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received May 1, 2003. In Final Form: August 5, 2003 The quadrilateral defect array is spontaneously formed after termination of the shear to the cholesteric emulsions, which are composed of water, surfactants, and cholesteric liquid crystals. We have here investigated the motion of water droplets in cholesteric media during the formation process of the defect array by using the laser scanning confocal fluorescence microscopy. A small amount of water-soluble fluorescent probe, Safranin O, was mixed in the cholesteric emulsions to detect the position of water droplets. After stopping the shear, the fluorescence intensity near glass surfaces increases rapidly, in contrast to the reduction in the midway between glass plates, which indicates that water droplets become preferably settled on the glass surface, not in the liquid-crystal medium. The spatial distribution of the droplets on the glass surface tends to be two-dimensionally uniform, independent of the period of the defect array and the cholesteric pitch. The condensation of water droplets onto the glass surface occurs only within 10 s after the shear, much more quickly than the formation of the defect array. On the basis of these experimental results, we discussed the effect of water droplets on the formation of the defect array, particularly focusing on the growth of the nuclei of point defects, which are initially produced on the glass plates by the homeotropic anchoring of the substrate surface.
Yokoyama Nano-structured Liquid Crystal Project. National Institute of Advanced Industrial Science and Technology. * To whom correspondence should be addressed. E-mail: yada@ nanolc.jst.go.jp.
microscopic length scale, but the longer range anisotropy is completely disrupted as a result of the droplet-induced distortions. Liquid-crystal dispersions can be expected to show further novel liquid-crystal structures and unique phenomena,8 owing to the interparticle and/or particleliquid crystal interactions. We have been studying a different type of liquid-crystal emulsion, consisting of water, surfactants, and cholesteric liquid crystals, so-called cholesteric emulsions.9-11 A cholesteric phase has a spontaneous long-range helical structure, although its local molecular ordering is identical to that of a nematic phase. Owing to the spontaneous helix, cholesteric liquid crystals show more complicated textures than nematics, such as fingerprints, focal conics, and polygonal textures.12,13 In cholesteric emulsions, we previously observed the spontaneous transformation of cholesteric textures from the shear-induced planar alignment to the regular quadrilateral defect array.9,10 The nucleation growth of point defects and their rearrangement to the defect array were examined by means of 2-dimensional fast Fourier transform (2D-FFT) image analysis.9 Moreover, we found the increase in the shear yield stress with the growth of point defects and discussed the generation mechanism of the elastic response in the defect array.10 In these previous researches, however, we did not obtain direct information about water droplets, since the droplets fragmentized by the shear could not be observed with a polarizing microscope for their small size below detectable limits, and there was only a little
(1) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (2) Lev, B. I.; Tomchuk, P. M. Phys. Rev. E 1999, 59, 591. (3) Poulin, P.; Weitz, D. A. Phys. Rev. E 1998, 57, 626. (4) Lubensky, T. C.; Petty, D.; Currier, N.; Stark, H. Phys. Rev. B 1998, 57, 610. (5) Monval, O. M.; Dedieu, J. C.; Krzywicki, T. G.; Poulin, P. Eur. Phys. J. B 1999, 12, 167. (6) Fukuda, J.; Yoneya, M.; Yokoyama, H. Phys. Rev. E 2002, 65, 041709. (7) Yamamoto, J.; Tanaka, H. Nature (London) 2001, 409, 321.
(8) Yamamoto, T.; Yamamoto, J.; Lev, B. I.; Yokoyama, H. Appl. Phys. Lett. 2002, 81, 2187. (9) Yada, M.; Yamamoto, J.; Yokoyama, H. Langmuir 2002, 18, 7436. (10) Yada, M.; Yamamoto, J.; Yokoyama, H. Langmuir 2003, 19, 3650. (11) Fukuda, J.; Lev, B. I.; Yokoyama, H. Phys. Rev. E 2002, 65, 031710. (12) Demus, D.; Richter, L. Textures of Liquid Crystals; Verlag Chemie: Weinheim, Germany, 1978. (13) Bouligand, Y. J. Phys. (Paris) 1972, 33, 715.
Introduction The system of colloidal particles dispersed in a liquidcrystal host1 has attracted significant interest in recent years, since new and unique physical behaviors are introduced by the combination of particles and anisotropic fluids. A single particle has an ability to distort the orientation of the adjacent liquid crystal and is expected, when the surface anchoring is sufficiently strong, to behave topologically as a core of an orientational defect yet with a much lower energy of formation because of its nonsingularity. The induced elastic distortion extends over a long distance in liquid crystals and creates novel interparticle interactions.2 Various types of topological defects, such as a hyperbolic hedgehog, boojums, and a Saturn ring, have been recently reported in dispersions of water droplets in nematic liquid crystals, so-called nematic emulsions.1-6 Particularly, water droplets with hyperbolic hedgehog defects form linear chains without direct contact with each other, owing to the balance of long-range dipolar attractive and short-range defect-mediated repulsive interactions, reported by Poulin et al.1 On the other hand, the “transparent nematic phase” was found in nematic microemulsions,7 where the nematic order exists in the † ‡
10.1021/la0347399 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/07/2003
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dependence of the radius of point defects or the yield stress on water concentration during the formation of the defect array.9,10 In this paper, the position of water droplets in the defect array and their motion during the formation process have been investigated with a laser scanning confocal fluorescence microscope. A small amount of water-soluble fluorescent probe, Safranin O, was mixed in the cholesteric emulsions to detect the position of water droplets. Confocal microscopes are useful to obtain three-dimensional images by piling up two-dimensional images from consecutive focal planes. On the other hand, a fluorescence microscopy has one problem about photobleaching14-18 to measure quantitative change of fluorescence intensity with time, though it is a highly efficient method for the positional detection of objects labeled with a fluorescent probe.17-19 We have examined the spatial distribution of water droplets in the three-dimensional structure of the defect array and attempted to examine a time lapse of fluorescence intensity under the condition where photobleaching can be ignored, for purposes of monitoring the motion of water droplets after stopping the shear. The role of the droplets in the formation of the defect array will be finally discussed from the results obtained with polarizing and laser scanning confocal fluorescence microscopes. Experimental Section Three types of cholesteric liquid crystals with different pitch (p ∼ 0.43, 1.3, and 11.1 µm) were prepared by mixing base nematics and a chiral dopant CB-15 (from Merck). Base nematics for 0.43 and 1.3 µm pitch samples are from Dainippon ink & Chemicals (Cr 11 °C N 116 °C I), and the isotropic-cholesteric phase transition temperatures TI-Ch are 62 and 82 °C, respectively. On the other hand, ZLI-1132 (N 75 °C I, from Merck) was used as base nematics for an 11.1 µm pitch cholesteric (TI-Ch ) 74 °C). These cholesteric pitches were evaluated by the Grandjean-Cano method. Water-in-cholesteric liquid-crystal emulsions were prepared by simply mixing 90 wt % didodecyldimethylammonium bromide (DDAB) aqueous solution, containing 250 µM water-soluble fluorescent probe, Safranin O (λem ) 565 nm), into the cholesteric liquid crystals. DDAB was used to stabilize small water droplets in the liquid crystal and to induce the homeotropic anchoring at the droplet surface. The weight concentration of the DDAB aqueous solution in the emulsion, Cw, was fixed at 3 wt %. In the isotropic phase, this system is equivalent to conventional waterin-oil microemulsions. The diameter of inverse micelles can be estimated to be ∼4 nm from the molecular length of DDAB (∼1 nm) and the concentration ratio of DDAB and water, assuming that both densities are equal to 1. TI-Ch in each sample became slightly lowered (∼1 °C) by containing the DDAB solution. When temperature goes down to the cholesteric phase, micron-scale water regions appear by macroscopic phase separation, as shown in Figure 1. The samples were sandwiched between a pair of glass plates, which had been treated with the surfactant (FS150, from Dainippon ink & Chemicals), to promote homeotropic orientation of liquid crystalline molecules to the glass surface. The sample thickness h was controlled at 20 or 50 µm by using cross-linked polystyrene particles as spacers. The lower glass plate is fixed on a hot stage, while the upper plate can be moved to apply a (14) Rigaut, J. P.; Vassy, J. Analyt. Quant. Cytol. Histol. 1991, 13, 223. (15) Oostveldt, P. V.; Verhaegen, F.; Messens, K. Cytometry 1998, 32, 137. (16) Ghauharali, R. I.; Hofstraat, J. W.; Brakenhoff, G. J. J. Microsc. 1998, 192, 99. (17) Lu, H. P.; Xun, L.; Xie, X. S. Science 1998, 282, 1877. (18) Go¨hde, W., Jr.; Fischer, U. C.; Fuchs, H.; Tittel, J.; Basche´, Th.; Bra¨uchle, Ch.; Herrmann, A.; Mu¨llen, K. J. Phys. Chem. A 1998, 102, 9109. (19) Velev, O. D.; Kaler, E. W.; Lenhoff, A. M. J. Phys. Chem. B 2000, 104, 9267.
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Figure 1. Cross-polarizing and fluorescence microscope images of the cholesteric emulsion (p ) 1.3 µm, Cw ) 3 wt %) containing Safranin O, before applying the shear (a and b, respectively). Temperature ) 30 °C, after cooling (2 °C/min) from I phase. Sample thickness ) 20 µm. Scale bar ) 50 µm. strong shear (>100 s-1) with tweezers. In these glass cells, the samples for observations were prepared by cooling from the isotropic phase, at a cooling rate of 2 °C/min. The polarizing and fluorescence images were obtained with a polarizing microscope (Eclipse E600W POL with 40× Plan Fluor objective, Nikon, Japan) equipped with a laser confocal scanner unit (CSU10, Yokogawa Electric, Japan), an image intensifier unit, and a CCD camera (C8600-3 and C4742-95 respectively, Hamamatsu Photonics, Japan). Confocal fluorescence and polarizing microscope modes can be switched instantaneously, and both fields of view are identical. The samples were excited by an argon ion laser (wavelength ) 488 nm, 532-BS-A04, Melles Griot) with excitation power of 50 mW/cm2. The exposure time of one frame scanning was fixed at 1 s to acquire sliced images sequentially at intervals of 1 µm along the normal direction and at 100 ms to examine the time dependence of fluorescence intensity. Image processing was carried out with IPLab Scientific Imaging Software (Scanalytics). The fluorescence intensity of each image was evaluated as the mean value in 240 × 170 pixels (∼1 pixel/um) by the software. Before the measurements, we confirmed the proportionality between the intensity and the concentration of the fluorescent probe in cholesteric emulsions.
Results and Discussion Spatial Distribution of Water Droplets in the Defect Array. Initially, we confirmed that the fluorescent probes remain in water droplets even in the mixture. Parts a and b of Figure 1 show respectively the cross-polarizing and the fluorescence images of the cholesteric emulsion (p ) 1.3 µm, Cw ) 3 wt %) before applying the shear. The temperature was held at 30 °C after cooling at 2 °C/min from the isotropic phase. The sample thickness was fixed at 20 µm, and these images were focused on the upper glass surface. The texture shown in Figure 1a is composed of cholesteric focal conics and water regions, which are formed by the macroscopic phase separation that occurs simultaneously at the I-to-Ch phase transition. In the fluorescence image (Figure 1b), water regions have a stronger fluorescence intensity per unit area, which is ∼23 times as large as that from the liquid crystalline region, and this contrast clarifies that the fluorescent probe is selectively concentrated in the water region. The image shown in Figure 1b did not apparently change with positions along the normal direction, which means the penetration of water regions between upper and lower glass surfaces. In the case of rapid cooling (>20 °C/min), water regions tend to have smaller area and higher number density than those in Figure 1, in such a way as to keep the total volume of water regions constant. Next, Figure 2 shows the cross-polarizing and the fluorescence images of the well-developed defect array, formed in 30 min after applying the strong shear to the texture shown in Figure 1. By comparing among fluorescence images of different focal planes, it is clear that fluorescence is more strongly emitted from around the upper and lower glass surfaces (Figure 2, b and f, respectively) than from the midway between glass plates
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Figure 2. Microscope images of the defect array (p ) 1.3 µm, Cw ) 3 wt %) at 30 °C, which was formed after the shear to the texture shown in Figure 1. Thickness ) 20 µm. Scale bar ) 50 µm. (a, c, e) Cross-polarizing microscope images and (b, d, f) fluorescence images. (a, b) Focused on the upper glass surface, (c, d) in the midway between glass plates, and (e, f) on the lower surface.
Figure 3. Cross-polarizing and fluorescence microscope images of the defect array (p ) 11.1 µm, Cw ) 3 wt %) at 50 °C (a and b, respectively). Focused on the upper glass surface. Thickness ) 50 µm. Scale bar ) 50 µm.
(Figure 2d). In Figure 2b,f, few water droplets can be clearly observed within optical resolution, while the distribution of fluorescence intensity is fairly uniform, independent of the period of the defect array shown in Figure 2a,e. These results indicate that small droplets fragmentized by the shear are uniformly distributed on both glass surfaces in the defect array. The same observation was performed in the defect array with a longer cholesteric pitch (p ) 11.1 µm), which can be detected as twice the period of fingerprint textures with a polarizing microscope. Figure 3a shows the concentric fingerprint patterns associated with the defect array;9 the comparison with Figure 3b indicates that the distribution of the droplets is also independent of the cholesteric pitch. These results reveal that there is no definite relation between the cholesteric texture and the position of water droplets in the defect array. The fluorescence intensity distribution along the normal direction in the emulsion was strongly affected with the temperature and the shear. Figure 4a shows the intensity profiles in the isotropic and cholesteric phases before applying the shear in a 20 µm thick sample (p ) 1.3 µm, Cw ) 3 wt %). The total intensity in each phase has been normalized as 1. The isotropic phase has a three-
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Figure 4. Effect of temperature and the shear on the distribution of fluorescence intensity along the direction normal to glass plates in the emulsion (p ) 1.3 µm, Cw ) 3 wt %). Thickness ) 20 µm. The total intensity under each condition was normalized as 1. (a) In I phase and Ch phase before the shear. In Ch phase, the dark and bright areas were observed in Figure 1b. The intensities of the dark and bright areas are allocated as 0.58 and 0.42, respectively, estimated from the intensity ratio per unit area and the area ratio of these observed regions. (b) Before and after the shear (in Ch phase). The profile before the shear is the sum of the data in the dark and bright areas shown in (a).
dimensionally uniform distribution of fluorescence intensity, as it is identical to the conventional microemulsion composed of inverse micelles dispersed in isotropic oil. In the cholesteric phase at 30 °C, formed after cooling from the isotropic phase, there appear two kinds of areas with clearly different brightness as shown in Figure 1b: the dark area of focal conics and the bright area of water regions. In Figure 4a, the intensities of the dark and bright areas are allocated as 0.58 and 0.42, respectively, estimated from the intensity ratio per unit area (1:23) and the area ratio of these observed regions (32:1). The bright area (i.e., the water region) shows a vertically uniform intensity distribution, in the same way as in the isotropic phase. On the other hand, the dark area (i.e., the liquid crystalline region) still has a considerable amount of fluorescence, which has a peak near the upper glass surface as shown in Figure 4a. This distribution in the dark area may be induced through the initial macroscopic phase separation, owing to the density difference between the liquid-crystal host and water droplets left in focal conics. (The droplets, which are 90 wt % DDAB aqueous solution, seem a little lighter than the liquid-crystal medium, from the fact that the transparent phase is settled on top of the liquid crystalline phase when phase separation occurs in a glass tube.) The profile in the dark area was not changed even in rapid cooling (>20 °C/min) within the experimental error.
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Figure 5. Time dependence of fluorescence intensity on the lower glass surface at 30 °C, in the emulsion (p ) 1.3 µm, Cw ) 3 wt %). Thickness ) 20 µm. (a) The defect array starts to be photoexcited at 50 mW/cm2 from t ) 0 s. (b) The shear was stopped at t ) 0 under continuous irradiation of the laser. A solid line shows a single-exponential curve; I/Imax ) 1 exp(-t/τw), where τw ) 2.4 s.
The shear effect on the intensity distribution is shown in Figure 4b. Here, the profile before the shear is the sum of the data in the dark and bright area shown in Figure 4a, and the profile after the shear was obtained in the defect array shown in Figure 2. After the shear, two distinct peaks of the intensity are found near both glass surfaces while the midway has little fluorescence, different from the moderate distribution before applying the shear. This variation of the intensity distribution shows that water droplets move from the midway to both surfaces between glass plates, in conjunction with the texture change by the shear.9 The breadth of the peaks after the shear was almost the same both in 20 and 50 µm thick samples. Here, we attempted to examine the time dependence of the intensity profile along the normal direction; however, it was impossible to obtain profiles for 30 s after stopping the shear, since the time for at least 30 s per one round was required to acquire a series of sliced images in our system of the confocal microscope. In the time range above 30 s after the shear, the intensity profile was not changed with time, though the texture transformation into the defect array was not yet finished, which will be discussed later. Motion of Water Droplets after Stopping the Shear. To monitor the motion of water droplets after the shear, we examined the time evolution of fluorescence intensity with the focus fixed at the lower glass surface. Figure 5 shows the data from 20 µm thick sample (p ) 1.3 µm, Cw ) 3 wt %) at 30 °C. We first checked the influence of photobleaching, as shown in Figure 5a. When the defect array starts to be photoexcited at 50 mW/cm2 from t ) 0 s, the fluorescence intensity remains constant for at least 1 min and then gradually decreases with time. This result allows us to conclude that photobleaching can be ignored within 1 min under our conditions. In case that the shear is applied and stopped to the emulsion under continuous irradiation (Figure 5b), the fluorescence intensity rapidly increases and reaches a constant value in 10 s. This behavior shows that water droplets are condensed onto the glass surface after the shear. The intensity increase
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Figure 6. Arrhenius plot of τw, obtained in the same way as Figure 5b. Sample; p ) 1.3 µm, Cw ) 3 wt %. The temperature was changed between 25 and 50 °C. Symbols and error bars correspond to the average and the scatter in measured values, respectively. Estimated activation energy ) 27.9 and 28.3 kJ/ mol at 20 and 50 µm thick, respectively.
shown in Figure 5b was repeatedly observed even under the shear applied to preformed defect arrays, which means that the shear can take the droplets off the glass plates and redisperse them into the cholesteric host. The data in Figure 5b could be fitted by a single-exponential form; I/Imax ) 1 - exp(-t/τw), where τw ∼ 2 s under this condition, defined as the specific time for the condensation of water droplets onto the glass surface. The Arrhenius plot of τw is provided in Figure 6, which was measured by changing temperature between 25 and 50 °C, in two kinds of thickness h; 20 and 50 µm for the same sample in Figure 5. The time-sweep measurement as shown in Figure 5b was repeated more than 10 times under each condition, and the average and the scatter in measured values were plotted as the symbols and the error bars in Figure 6, respectively. τw tends to become larger with lowering of temperature in both thicknesses. On the assumption of typical Arrhenius-type relation, we evaluated the activation energy E of τw from the slopes of the fitting lines; E ∼ 27.9 and ∼ 28.3 kJ/mol at 20 and 50 µm thick, respectively. These values are in good agreement with that of the flow viscosity η for the pure cholesteric used as the medium; E ∼ 27.4 kJ/mol. (Here, η was measured with a cone and plate geometry at a shear rate, 300 s-1, in the high-shear Newtonian region.) τw is also changed by the sample thickness, which should depend on the average drift distance of water droplets down to the lower glass surface. On the other hand, Figure 7 shows the relation between τw and η for two samples with different pitch, 1.3 and 0.43 µm, in the temperature region between 25 and 50 °C. (The sample thickness was fixed at 20 µm.) τw is proportional to η, while the pitch dependence of this relation is not detected in Figure 7. These results in Figures 6 and 7 inform that the mobility of water droplets is dominated simply by the viscosity of the cholesteric medium, and not directly affected by the pitch or the texture change to the defect array, which should be introduced by the droplets. We here compare the time scales between the condensation of water droplets and the formation of the defect array. Figure 8 shows the time dependence of the radius
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Figure 9. Predicted scheme for the motion of water droplets and the growth of point defects after the shear. The layer shows cholesteric helix. (a) Immediately after the shear (generation of nuclei of the defects on the homeotropic-anchoring glass surface.) (b) Condensation of the droplets onto the glass surface. (c) Appearance of the defects with the critical size. (d) Spontaneous growth of the defects.
Figure 7. Relation between τw and the shear-flow viscosity (at 300 s-1) of pure cholesterics, which were used as disperse media. Thickness ) 20 µm. The temperature was changed between 25 and 50 °C.
Figure 8. Formation process of the defect array after stopping the shear (t ) 0), under the same condition as that in Figure 5b. Filled squares and open circles show the average radius and the number density of point defects, respectively.
and the number density of point defects under the same condition as in Figure 5b, which reflects the formation process of the defect array. These qualitative properties in Figure 8 were already explained in our previous paper.9 Immediately after stopping the shear, defects appear from the shear-induced planar texture, and their average radius and density rapidly increase together via the nucleation growth20 process of isolated point defects. After defects fill up the available space, the radius increases slowly while the density decreases, and they approach certain fixed values in a long time region, which is the rearrangement process of irregularly distributed point defects to the defect array. From the above information, Figure 8 identifies two kinds of characteristic times: t ∼ 30 s for isolated point defects to fill all the space and t ∼ 1000 s for the regular arrangement of point defects to be completed. Obviously, both characteristic times are sig(20) Feher, G.; Kam, Z. Methods Enzymol. 1985, 114, 77.
nificantly larger than the time necessary for the condensation of water droplets (τw ∼ 2 s), which explains why the mobility of the droplets is not affected by the texture change (Figures 6 and 7). Water droplets attached to the glass surfaces are virtually unable to distort the cholesteric planar texture sufficiently because of the same homeotropic condition as bare homeotropic-anchoring glass surfaces, which did not affect the shear-induced planar alignment of pure cholesterics (in case that the pitch is much smaller than the sample thickness). For this reason, the effect of water droplets on the formation of the defect array should appear only at the initial process in the nucleation growth of point defects. Contribution of Water droplets to the Formation of the Defect Array. Previously, we considered the role of water droplets during the formation of the defect array as follows.9 The growth of one point defect after the shear requires the cooperative and long-range rearrangement of liquid crystalline order, from the planar alignment to the homeotropic orientation. In pure cholesterics without water droplets, point defects cannot efficiently develop after the shear only with the aid of the homeotropic anchoring of glass plates, which indicates the presence of a substantial energy barrier to disrupt the planar alignment. In cholesteric emulsions, on the other hand, the shear-induced planar texture includes many defect cores, which are water droplets with homeotropic anchoring at the interface. Therefore, we considered that the distortion of liquid crystalline order around the droplets reduces the height of the energy barrier to create critical nuclei of point defects, which leads to the growth of point defects. The experimental results shown in Figures 5b and 8 indicate that water droplets influence the liquid crystalline order only for several seconds immediately after stopping the shear, which seems to be a reasonable time region to pass the energy barrier in the nucleation growth process of point defects.20 Moreover, Figures 5b and 8 suggest that the subsequent process, including the further growth of isolated defects and their array formation, should not be contributed by water droplets. This suggestion agrees with our past experimental results of the defect array,9,10 little influence of water concentration on the size of point defects or the shear yield stress in the rearrangement process of contacting defects to the defect array, when the water content is sufficiently high. On the basis of the above consideration, we described the scheme for the formation process of the defect array in Figure 9. Water droplets are basically difficult to disperse in cholesteric media, from the fact that macroscopic phase separation occurs as shown in Figure 1. The
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application of a strong shear to the emulsions compellingly fragmentizes and disperses water droplets into the cholesteric planar texture, which is also induced by the shear. This is the initial state for the formation of the defect array (Figure 9a), having a higher free energy than the pure cholesterics. Immediately after stopping the shear, the nuclei of point defects appear on the glass surface owing to its homeotropic anchoring, and simultaneously water droplets begin to be removed from the inside of the cholesteric helix for the reduction of the free energy of liquid crystalline order. When water droplets approach the nuclei generated on the glass surface, the distortion of the molecular orientation around the nuclei is promoted by the surface anchoring of the droplets, which introduces the expansion of point defects (Figure 9b). By the time all droplets reach the glass surface, some point defects already have the size enough to grow themselves automatically (that is, have overcome the energy barrier to create critical nuclei). Therefore, the subsequent growth of the defects can spontaneously progress in a long distance, even if water droplets do not exist in the inside of the cholesteric medium at that time (Figure 9c,d). The droplets thereafter do not contribute to the rearrangement of point defects to the defect array, which should be dominated by minimization of the distortion energy of the cholesteric helices, as stated in our previous paper.9 This scheme shown in Figure 9 is consistent with our experimental results: the difference of the time scales between the condensation of the droplets and the formation of the defect array (Figures 5b and 8), the simple dependence of τw on the medium viscosity (Figures 6 and 7), and little dependence of the defect array formation on water concentration.9,10 Thus, we can explain that water droplets play a role to create nuclei with the critical size20 for the nucleation growth of point defects. Finally, we consider a possibility of wetting of water droplets on the glass surface. In Figure 2b,f, it is impossible to judge whether small droplets exist intact on the plate or turn into a wetting layer. Here, we already know that water droplets localized on the glass surface in the preformed defect array can be redispersed into cholesteric
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mediums by applying the shear, from the repeatability of the increase in fluorescence intensity shown in Figure 5b. Even if the fixed wetting layer exists on the glass surface, the layer will not influence the nucleation of point defects because of just the same condition as the bare homeotropicanchoring glass surface (which was treated with FS150, a dedicated surfactant to promote homeotropic orientation). From this viewpoint, it is reasonably considered that the condensation process of water droplets onto both glass surfaces is essential for the nucleation growth of point defects, which leads to the formation of the defect array. Conclusions By using laser scanning confocal fluorescence microscopy, the motions of water droplets containing a fluorescent probe were monitored during the formation of the defect array in cholesteric emulsions. After stopping the shear, the droplets dispersed by the shear are expelled from the inside of the cholesteric medium to the glass surfaces, prior to the growth of point defects. This result means that water droplets influence the liquid crystalline order only for the initial process in the nucleation growth of point defects. Water droplets with a homeotropic anchoring surface play an essential role to create nuclei of point defects with the critical size by distorting molecular orientation around the nuclei when the droplets approach the glass surface after the shear. The following processes, including the further growth of isolated point defects and their rearrangement to the defect array, are not influenced by the droplets but are dominated by the deformation of liquid crystalline order, in such a way as to minimize the distortion energy of the cholesteric helix. The spontaneous formation of the defect array may thus be explicable from both aspects of the molecular rearrangement of cholesterics and the motion of water droplets. Acknowledgment. The authors gratefully acknowledge Prof. B. I. Lev of the National Academy of Sciences of Ukraine for his helpful comments and useful discussions. LA0347399
