Liquid crystalline tactoids: ordered structure, defective coalescence and evolution in confined geometries

Abstract

Tactoids are liquid crystalline microdroplets that spontaneously nucleate from isotropic dispersions, and transform into macroscopic anisotropic phases. These intermediate structures have been found in a range of molecular, polymeric and colloidal liquid crystals. Typically only studied by polarized optical microscopy, these ordered but easily deformable microdroplets are now emerging as interesting components for structural investigations and developing new materials. In this review, we highlight the structure, property and transformation of tactoids in different compositions, but especially cellulose nanocrystals. We have selected references that illustrate the diversity and most exciting developments in tactoid research, while capturing the historical development of this field.

This article is part of a discussion meeting issue ‘New horizons for cellulose nanotechnology’.

1. Introduction

Since the pioneering work of Zocher on vanadium pentoxide sols in 1925 [1], tactoids, which are liquid crystalline microdroplets formed in isotropic phases, have been observed in examples of molecular, polymeric and colloidal lyotropic liquid crystals, such as tobacco mosaic viruses [2], iron oxyhydroxide nanorods [3], polypeptides [4] and cellulose nanocrystals [5]. As the phase transitions in these systems were found to be mediated by liquid crystalline tactoids, they have received considerable attention in the past 90 years. Although significant results have been obtained in studies of the formation [6], morphology [7], structure [8] and other aspects of tactoids, many properties of these highly ordered, deformable microdroplets remain unclear.

2. Formation of tactoids in isotropic phases

When an initially isotropic dispersion (below the critical concentration) is concentrated by evaporation, one can observe the formation of tactoids as anisotropic microdroplets by polarized optical microscopy (POM). Although the direct and real-time monitoring of mesogens during the emergence of a liquid crystalline tactoid is still impossible, this process has been theoretically investigated since the 1930s [6]. The formation of a tactoid needs both an attractive force to gather together mesogens into a microdroplet and a repulsive force to arrange these mesogens into liquid crystalline order. These interactions are typically modelled by the Derjaguin–Landau–Verwey–Overbeek theory, where the electrostatic repulsion between charged mesogens is balanced by the long-range van der Waals forces [9,10]. As an alternative, Langmuir [6] showed that the Coulombic attraction between mesogens and oppositely charged intervening counterions is enough to balance the repulsive interactions.

Although the above thermodynamic models can partly explain how tactoids are created by competing repulsive and attractive forces, it is still unclear whether these liquid crystalline microdroplets have any unique intrinsic properties that distinguish them from the isotropic environment. Are the mesogens in a tactoid somehow different from those in the nearby isotropic phase? This remains a difficult problem to address owing to the lack of direct microscopic observations, but studies on phase-separated lyotropic liquid crystals may give some hints as phase separation in these systems is usually mediated by the coalescence and sedimentation of tactoids.

In a study by Hirai et al. [11] on phase-separated cellulose nanocrystal suspensions, the rod-shaped mesogens in the liquid crystalline phase were found to have significantly greater lengths and higher aspect ratios than those in the isotropic phase. This is consistent with the theoretical studies by Lekkerkerker et al. [12], who investigated the phase separation in a dispersion of rod-shaped mesogens of different lengths based on the Onsager theory, and found a higher fraction of the longer rods in the liquid crystalline phase than in the isotropic phase. As it has been confirmed that the phase transitions in cellulose nanocrystal suspensions are mediated by tactoids, which kinetically transfer mesogens from one phase to the other, the enrichment of high-aspect-ratio mesogens in the long-range liquid crystalline phases may have occurred through the nucleation of tactoids.

According to the Onsager theory [13], the critical concentration for phase separation in a rod dispersion decreases with the aspect ratio of the rods. As the formation of a liquid crystalline tactoid is actually a microscopic phase separation process that happens when the local critical concentration is reached, if an initially isotropic dispersion of rod-shaped mesogens is slowly and homogeneously concentrated, tactoids will first form in microdomains rich in high-aspect-ratio mesogens due to a lower local critical concentration. This will selectively collect these high-aspect-ratio mesogens into the macroscopic liquid crystalline phase through coalescence and sedimentation.

3. Shapes and director field configurations of tactoids

As previously demonstrated by Kaznacheev et al. [7,14] and Prinsen & van der Schoot [15] in their theoretical studies, the shape of a large tactoid is determined by the competition between the surface energy of the anisotropic/isotropic interfacial tension [16,17], which favours a spherical shape, and the elastic energy of the liquid crystalline phase, which tends to elongate the tactoid. On the other hand, a small tactoid is shaped by the competition between the surface energy and the anchoring energy, where the latter is caused by the deviation of the director field at the tactoid boundary from the tangential orientation.

Experimentally, the morphology and director field configuration of a tactoid can be examined by POM because the alignment of mesogens leads to birefringence. Nematic tactoids formed in lyotropic liquid crystals of ribbon-like or rod-like mesogens, such as vanadium pentoxide (figure 1a), aluminium oxyhydroxide (figure 1b) [18], tobacco mosaic viruses (figure 1c) or carbon nanotubes (figure 1d) [19], are prolate spindle-shaped or spheroidal microdroplets with circular arc boundaries (figure 1a). These tactoids display a continuous range of director field configurations from homogeneous to bipolar with increasing tactoid size (figure 1d). Tactoids formed in smectic liquid crystals such as fd virus suspensions [20] are spindle-shaped microdroplets with a number of smectic rings periodically spaced along the long axes (figure 2).

Figure 1.

Tactoids formed in nematic liquid crystalline dispersions of (a) vanadium pentoxide, (b) aluminium oxyhydroxide and (c) tobacco mosaic viruses are spindle-shaped microdroplets. (d) Schematic representations of nematic tactoids with homogeneous, intermediate and bipolar director field configurations formed in carbon nanotube dispersions. ((a) Reproduced with permission from Kaznacheev et al. [7]. Copyright © 2002 Springer. (b) Reproduced with permission from Zocher & Török [18]. Copyright © 1960 Springer. (c) Reproduced with permission from Bawden et al. [2]. Copyright © 1936 Macmillan. (d) Reproduced with permission from Jamali et al. [19]. Copyright © 2015 American Physical Society.)

Figure 2.

Smectic tactoids formed in fd virus suspensions are prolate microdroplets with periodically spaced smectic rings. (Reproduced with permission from Dogic & Fraden [20]. Copyright © 2001 Royal Society.)

In discotic nematic liquid crystals like colloidal gibbsite suspensions [21], large tactoids are spherical microdroplets showing a Maltese cross pattern when observed between crossed polarizers, which indicates a radial director field with a hedgehog defect (topological charge + 1) in the centre. On the other hand, small tactoids are oblate spheroids with an axial director field configuration due to the homeotropic (perpendicular) anchoring of the disc-shaped mesogens to the boundary. Therefore, tactoids of intermediate size are usually asymmetric oblate with an off-centre ring disclination (figure 3).

Figure 3.

POM images and director field configurations of discotic nematic tactoids formed in colloidal gibbsite dispersions. (Reproduced with permission from Verhoeff et al. [21]. Copyright © 2011 American Chemical Society.) (Online version in colour.)

Chiral nematic (cholesteric) tactoids, such as those formed in polypeptide solutions (figure 4a) [4] or cellulose nanocrystal suspensions (figure 4b) [5], are spherical or ellipsoidal microdroplets with periodic birefringent bands. Under planar (parallel) anchoring conditions, the periodic bands are arranged into an onion-like structure consisting of multiple concentric spherical shells with radially oriented helical axes [22]. Flat periodic bands can be observed when the anchoring of the mesogens at the anisotropic/isotropic interface is homeotropic or weak, which usually happens in the case of colloidal liquid crystals like cellulose nanocrystals.

Figure 4.

(a) Tactoids observed in polypeptide solutions are spherical microdroplets with concentric chiral nematic bands indicating radially oriented helical axes. (Reproduced with permission from Robinson [4]. Copyright © 1956 The Royal Society of Chemistry.) (b) Tactoids formed in cellulose nanocrystal suspensions have flat chiral nematic bands. (Reproduced with permission from Revol et al. [5]. Copyright © 1992 Elsevier.)

Because of the subtle balance between the surface tension, the surface anchoring condition and the long-range elasticity of the liquid crystalline phase, the shape and director field configuration of tactoids are very sensitive to a range of chemical and physical factors, such as chiral additives or external forces [23–25].

4. Capture of liquid crystalline tactoids in solid matrices

Until a few years ago, structural studies of tactoids were mainly based on optical microscopy or other non-destructive testing methods due to the highly deformable nature of these fluid microstructures. Unfortunately, these techniques are not usually capable of revealing the arrangement of mesogens due to their limited resolution. A possible solution is to capture and stabilize these highly ordered microdroplets by introducing a solid-state matrix into the system, during which the ordered structures of tactoids should be well maintained without distortions.

Recently, we successfully captured and solidified liquid crystalline tactoids in cross-linked polyacrylamide matrices by rapid photopolymerization [8]. The fine structure of the captured tactoids was stabilized by the polymer matrix and then investigated by electron microscopy at the resolution of individual mesogens. As revealed by scanning electron microscopy (SEM) of the cross-sections of the polymerized samples, tactoids formed in cellulose nanocrystal suspensions are liquid crystalline microdomains with spherical or ellipsoidal boundaries that are clearly distinguishable from the isotropic phase (figure 5). Small tactoids seem to be unwound nematic due to the boundary conditions because cellulose nanocrystals in them are uniformly aligned (figure 5a–c). On the other hand, large tactoids have a left-handed helical chiral nematic structure appearing as a series of flat periodic bands (figure 5d–i), each of which represents a half-helical pitch, where the mesogens twist by 180° from one end to the other. By creating an intersection of two fracture surfaces that are perpendicular to each other (figure 6a), the chiral nematic structure of a tactoid could be examined from multiple angles (figure 6b–i). In the isotropic phase, mesogens are randomly arranged without any preferred orientations, consistent with the POM observations [8].

Figure 5.

Liquid crystalline tactoids were solidified in cross-linked polyacrylamide matrices by photopolymerization and observed by electron microscopy from cross-sections. (a) A tactoid newly formed in a cellulose nanocrystal suspension is an ellipsoidal nematic microdomain, where the mesogens are uniformly aligned, as shown at higher magnification in (b). This tactoid is separated from the isotropic phase by an arc-shaped sharp boundary (c). (d) A tactoid with only one chiral nematic band, where the mesogens twist by 180° from one end to the other. A larger tactoid with three chiral nematic bands is shown in (e), which has a half-helical pitch of about 3.7 µm. (f) SEM image of a tactoid with four chiral nematic bands. The images in (g–i) are expanded views of the tactoids shown in (d–f), respectively. Scale bars: (a) 1 µm, (b,c) 200 nm, (d) 2 µm, (e) 5 µm, (f) 5 µm, (g) 500 nm, (h) 500 nm and (i) 1 µm. (Adapted from Wang et al. [8].)

Figure 6.

SEM images of a chiral nematic tactoid sitting at a right-angle edge, which was observed from the top (a–c), left (d,g), front (e,h) and right (f,i) sides. In the preparation of this sample, the dried hydrogel with embedded tactoids was broken with a hammer, revealing a tactoid on an edge. Scale bars: (a) 10 µm, (b) 2 µm, (c) 1 µm, (d) 500 nm, (e) 1 µm, (f) 1 µm, (g) 200 nm, (h) 500 nm and (i) 300 nm. (Adapted from Wang et al. [8].)

5. Coalescence of tactoids and formation of topological defects

The fusion of tactoids has been observed in many lyotropic liquid crystals, such as vanadium pentoxide [7], fd viruses [26], cellulose nanocrystals [8], collagens [27] and F-actin [28], where several smaller tactoids merge together to form a larger one (figure 7). This coalescence process usually generates topological defects in the director field of the resulting tactoid because it is the combination of multiple liquid crystalline microdomains with initially random orientations. These defects could be healed by the reorientation of the director field as they are thermodynamically unstable, but, in some cases, the defective structures remain in the fused tactoid for a long time because of the slow relaxation rate of the mesogens.

Figure 7.

POM images showing the coalescence of chiral nematic tactoids in a cellulose nanocrystal suspension [8]. (Online version in colour.)

To understand the fingerprint textures in solid films with chiral nematic order [29], we studied the coalescence of liquid crystalline tactoids in cellulose nanocrystal suspensions by SEM after capturing the systems in polyacrylamide matrices [8], where we observed the contact between multiple tactoids (figure 8a–c) as well as defective tactoids with dislocated or folded chiral nematic bands (figure 8d,e). Note that the defects are in the helical orientation field, not necessarily in the director field [22]. As tactoids have a higher density than the isotropic phase due to a more efficient packing of the mesogens, they will gradually settle to the bottom of the suspension (figure 8f) and eventually coalesce into a macroscopic liquid crystalline phase containing a large number of kinetically arrested fusion defects (figure 8g–i).

Figure 8.

(a) SEM image showing the initiation of coalescence between two tactoids. Fine structure of the contact point is shown in (b) and (c). (d,e) Defective tactoids with dislocated or folded chiral nematic bands. (f) Tactoids sink to the bottom of the suspension (i.e. the lower part of this SEM image) due to a higher density, and coalesce into a long-range chiral nematic phase. (g) SEM image showing defects in the helical orientation field of a long-range liquid crystalline phase, which appear as an array of folded chiral nematic bands (h). Fine structures of these defects are shown in (i). Scale bars: (a) 10 µm, (b) 1 µm, (c) 300 nm, (d,e) 10 µm, (f) 30 µm, (g) 10 µm, (h) 3 µm and (i) 1 µm. (Adapted from Wang et al. [8].)

6. Alignment of tactoids and elimination of topological defects

Soon after the discovery of liquid crystalline tactoids in vanadium pentoxide sols, Zocher & Jacobsohn [30] observed the alignment of these spindle-shaped microdroplets in a magnetic field. Vanadium pentoxide tactoids can be oriented in a relatively weak magnetic field (approx. 0.3 T), which prevents the occurrence of topological defects during the coalescence and thus leads to the formation of a fully aligned single nematic domain [31]. Moreover, Kaznacheev et al. [7] found that a magnetic field also stretches tactoids in its direction (figure 9a,b), especially in the case of large tactoids. This phenomenon is associated with the anisotropic magnetic susceptibility of the mesogens, which enables the magnetic field to orient the director of the liquid crystal tactoids in its direction.

Figure 9.

Nematic tactoids formed in vanadium pentoxide sols can be aligned and stretched by an external magnetic field, as shown in (a) and (b), respectively. (Reproduced with permission from Kaznacheev et al. [7]. Copyright © 2002 Springer.) Chiral nematic tactoids formed in cellulose nanocrystal suspensions can also be oriented in a magnetic field with their periodic bands perpendicular to the field direction (c), resulting in a nearly perfect long-range chiral nematic phase (d). (Reproduced with permission from Revol et al. [32]. Copyright © 1994 Taylor & Francis Ltd.)

Chiral nematic tactoids formed in cellulose nanocrystal suspensions were first aligned in an external magnetic field of 7 T [32] (see [33] for studies with weaker magnetic fields), where the periodic bands of both the tactoids and the resulting nearly perfect long-range liquid crystalline phases are oriented perpendicular to the magnetic field direction (figure 9c,d). It should be noted that, in either vanadium pentoxide sols or cellulose nanocrystal suspensions, the application of a magnetic field does not induce any observable birefringence in the isotropic phase, which indicates that the orientation of the liquid crystalline phase may require some cooperative interactions between the mesogens [31]. In our hypothesis, the magnetic torque on a single mesogen would be too weak to overcome the steric hindrance of nearby mesogens as they are randomly oriented or even entangled in the isotropic phase.

Another way to control the orientation of tactoids in a lyotropic liquid crystal is to apply a shear force. It has been reported that the helical axes of chiral nematic tactoids formed in cellulose nanocrystal suspensions can be vertically oriented in a horizontal shear flow [34], which significantly improves the uniformity and therefore the optical properties of the dried films.

7. Confinement of chiral nematic tactoids in spherical or spherical shell geometries

The behaviour of liquid crystals in confined geometries is essential both for understanding the topological properties of these ordered soft matter [35–37] and for the development of new optical devices [38]. However, because most previous studies were based on molecular thermotropic liquid crystals with fast phase transition processes, the intermediate states in these systems were difficult to distinguish and elucidate. New insights may be provided by lyotropic liquid crystals as they usually have much slower kinetics that would enable tracking of the phase transitions in geometrical confinements during a reasonably long period of time.

In a recent study, we investigated the evolution of chiral nematic liquid crystalline tactoids confined in spherical microdroplets with diameters from tens to hundreds of micrometres [39], which are comparable to the sizes of tactoids themselves. The microdroplets were prepared by emulsifying a thoroughly homogenized aqueous suspension of cellulose nanocrystals in a non-polar organic solvent. Initially, newly formed tactoids are small enough to avoid the confinement effects of the microdroplet boundaries, thus they have flat parallel chiral nematic bands similar to those formed in bulk suspensions (figure 10a). As time passes, more tactoids are generated in the isotropic phase, they coexist in the same microdroplet (figure 10b,c) and they grow by coalescence. When a tactoid is large enough to be affected by the spherical confinement of the microdroplet boundary, its chiral nematic bands will be bent to accommodate the water/oil interface (figure 10d), which leads to the formation of a concentric spherical multi-shell structure with radially oriented helical axes (figure 10e,f).

Figure 10.

POM images showing the evolution of chiral nematic liquid crystalline tactoids in spherical confinement. (a) A newly formed tactoid has flat chiral nematic bands unaffected by the confined geometry due to its small size. (b,c) Over time, more tactoids appear in the isotropic phase. Multiple tactoids coexist in the same microdroplet, and coalesce into larger ones. (d) The chiral nematic bands of a tactoid will be bent to accommodate the spherical confinement of the microdroplet when it is large enough. (e,f) After complete phase separation, integrated tactoids with concentric spherical chiral nematic bands will be formed in the microdroplets. Evolution time and diameter of these microdroplets: (a) 0.5 h, 118 µm; (b) 1 h, 58 µm; (c) 3 h, 64 µm; (d) 6 h, 78 µm; (e) 9 h, 57 µm; and (f) 12 h, 114 µm. (Reproduced with permission from Wang et al. [39]. Copyright © 2016 Wiley-VCH Verlag GmbH & Co.) (Online version in colour.)

We further examined the fine structures of the chiral nematic tactoids confined in spherical geometries by electron microscopy [40]. Polyacrylamide precursors were added to the cellulose nanocrystal suspensions prior to emulsification, enabling us to solidify the liquid crystal microdroplets by photopolymerization. The chiral nematic bands of the tactoids usually adopt a spiral structure with a topological defect in the core (figure 11i), as confirmed by both cross-sectional SEM (figure 11a–f) and confocal laser scanning microscopy (figure 11g,h). It remains unclear how the spherical geometry of the microdroplet boundary influences the structure of the tactoid, because, in many cases, the tactoid is surrounded by a thick isotropic shell that isolates it from the boundary of the microdroplet (figure 11h). Besides the concentric circle patterns (figure 12d), tactoids with flat or arc-shaped chiral nematic bands were also observed in some microspheres (figure 12a–c), which may have been captured at different growth stages.

Figure 11.

(a) Cross-sectional SEM image showing a chiral nematic tactoid in the centre of a microsphere. This tactoid consists of two chiral nematic bands arranged in a spiral pattern (b) with a disclination defect in the core region (c). (d,e) SEM images showing the left-handed helical chiral nematic structure in the periodic bands of this tactoid. The arrangement of individual mesogens can be clearly observed at high magnification (f). (g) Confocal microscopy image of a microsphere showing a tactoid in the centre with spiral chiral nematic bands. The overall organization of the chiral nematic bands in this tactoid is a multi-shell spherical structure, as revealed by the three-dimensional reconstructed image (h). A simplified model for these microspheres is depicted in (i), which shows the radially orientated helical axes of the chiral nematic tactoid as well as its defective core region. Scale bars: (a) 10 µm, (b) 5 µm, (c) 2 µm, (d,e) 500 nm, (f) 200 nm and (g) 20 µm. (Reproduced with permission from Wang et al. [39]. Copyright © 2016 Wiley-VCH Verlag GmbH & Co.) (Online version in colour.)

Figure 12.

Liquid crystalline tactoids with chiral nematic bands arranged in flat (a), bent (b,c) or concentric circular (d) patterns are observed in different microspheres; these structures may represent different stages of the growth of tactoids in spherical confinement. The evolution of tactoids is also affected by the size of the confined space. Tactoids formed in relatively small microspheres (one to two times of the helical pitch) usually have an isotropic core surrounded by a liquid crystalline shell of a few chiral nematic bands (e). Medium-sized microspheres (five to seven times of the helical pitch) have integrated tactoids with concentric spherical multi-shell structures (f). On the other hand, tactoids cannot be efficiently integrated in large microspheres (more than 10 times larger than the helical pitch) due to the long distances between them (g). POM images in (e–g) were taken at the same magnification. Diameter of microspheres: (a) 85 µm, (b) 136 µm, (c) 142 µm and (d) 141 µm. (Reproduced with permission from Wang et al. [39]. Copyright © 2016 Wiley-VCH Verlag GmbH & Co.) (Online version in colour.)

The structure of tactoids evolved in confined geometries is also affected by the relative size of the space compared with the chiral nematic pitch (approx. 20 µm). Tactoids confined in relatively small microspheres (30 µm in diameter, from one to two times of the helical pitch) usually have an outer shell of a few chiral nematic bands and an inner isotropic kernel (figure 12e). Medium-sized microspheres (125 µm in diameter, about five to seven times of the helical pitch) generally include a single tactoid with a concentric spherical multi-shell structure surrounded by an isotropic shell (figure 12f). In more extreme cases, microspheres with much larger sizes (approx. 220 µm, more than 10 times larger than the helical pitch) always contain multiple discrete tactoids that have not been efficiently integrated (figure 12g), possibly due to the rather long distances between them.

In some cases, a gas bubble could be trapped in the centre of a microdroplet, thus the evolution of the chiral nematic tactoids was confined in a spherical shell geometry between the water/oil interface and the bubble surface, resulting in a hollow microsphere with a chiral nematic shell after photopolymerization (figure 13). The chiral nematic bands adopt a concentric arrangement in the shell as revealed by SEM (figure 13a), and cellulose nanocrystals exhibit planar anchoring at both the water/oil interface as well as the liquid/gas interface (figure 13b–d).

Figure 13.

(a) SEM image showing a hollow microsphere with chiral nematic structures in the shell. The chiral nematic arrangement of the mesogens and the planar anchoring at interfaces can be observed at high magnification in (b–d).

8. Conclusion and perspectives

In this article, we have reviewed the 90 year history of liquid crystalline tactoids, emphasizing the structure, transformation and manipulation of these highly ordered microdroplets. Tactoids play a significant role in the evolution of lyotropic liquid crystals by bridging the disordered and ordered phases through a kinetic mechanism. They have unique metastable structures determined by the subtle balance between several internal and external factors such as the elastic energy, the anchoring conditions and the interfacial tension between the two phases. Moreover, the director field configurations of tactoids are easy to map because they are the smallest units of lyotropic liquid crystalline phases. All these features make them ideal models for understanding the behaviour of liquid crystals.

Some aspects of tactoids have not been discussed here. For example, in a few cases, discrete isotropic microdomains could be observed in a continuous long-range liquid crystalline phase, which are sometimes referred to as atactoids [1,41] or negative tactoids [40]. So far they have only been reported in a limited number of studies, and the origin, structure and behaviour of these objects are still unclear.

The manipulation of tactoids is key to the development of novel materials and devices based on liquid crystals with improved properties. The alignment of chiral nematic tactoids (e.g. those formed in cellulose nanocrystal suspensions) in an external field would be a feasible way to produce solid films with perfect helical structures, which might be used as low-cost circular polarizers or chiral filters. Encapsulation of tactoids in confined spaces may have potential applications in displays, optical sensors or microlasers.

Data accessibility

This article has no additional data.

Authors' contributions

Both authors have contributed to the preparation of this review article.

Competing interests

We declare we have no competing interests.

Funding

The present work was financed by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada. P.-X.W. is grateful to the University of British Columbia for a 4YF Fellowship.