Our first step was to observe the phase behavior of the CNC suspensions. When CNCs are suspended in water above a critical concentration, they can self-assemble into a lyotropic cholesteric LC phase20,21. The attractive and repulsive intermolecular interactions and the balance between them all play a role in governing the thermodynamic stability of a CNC colloidal suspension as well as its ability to self-assemble into an LC. Presumably, the attractive interactions are caused by van der Waals forces, whereas the repulsive forces originate from not only short-range steric repulsions but also longer-range steric or electrostatic ones63. Although at low concentrations, CNC nanorods orient rather randomly, thereby forming an isotropic phase, sufficiently high concentrations often locally promote intermolecular alignment between individual nanorods. Thus, when their concentration is increased, the samples must undergo a first-order phase transition while transitioning from an isotropic phase to a cholesteric phase, by way of an intermediate regime wherein both phases coexist. The ordered phase can be identified by characteristic cholesteric fingerprint patterns using a POM.
Figure 2(a) illustrates the phase behavior of CNC suspensions as a function of the CNC concentration, which exhibits a clear phase transition from pure isotropic to biphasic and anisotropic LCs with the increasing concentration, as expected. The phase diagram in Figure 2(b) shows the proportion of the anisotropic phase as a function of the total CNC concentration in the suspension, which can be calculated as the ratio of the volume of the anisotropic phase to the total sample volume. In the biphasic regime, the suspension separates into two parts after the precipitate is allowed to settle in the tube: the upper phase is isotropic, and the lower one is anisotropic. This biphasic regime appears at CNC concentrations from ~4% to ~10%, beyond which the suspension exhibits only anisotropy.
In the LC phase, the cholesteric order of the CNCs is characterized by locally oriented nanorods along an average common direction63. The average orientation of molecules in a small volume is characterized as the director , which spatially rotates about an axis and forms a helicoidal structure with respect to . The CNCs assemble in the left-handed helical direction, which is caused by the chiral interactions between the nanorods; however, the intrinsic mechanism underlying the formation of the helicoidal structure is still under debate64–67. In a helicoidal structure formed by , the distance required for to rotate a full 360° is defined as the helical pitch . CNCs intrinsically generate optical properties when their helical pitches vary only within a certain range.
The helical structures of exhibit a characteristic “fingerprint texture” when the helical axis is parallel to the substrate surface, which means that the LCs must be vertically aligned to observe these fingerprint textures. Measuring the line distance of the fingerprint texture gives half of the helical pitch in the cholesteric LC phase. The texture observed through a POM is composed of planar and fingerprint textures, wherein the inclination of varies from orthogonal to parallel to the plane of the sample, with various disclinations. A POM can only be used to observe helical pitches from 2 to 5 µm; however, when the helical pitch approaches the wavelength of visible light, it becomes difficult to observe the fingerprint textures; thus, shorter pitches cannot be determined using this method.
Our purpose is to attain uniformly colored CNC films. To this end, the helical pitches values must be short enough to reflect visible light, and the alignment of the molecules must be controlled, which in turn controls helical axes; however, we have encountered difficulties in satisfying both requirements. The helical pitch of CNCs in aqueous suspensions is ~2 µm at most, as observed in Figure 2, and the suspensions are colorless; therefore, even if we deposit a CNC film from these suspensions on a glass substrate via spin-coating or a dipping process, the film does not exhibit iridescence, and its color is rather transparent.
Thus, the helical pitch of CNC suspensions must be shortened to observe structural color in CNC films. In general, the pitch is affected by several factors involving different mechanisms, for example, adding a nonvolatile additive or cosolvent is known to influence the equilibrium pitch of cholesteric suspensions, and adding D-glucose leads to a blue shift24. However, the success of these previous strategies has not been reproducible in our laboratory, even with D-glucose and similar additives such as d-fructose and d-sucrose, and a reliable way to control the pitch has not yet been established and remains an area of active research68.
Fortunately, during the course of our experiments on phase transitions in CNC suspensions, we unexpectedly observed a partially colored film that had dried from a droplet of a CNC suspension that accidentally dripped onto a desk. Inspired by this coloration, we began to examine some intentionally made droplets from CNC suspensions. Figure 3 compares two cases of differently dried CNC suspension droplets: one droplet was dried into a film in the ambient air and the other under higher humidity (~70–80%), which tends to allow the droplets to deform during drying. In the former, multiple colors form a rainbow-like “coffee-ring” stain from red to blue from the outer to the inner region of the droplet, with a bluish color appearing to be more dominant and uniform in the film interior, the observation of which is consistent with ref 60. During the drying process of the CNC suspension droplets, these coffee-ring stains can form because of the capillary flow caused by contact line pinning61,69 (i.e., the tendency of the outline of the droplet to remain in place). Initially, the contact line of the droplet is pinned, and the CNCs are homogeneously distributed; however, as water evaporates at the pinned contact line, this evaporation induces capillary flow, which tends to draw the CNCs toward the perimeter while the contact line gradually recedes toward the center. This mechanism is how the coffee-ring effect is generally understood60. These colors can be observed without a pair of polarizers and therefore must be structural coloration derived from the various helical pitches of the CNCs, suggesting that their pitch differs between the suspension and the resulting films and that the pitch in suspension gradually shortens as the droplet evaporates.
Obviously, increasing the temperature promotes the evaporation of water from the droplets, which in turn makes the flow in the droplets more active. This more active flow during evaporation can be assumed to kinetically hinder the transition to a thermodynamically stable pitch, which would be equivalent to the kinetic arrest of molecular motions into a gel-like glassy state68. Further, even if the pitch falls into the visible range under this kinetic arrest, the resulting colors may eventually disperse owing to the disturbed flow in the droplets. Figure 4 compares the textures of some films made from droplets of aqueous CNC suspensions with different concentrations dried at various constant temperatures. The biphasic suspension, i.e., the 7% CNC suspension, provides the most iridescent films under our conditions. This result contradicts previous findings that a sufficiently high concentration to ensure full LC properties is preferable in order to promote a uniform helical orientation perpendicular to the film plane58. By contrast, with the 10% CNC suspension, remarkably bright whitish textures increasingly appear in the films as the temperature increases, and very weak coffee-ring stains appear after drying at room temperature (~20 °C). Further, some domains with dimensions ranging from submicrons to millimeters can be observed, presumably because another factor, such as viscosity and hence mass transfer, plays a role in forming a film as water evaporates from the suspension. The film of the 2% CNC suspension also exhibits coffee-ring stains after drying at room temperature, but they can reasonably be considered weaker than those in the film made from the 7% CNC suspension. Owing to the isotropic nature of the 2% CNC suspension, the major central part of the film is dark. At higher temperatures, lightly colored textures can be observed against the dark background, which resemble an activated trace of mass transfer. Overall, as the temperature increases, the colors in the films mix over a larger area, and mixed stains appear rather than coffee-ring stains, thus validating our predictions of active flow in the droplets based on existing knowledge. When dried at room temperature, however, the texture of the films changes from mixed stains to coffee-ring stains.
Humidity is another parameter that influences the rate of water evaporation from aqueous CNC suspensions and hence affects the quality of their films. The moisture content in air is defined as humidity, which is expressed as the ratio of the amount of water vapor in the air to that of the saturated water vapor at a certain temperature. Obviously, low humidity means that air can take up more water vapor, thus increasing the evaporation rate from the suspensions. On the other hand, high humidity means that air already contains a great deal of moisture; thus, water evaporates more slowly. Figure 5 compares films made from CNC suspension droplets dried under various constant humidities. As with the temperature-controlled films, the biphasic 7% CNC suspension exhibits the most iridescent texture among the films. As the controlled humidity increases, the coffee-ring stains extend further from the edge toward the inner part of the droplets. At the highest humidity, the blue color spreads widely and evenly at the center of the droplet.
In terms of iridescence, none of the films dried from droplets of the 2% and 10% CNC suspensions show sufficient color. However, weak coffee-ring stains in the films made from the 2% CNC suspension clearly tend to extend further toward the center of the droplet as the humidity increases, while appearing dark around the center of the droplet, which must be due to the isotropic nature of the 2% CNC suspension. Meanwhile, the droplets made from the 10% CNC suspensions appear bright whitish rather than as colorful coffee-ring stains, suggesting that mass transfer was inefficient during drying, presumably owing to the high viscosity of this concentrated suspension. In addition, this finding suggests that the helical pitch of the CNCs varies in the visible spectral region, and the CNC phase changes into a glassy state before reaching thermodynamic equilibrium.
As another way to control the rate of water evaporation, we also attempted to increase the ionic strength of CNC suspensions using NaCl; however, adding NaCl drastically changed the suspensions, which immediately became opaque, indicating aggregation. The CNC suspensions must be in an electrostatically stabilized colloid state, wherein the delicate balance of the interaction potential among the colloids can be destroyed owing to the ionic strength. In other words, the CNC suspensions were thermodynamically in a quasi-stable state. The energy barrier from the quasi-stable state to the globally stable state, namely, the aggregated state, can be lowered at higher ionic strengths.
Further, to determine whether a more uniformly blue film can be attained, the humidity-controlled drying process was combined with orbital shaking, which is known to effectively form uniform CNC films. Figure 6 shows two droplet films dried under two constant high humidity values while being orbitally shaken. The blue region more uniformly extends from the edge to the center of the droplet when combining high-humidity drying with orbital shaking, suggesting that these conditions indeed effectively enhance the uniformity of the blue region in the film. Figure 7 validates this uniformity by comparing the reflection spectra of two extreme cases: one is from the film made dried from the 7% CNC suspension without orbital shaking under 24% humidity, and the other formed from the same concentration during orbital shaking under 98% humidity. The latter indeed shows reflection peaks caused by selective reflection in the blue region at the positions from the edge to the center of the film, whereas reflections from the same positions in the former shift from the yellow to blue region.
As previously reported58, orbital flow may enable the distortion of tactoids, thereby forming microdomains of CNCs while breaking the symmetry of these tactoids such that their vertical helical axes are oriented. Although this process is plausible, another possible mechanism should also be considered, namely, that orbital flow breaks capillary flow. The latter is directly correlated with water evaporation. Drying under high humidity while being orbitally shaken uniformly enhances the effect of the slow evaporation over the entire area of the droplet. Indeed, under high-humidity drying combined with orbital shear, the blue region spreads over the entire film.
The various colors observed in the CNC droplet films from red to blue are indeed structural color originating from the periodic CNC structure, and these colors vary with the helical pitch in the CNC films. Figure 8 shows the reflection spectra of some of the films presented in Figure 7, which were recorded using circularly polarized light as the input. Evidently, the left-handed helical pitch of the CNC reflects left-handed circularly polarized light (LPL) to a greater degree than it reflects right-handed polarized light (RPL). Unlike films of well-aligned cholesteric LCs, our CNC droplet films are obviously non-uniform in terms of thickness, helical pitch, and the orientations of their microdomains. Presumably, these intricate but non-uniform properties may depolarize the incoming light, and the LPL of the depolarized light would be preferentially reflected from the CNC films.
Our systematic observations above enable the development of a model to elucidate these results. One important finding is that the CNC helical pitch continually decreases as the CNC films form from the suspensions, suggesting that the helical pitch changes as water evaporates from the suspension droplets. In particular, temperature and humidity influence the rate at which water evaporates from aqueous suspensions, which in turn affects the CNC helical pitch, the dispersion of the CNC molecules, and the alignment of the helical axes in the films, as mentioned above. These considerations allow us to compare two types of LCs: thermotropic and lyotropic. In thermotropic LCs, the motions of rod-like and disk-like molecules in the fluid state can be frozen into a solid state depending on how rapidly the temperature decreases70. These materials can vitrify via the kinetic arrest of molecular motions in the fluid state, which dictates the degree of order in the resulting glassy state. The relationship between thermotropic LCs and temperature can be naturally observed in other common materials, including organic semiconductors, small molecules, and polymers; for example, good crystallinity and high molecular order both require a gradual decrease in temperature.
Analogously, the solvent (water in our case) plays a role akin to that of temperature in lyotropic LCs, including those formed in aqueous CNC suspensions. Figure 9 illustrates the qualitatively established model, wherein the water evaporation rate governs the degree of the helical pitch and molecular order. When water evaporates rapidly from the CNC suspension droplets, their helical pitch cannot be considered to change in a thermodynamically equilibrated manner24,60. In other words, CNCs can vitrify via the kinetic arrest68,71–73 of molecular motions in the fluid state. Considering that the formation of a uniformly colored film is based on the packing and alignment of molecules in the film, the molecular organization of CNCs, which are glass-forming LCs, can freeze during the drying process from the fluid state into a solid glass. By contrast, sufficiently slow water evaporation from the CNC suspension droplets can maintain thermodynamic equilibrium, and the CNC helical pitch shortens into the visible range while gradually following this equilibrium state, thereby exhibiting iridescence.