Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 1845−1851
pubs.acs.org/JPCL
Structure Evolution and Drying Dynamics in Sliding Cholesteric Cellulose Nanocrystals Guang Chu, Rita Vilensky, Gleb Vasilyev, Patrick Martin, Ruiyan Zhang, and Eyal Zussman* NanoEngineering Group, Faculty of Mechanical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel S Supporting Information *
ABSTRACT: The study of colloidal liquid crystals (LCs) reveals fundamental insights into the nature of ordered materials, giving rise to emergent properties with fascinating applications in soft matter nanotechnology. Here we investigate the shape instabilities, layer undulations, dynamic assembly, and collective behaviors in evaporating a cellulose nanocrystal-based cholesteric LC drop. During the drying process, the drop edges are pinned to the substrate with spontaneous convective flow occurring along the drop, which leads to nonequilibrium sliding of the individual cholesteric fragment with active ordering as well as hydrodynamic fluctuations and flow transitions in the bulk cholesteric phase.
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cholesteric liquid-crystalline phase above a threshold concentration, with their rods oriented locally uniform along a common direction.18 Moreover, drying a CNC suspension can induce a phase transition from isotropic to cholesteric with its ordering retained in a solid film after the suspension completely dries, providing a robust matrix for optical sensor, templating, and assembling nanoparticles.19−24 Due to the coffee-stain effect, some recent studies have shown that accumulation of CNCs occurs at the edge of the evaporating droplet, resulting in gradients across the droplet center to its border, both in the concentration of CNC nanoparticles and photonic structural colors.25−27 However, the dynamic assembly details of CNCs during evaporation has so far not been elucidated. Thus, the drop-drying effect with CNC cholesteric LCs is still very attractive for further investigation. In this contribution, we bridge the gap from static to dynamic assembly in cholesteric LCs by introducing the coffee-stain system based on a CNC drop. We explored the evaporation dynamics, ordering transition, pitch deformation, collective behavior, deposition pattern, and temporal evolution of the LC structure during the entire drying process. It showed that the CNCs aggregated and self-assembled into twisted filaments and cholesteric fragments in dimethylformamide (DMF), demonstrating a flexible and stretchable mesophase. When under evaporation, these fragments could move, fold, and unfold during the convective flow, generating dynamic ordering and structural transition. Additionally, topological defects in cholesteric fragments could be preserved in both dynamic and static states. Drying a pure cholesteric LC drop led to
drying drop of colloidal suspension is a complex and farfrom-equilibrium system that exhibits a rich phenomenology such as the coffee-stain effect, Marangoni flow, complex deposition patterns, and phase concentrating at the drop edge.1−4 This phenomenon has been used to control the assembly of nanoparticles, molecules, and polymers during the evaporation process,5−7 providing a versatile medium for both fundamental science and engineering ideas that has practical applications in coating, printing, and surface patterning.8−11 A colloidal liquid crystal (LC) contains solid particles suspended in fluids that combines the features of colloids and anisotropy, giving rise to a highly ordered long-range structure at equilibrium.12 When evaporation occurs in colloidal LCs, their droplet contact lines become pinned to the substrate and generate a radial outward capillary flow. Therefore, the suspended particles are transported to the perimeter of the drop, concentrate, and dry along the edge, generating a “coffeering” stained structure.13 The coffee-stain effect in rod-like particle suspensions has intrinsic interests in colloidal LCs. Upon evaporation, the suspension will spontaneously transform into an anisotropic phase in which the rod-like particles are entropically favored, resulting in lyotropic LCs.14 During the flowing stage, colloid particles are dynamically self-assembled with the liquid-crystalline ordering sustained, denoting a process that the structure forms when the evaporation system is driven by continuous energy input and dissipation.15 Cellulose nanocrystals (CNCs) can be produced by controlled acid hydrolysis of cellulose microfibrils, exhibiting rigid, rod-like crystallites.16,17 When sulfuric acid is employed, sulfate ester groups are covalently attached to the surface of CNC nanoparticles and make them negatively charged, allowing the formation of a stable aqueous suspension. The colloidal CNCs can spontaneously self-organize into a © XXXX American Chemical Society
Received: March 2, 2018 Accepted: March 27, 2018 Published: March 27, 2018 1845
DOI: 10.1021/acs.jpclett.8b00670 J. Phys. Chem. Lett. 2018, 9, 1845−1851
Letter
The Journal of Physical Chemistry Letters disruption of the cholesteric ordering due to the intensive flow. As evaporation progressed, the flowing speed decreased and the cholesteric structure was reformed and highly aligned along the flowing direction. Eventually, the bulk cholesteric ordering was preserved and deposited into the film with its pitch director pointed to the center of the drop. Our work not only demonstrates an applicable strategy for constructing novel macroscopic materials that integrates LC technology with the coffee-stain effect but also expands the idea for dynamic assembly in conventional colloids. The CNCs were prepared by sulfuric acid hydrolysis followed by purification and freeze-drying, showing an average length and diameter of 160 and 8 nm, respectively (Supporting Information, Figure S1). CNC powder was redispersed in DMF by strong stirring and sonication, generating a transparent suspension with a concentration of 6.0 wt % (Figure 1a). After
(Supporting Information, Figure S5). In a cross-polarized optical microscopy (POM) image, twisted filaments were observed after allowing the CNC suspension to stand for 8 h, with the corresponding sizes ranging from 100 to over 500 μm (Figures 1c and S6). The dark regions corresponded to the sections of the filaments where the CNCs pointed perpendicular to the image plane. In comparison, bright regions indicated the sections of filaments where the CNCs were in the image plane and exhibited strong structural anisotropy. These results are very similar to the recently reported DNA origami filaments.28 After standing 12 h, the CNC suspension changed from transparent to cloudy, generating amounts of micrometer-sized cholesteric fragments with a helical pitch of 12 μm (Figure 1d), indicating the formation of a polydomain mesophase. Cryo-scanning electron microscopy (Cryo-SEM) images of the fragments confirmed the assembly and aggregation of CNCs as well as their 3D irregular shape (Supporting Information, Figure S7). Because of morphology differences, we can distinguish the cholesteric fragments from CNC tactoids. The tactoids are spherical, ellipsoidal, or spindle-shaped anisotropic droplets that nucleated with sharp edges in an isotropic suspension.18,29,30 Besides, these cholesteric fragments could be dismissed into bundles of filaments under high shear stress, which implied the evolution trace from filaments to fragments (Supporting Information, Figure S8). Finally, over time, the sediment fragments led to bulk phase separation between an upper transparent and a bottom cloudy phase, generating a cholesteric liquid-crystalline CNC.31,32 The helical pitch in the anisotropic bottom phase was 10 μm, which was similar to the cholesteric fragments in suspension (Figure 1e). The corresponding CNC concentrations and volume fractions in the top and bottom phases were 3.3 and 7.8 wt % and 0.0194 and 0.0467, respectively. The temporal evolution of the drying effect in CNC suspension was studied by tracking the evaporation process through POM images and videos. An aged CNC suspension (12 h, 50 μL) was deposited on a hydrophilic glass slide and airdried under ambient conditions (Figure 2). The evaporating drop featured a curved air−fluid interface and had a spherical cap shape, with the contact angle and droplet’s diameter at 15° and 8 mm, respectively (Supporting Information, Figure S9). Initially, the contact line was indeed pinned and generated outward capillary flow from the drop’s center to its edges.1 Due to the nonuniform evaporation-induced temperature gradients, inward Marangoni flow was observed at the drop free surface (Figure 2a). The combination of capillary flow and Marangoni flow formed strong convection and was prominent in organic solvents but difficult to generate in aqueous suspension.33,34 During evaporation, massive cholesteric fragments were observed inside of the isotropic CNC suspension, exhibiting distinct flow-induced shape instabilities (Figure 2b and Movie S1). When viewed under cross-polarized light, these fragments exhibited a typical fingerprint morphology, with the outlines blurred by the curved interface of the drop. The area of the fragment could be larger than 100 μm2 with varying shapes, such as sheet, bundle, arch, circle, semicircle, etc. Owing to the convective flow, the CNC cholesteric fragments from the drop interior were first transported to its perimeter and then gradually moved back (Figure 2c). The corresponding moving speeds of these fragments for the outward and inward direction were 35 and 22 μm/s, respectively. An asymmetric hydrodynamic shear stress was applied on these fragments during the
Figure 1. (a) Photograph of the phase separation process of the macroscopic CNC suspension into a cholesteric phase and an isotropic phase. (b) Growing of the CNC aggregates diameter up increasing time. (c) POM image of an individual CNC filament with twisted morphology. (d) POM image of the CNC cholesteric fragments suspended in solution. (e) POM image of the CNC cholesteric phase sealed in a tube.
sonication, the CNC suspension was entirely isotropic, exhibiting low viscosity and concentration-dependent high fluidity (Supporting Information, Figures S2−S4). After equilibration, the dynamic light scattering (DLS) measurement showed that the CNC nanorods in DMF self-organized into aggregates, and the sizes began to grow at a rate that increased over time (Figure 1b). As time increased further, these aggregates grew from particles into 1D birefringent filaments ∼5 μm in diameter and ∼100 μm in persistence length 1846
DOI: 10.1021/acs.jpclett.8b00670 J. Phys. Chem. Lett. 2018, 9, 1845−1851
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The Journal of Physical Chemistry Letters
Figure 2. (a) Schematic diagram of the evaporation process. (b) POM image of the CNC cholesteric fragment inside of the drop. (c) Moving process of a cholesteric fragment first from the drop center to its rim and then back. Time interval, 2 s. (d,e) Series of magnified POM images; the CNC cholesteric fragment constantly changes (rotate and fold) its structure and texture during the hydrodynamic flow, showing a highly dynamic state of the cholesteric ordering. Time interval, 2 s. (f) Structure of the regular cholesteric LC, where p is pitch, n is director, and χ is helical axis direction. (g) Sketch of the Helfrich−Hurault instability of the cholesteric layers under hydrodynamic force applied at the walls in a vertical direction. (h) Sketch of the pitch−splay, or layer undulation, instability in sliding cholesteric fragments. Black lines show the projection of the twisted director field onto the plane containing the splayed pitch axis (red lines). The black arrows show the asymmetric hydrodynamic stress, which acts to increase the distortion and drives the instability.
between the surrounding fluid medium and cholesteric ordered CNC aggregates. However, in contrast to rotating, folding of the fragment usually occurred in the 2D fragment sheet. It showed a typical fingerprint texture with semicircle morphology, exhibiting layer undulations and incomplete Schlieren texture during the flow. When the fragment was rotated by 180° and rebound with a relative moving direction opposite its original, then the sheet started to bend into a scroll that exhibited the extensile behavior of the CNC cholesteric layers. Besides, it was worth noting that both rotating and folding of the fragment could lead to a dynamic bend−splay deformation of the cholesteric texture, showing a continuous change from one perturbed structure to another. Figure 2f−h illustrates the structure evolution of the cholesteric fragment during the hydrodynamic flow. Usually, the CNC nanorods cause a twist in the nematic structure under the static state. They consist of quasi-nematic layers, whose individual directors are turned by a fixed angle upon proceeding from one layer to the next. The layers are turned equivalently by an angle of 2π, and the distance between two adjacent layers defines the pitch of the helical structure (Figure 2f). In this case, periodic modulation in the cholesteric fragment carries the director through a continuous family of symmetry-equivalent directions. However, if the fragments are dynamically moving in the fluid flow, the resulting asymmetric hydrodynamic interactions between the fluid and fragments can generate
collective movement (see Supporting Information), leading to structure transition, shape instability, and inner orientation changing of the helical director. In view of this phenomenon in the fragment, we therefore refer to this remarkable active moving state of the CNC fragment as “sliding cholesteric”. Compared with bulk LCs, the flow-induced sliding of cholesteric fragments depends on their intrinsic stretchable and flexible properties as well as their irregular shapes. Convective flow generates driving forces between different fractions of the same fragment, which leads to complex motion states of the monolithic fragment and fluctuations in its helical director. Figure 2d,e shows image sequences of an individual fragment in the motion, which highlights the motion states of rotating and folding, respectively (see Movies S2 and S3 in the Supporting Information). In one case, the 3D fragment rotated anticlockwise along the flow with four dark brushes twined around it. The brushes could be ascribed to the distorted Schlieren textures, which corresponded to the extinction orientation of the aligned CNC nanorods. The singularity point, where four brushes met, was called a disclination and was dynamically shifted from the center to the surface of the fragment when the fragment was rotating, indicating collective orientation variation of the wave vectors inside of the fragment. Furthermore, the helical pitch almost remained constant (14 μm) with its fingerprint textures dynamically undulated along the rotating direction, implying hydrodynamic interactions 1847
DOI: 10.1021/acs.jpclett.8b00670 J. Phys. Chem. Lett. 2018, 9, 1845−1851
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The Journal of Physical Chemistry Letters
Figure 3. (a) POM image of the CNC tactoids during evaporation. (b) Sequence of POM images demonstrating retention of CNC tactoids at the contact line. Time interval, 1.5 s. POM images of the accumulations of the CNC tactoids (c) without and (d) with a full-wavelength (530 nm) retardation plate with the slow axis (γ) marked by a yellow arrow.
Figure 4. POM images of island-distributed cholesteric fragments surrounded by an isotropic phase obtained (a) without and (b) with a 530 nm retardation plate. Defects in cholesteric fragments show dislocations (c), disclinations (d), and grain boundary (e).
critical concentration, CNCs aggregated and formed shortrange ordered tactoids. Capillary flow brought the suspended tactoids from the drop center to its edge and then moved it back due to Marangoni flow (Figure 3a and Movie S4). The outward and inward moving speeds of the CNC tactoids were 22 and 18 μm/s, respectively. This speed difference led to the detaining of tactoids at the contact line, rotating during the convective flow (Figure 3b and Movie S5). When evaporation went further, the viscosity of the CNC suspension increased and the contact line started to retract (Supporting Information, Figures S10 and S11). Instead of fusing into the cholesteric mesophase, the tactoids heaped up at the rim of the drop, giving rise to a domain pattern (Figure 3c). In contrast, when the sample was viewed in the presence of a full-wave
layer undulations and distortions in the cholesteric plane, exhibiting bend−splay instabilities. It will apply a hydrodynamic stress on the dislocation in the direction perpendicular to the plane and give a wavy form to the bend−splay distortion in the cholesteric layer (Figure 2g,h). Thus, the flow-induced deformation of the cholesteric fragment includes a bend distortion of the director field that is directed along the pitch axis and a pure splay distortion of the pitch or a bend of the cholesteric layer. This layer undulation of the CNC fragment is similar to the Helfrich−Hurault instability, which is obtained in bulk cholesteric LCs in response to an external field or hydrodynamic stress.35,36 We also experimentally tracked the phase transition in a CNC drop. When the isotropic suspension was evaporated to 1848
DOI: 10.1021/acs.jpclett.8b00670 J. Phys. Chem. Lett. 2018, 9, 1845−1851
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The Journal of Physical Chemistry Letters
Figure 5. Evaporation of a pure cholesteric LC drop with convective laminar (a) and vortex (b) flow. (c) Retracting of the contact line as evaporation progressed with shifting of the laminar flow from the drop edge to its interior. The gray and yellow arrows exhibit the outward and inward flow directions, respectively. Collective motion of large area of the cholesteric phase (d) and defects (e) when the moving speed of the fluid decreased. (f) Schematic diagram of the collective motion of a CNC cholesteric LC with its pitch axis along the moving direction. (g) POM image of the static cholesteric LC drop, showing a fingerprint texture along the moving direction. (h) POM image of the dried cholesteric drop with marblelike texture. (i) SEM image of the surface undulations at the rim that propagate inward to the drop center.
Information, Figure S13). This allowed us to deduce the rolling 3D shape of these cholesteric fragments, which agreed well with its corresponding morphology patterns (Supporting Information, Figure S14). Figure 5 shows the drying process of a pure cholesteric LC derived from the bottom phase. Initially, a drop of the cholesteric LC with a volume of 50 μL was cast onto a glass slide and evaporated at ambient condition. The droplet’s diameter and contact angle were 5 mm and 23°, respectively (Supporting Information, Figure S15). Compared with the aged CNC suspension, the low scalability of the cholesteric LC drop was mainly due to its high viscosity, which prevented fluid spreading on the glass slide (Supporting Information, Figure S16). During the starting stage of drying, the contact line was pinned, and a bulk LC from the drop interior was first transported to the perimeter and then moved back due to the convective flow. Two types of motion were observed for cholesteric LC; one was the convective laminar flow, and the other was a convective vortex (Figure 5a,b and Movies S6 and S7). Both of them were located at the edge of the drop, with the LC in the drop interior kept static. The laminar flow was due to the combination of geometrical constraint and a temperature-induced Marangoni effect. The LC fluid was first squeezed outward by capillary force to compensate for evaporation and then moved along the surface tension gradients back to the drop interior. The moving speed of the outward flow (33 μm/s) was higher than its inward one (21 μm/s), which revealed retention of the LC at the contact line. The vortex was usually observed at the initial stage of drying with a high LC fluid moving speed. This turbulence could be ascribed to the interplay between the increasing fluid viscous stresses during LC motion and the confinement space of the drop, resulting in a high Reynolds number in the LC, in agreement with reported hydrodynamic numerical simulations
retardation plate between crossed polarizers, discrete blue and orange tactoids were observed and indicated phase shifting (Figure 3d). For CNCs with ne = 1.595 and no = 1.534, ne and no are extraordinary and ordinary refractive indices, respectively.37,38 The positive refractive index anisotropy (Δn = ne − no) along with the interference colors revealed the differences in orientation as well as the surface topography (Δn ≈ λ/d) in stacking of the tactoids. At the final drying stage, the cholesteric fragments gathered at the center of the drop and stopped moving, forming a polydomain region in which the fragments were islanddistributed and radially aligned in a direction from the interior to the rim of the drying drop (Figure 4a,b). The helical axes were oriented along different directions in each domain, with their periodicities of the fingerprint texture in corresponding domains slightly different when the sample was examined with a full-wave plate (Figure 4b). This might be due to the different oblique and in-plane orientations of the helical axes as well as sometimes slightly different surface topography in domains of cholesteric fragments. In magnified individual domains, massive topological defects such as dislocations, disclinations, and grain boundaries were observed inside of the fragments, as revealed by POM images shown in Figure 4c−e. The deformations of cholesteric phases at both short-range and long-range (as compared to the cholesteric pitch) scales were in accordance with the predictions of elastic theory, where the size of the fragment was much larger than its pitch, and exhibited strongly twisted cholesterics.39 It was worth noting that the topological defects were derived from each individual sliding cholesteric fragment rather than emerging from adjacent fragments during the drying process (Supporting Information, Figure S12). When the fragment was rotated in the presence of a full-wave retardation plate, continuous switching of alternated blue and orange fingerprint domains was observed (Supporting 1849
DOI: 10.1021/acs.jpclett.8b00670 J. Phys. Chem. Lett. 2018, 9, 1845−1851
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about the cholesteric LC with high activity.40 The turbulent flow could transform into laminar flow as evaporation proceeded. Both flows could disrupt the cholesteric order under high shear stress, with the resulting director field and flow field resembling that of a nematic LC at large activity. The LC fluid motion continued to decrease because of the evaporative solvent loss, which finally led to shifting of the convective flow inward from the drop edge to its interior (Figure 5c and Movie S8). During this drying stage, the director field of the LC was highly aligned with its orientation parallel to the flow direction that revealed the flow-induced dynamic assembly of the CNC (Supporting Information, Figure S17). Furthermore, when the moving speed of the convective flow was decreased to the critical threshold (∼10 μm/s), a large area of the fingerprint structure was observed along with the flow direction, which indicated the formation of periodic cholesteric ordering (Figure 5d,e and Movies S9 and S10). The deformations of cholesteric ordering led to topological defects in the fingerprint structure, which was also preserved during the fluid motion. Figure 5f is a schematic diagram of the cholesteric LC moving with its pitch axis along the flow direction. Here, we assume that the cholesteric LC is subjected to an imposed flow parallel to the pitch axis with the helical phase kept constant. The coupling between the director and velocity field led to strong non-Newtonian fluid behavior.41 Finally, the fluid motion stopped, and the radial flow stress resulted in longrange fingerprint structures at the rim propagated inward to the center of the drop, giving rise to highly oriented helical domain patterns with a pitch of 11 μm (Figure 5g). With continued evaporation, the drop was cast onto a film. Figure 5h,i shows the POM and SEM images of the dry film, respectively. The film displayed a marble-like texture, typical of cholesteric order but without a fingerprint structure, which is due to shrinking of the helical pitch. The morphology of the film showed some surface undulations along the flow direction that radially pointed to the center, with its helical structure lying on the substrate (Figures 5i and S18). The birefringence texture and surface undulations of the dry film suggested that, although the convective flow rendered influences on the cholesteric LC at macroscopic scale, their presence did not totally destroy the ordering, only left some traces. In summary, we describe the dynamic assembly of a drying CNC suspension drop, showing both flow-induced sliding of the cholesteric fragment and collective behavior of the bulk phase. During solvent evaporation of an aged CNC suspension, the shape instabilities, layer undulations, birefringent textures, topological defects, phase transition, and temporal evolution of the cholesteric fragment are tracked and studied in detail. However, when the cholesteric fragments settlement into the bulk LC, upon drying of the LC in a drop, its cholesteric ordering is first destroyed by the highly active convective flow and then reformed and aligned along the flow direction when the flowing speed decreases. This mode of dynamic assembly in cholesteric LCs expands our knowledge to realize the transitions and fluctuations in a new type of active matter. We believe that the fundamental ideal presented in this work will be heuristic for other colloidal LC systems with different phases, such as nematic, smectic, and blue phases with controlled chirality, which deserves further investigation and achieves more sophisticated dynamic LC ordering.
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b00670. Additional experimental details and figures (e.g., SEM images, POM images and rheology data) (PDF) Movie S1 showing the collective motion of CNC cholesteric fragments flowing from the drop center to the contact line and then moving back due to the Marangoni flow, showing flow-induced sliding instability (AVI) Movie S2 showing rotating of the 3D cholesteric fragment during the flow, exhibiting structure transition, shape instability, and inner orientation changes of the helical director (AVI) Movie S3 showing folding of the 2D cholesteric fragment layer during the flow under hydrodynamic stress (AVI) Movie S4 showing the collective motion of CNC tactoids swimming from the drop center to the rim and then moving back as evaporation progressed (AVI) Movie S5 showing that the CNC tactoids were retained at the contact line and rotated during the convective flow (AVI) Movie S6 showing the cholesteric LC first move outward due to the capillary flow and then moving inward due to the corresponding Marangoni flow, showing a convective laminar flow inside of the whole drop (AVI) Movie S7 showing the convective vortex flow of the cholesteric LC drop located at the edge of the drop with symmetry distribution (AVI) Movie S8 showing shifting of the convective flow from the drop edge to its interior due to the increase of viscosity and decrease of LC fluid motion as evaporation progressed (AVI) Movie S9 showing the collective motion of a large area of cholesteric LC ordering, showing typical fingerprint texture along the flow direction (AVI) Movie S10 showing the defect in cholesteric LC during the collective motion when the moving speed is low (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Guang Chu: 0000-0003-1538-5276 Ruiyan Zhang: 0000-0002-6739-2075 Eyal Zussman: 0000-0002-4310-6548 Author Contributions
G.C. prepared the cellulose liquid crystal and carried out the POM, SEM, and rheology measurements. G.C. designed and led the project. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Russell Berrie Nanotechnology Institute (RBNI), the Israel Science Foundation (ISF Grant No. 286/15). E.Z. acknowledges financial support of the 1850
DOI: 10.1021/acs.jpclett.8b00670 J. Phys. Chem. Lett. 2018, 9, 1845−1851
Letter
The Journal of Physical Chemistry Letters
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Winograd Chair of Fluid Mechanics and Heat Transfer at Technion.
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DOI: 10.1021/acs.jpclett.8b00670 J. Phys. Chem. Lett. 2018, 9, 1845−1851
