Aspect ratio and polydispersity dependence of isotropic-nematic transition in discotic suspensions

Dynamic light scattering analysis of size-selected graphene oxide 2D colloids fractioned via liquid crystal phase separation

Exfoliated platelets of graphene oxide (GO) can be considered as polydisperse 2D colloids that form nematic colloidal liquid crystal phases in aqueous suspension even at very low concentrations thanks to their extremely high aspect ratios. However, with the rapidly emerging scientific interest in these GO-based liquid crystals, it became clear that the precise analysis and control of the GO sheet size distribution is essential, both for their scientific understanding and for potential applications, e.g., in optoelectronic devices, nanocomposites, or catalysis. In this work, we show that the mean effective (hydrodynamic) GO platelet width can be determined from the translational diffusion coefficient with depolarized dynamic light scattering by using a model for circular, infinitely thin disks. We further studied the phase separation process of biphasic isotropic-nematic GO dispersions and developed a simple fractionation protocol, which can be used to prepare relatively monodisperse fractions of GO sheets with widths ranging from 2.0-12.4 μm. Overall, we expect that the combined application of these relatively simple fractionation and analysis methods will advance the fabrication of well-defined and size-selected GO-based systems.

Multiresponsive Polymer Nanocomposite Liquid Crystals Having Heterogeneous Phase Transitions for Battery-Free Temperature Maintenance Indicators

Multiresponsive functional materials that respond to more than one external stimulus are promising for novel photonic, electronic, and biomedical applications. However, the design or synthesis of new multiresponsive materials is challenging. Here, this work reports a facile method to prepare a multiresponsive colloidal material by mixing a liquid-crystalline 2D nanocolloid and a functional polymer colloid. For this purpose, electrically sensitive exfoliated α-ZrP 2D nanocolloids and thermosensitive block copolymer colloids that are dispersed well in water are mixed. In the liquid-crystalline nanocomposite, nematic, antinematic, or isotropic assemblies of α-ZrP, nanoparticles can be electrically and selectively obtained by applying electric fields with different frequencies; furthermore, their rheology is thermally and reversibly controlled through thesol-gel-sol transition. The nanocomposite exhibits a solid gel phase within a predesigned gel temperature range and a liquid sol phase outside this range. These properties facilitate the design of a simple display device in which information can be electrically written and thermally stabilized or erased, and using the device, a battery-free temperature maintenance indication function is demonstrated. The proposed polymer nanocomposite method can enrich the physical properties of 2D nanocolloidal liquid crystals and create new opportunities for eco-friendly, reusable, battery-free electro-optical devices.

Fine tuning the structural colours of photonic nanosheet suspensions by polymer doping

Aqueous suspensions of nanosheets are readily obtained by exfoliating low-dimensional mineral compounds like H₃Sb₃P₂O₁₄. The nanosheets self-organize, at low concentration, into a periodic stack of membranes, i.e. a lamellar liquid-crystalline phase. Due to the dilution, this stack has a large period of a few hundred nanometres, it behaves as a 1-dimensional photonic material and displays structural colours. We experimentally investigated the dependence of the period on the nanosheet concentration. We theoretically showed that it cannot be explained by the usual DLVO interaction between uniform lamellae but that the particulate nature of nanosheet-laden membranes must be considered. Moreover, we observed that adding small amounts of 100 kDa poly(ethylene oxide) (PEO) decreases the period and allows tuning the colour throughout the visible range. PEO adsorbs on the nanosheets, inducing a strong reduction of the nanosheet charge. This is probably due to the Lewis-base character of the EO units of PEO that become protonated at the low pH of the system, an interpretation supported by theoretical modeling. Oddly enough, adding small amounts of 1 MDa PEO has the opposite effect of increasing the period, suggesting the presence of an additional intermembrane repulsion not yet identified. From an applied perspective, our work shows how the colours of these 1-dimensional photonic materials can easily be tuned not only by varying the nanosheet concentration (which might entail a phase transition) but also by adding PEO. From a theoretical perspective, our approach represents a necessary step towards establishing the phase diagram of aqueous suspensions of charged nanosheets.

Mesoscopic Architectures Made of Electrically Charged Binary Colloidal Nanosheets in Aqueous System

Inorganic layered materials can be converted to colloidal liquid crystals through exfoliation into inorganic nanosheets, and binary nanosheet colloids exhibit rich phase behavior characterized by multiphase coexistence. In particular, niobate-clay binary nanosheet colloids are characterized by phase separation at a mesoscopic (∼several tens of micrometers) scale whereas they are apparently homogeneous at a macroscopic scale. Although the mesoscopic structure of the niobate-clay binary colloid is advantageous to realize unusual photochemical functions, the structure itself has not been clearly demonstrated in real space. The present study investigated the structure of niobate-clay binary nanosheet colloids in detail. Four clay nanosheets (hectorite, saponite, fluorohectorite, and tetrasilisic mica) with different lateral sizes were compared. Small-angle X-ray scattering (SAXS) indicated lamellar ordering of niobate nanosheets in the binary colloid. The basal spacing of the lamellar phase was reduced by increasing the concentration of clay nanosheets, indicating the compression of the liquid crystalline niobate phase by the isotropic clay phase. Scattering and fluorescence microscope observations using confocal laser scanning microscopy (CLSM) demonstrated the phase separation of niobate and clay nanosheets in real space. Niobate nanosheets assembled into domains of several tens of micrometers whereas clay nanosheets were located in voids between the niobate domains. The results clearly confirmed the spatial separation of two nanosheets and the phase separation at a mesoscopic scale. Distribution of clay nanosheets is dependent on the employed clay nanosheets; the nanosheets with large lateral length are more localized or assembled. This is in harmony with larger basal spacings of niobate lamellar phase for large clay particles. Although three-dimensional compression of the niobate phase by the coexisting clay phase was observed at low clay concentrations, the basal spacing of niobate phase was almost constant irrespective of niobate concentrations at high clay concentrations, which was ascribed to competition of compression by clay phase and restoring of the niobate phase.

Hierarchically Ordered α-Zirconium Phosphate Platelets in Aqueous Phase with Empty Liquid

Platelets of α-zirconium phosphate (α-ZrP) obtained from the reflux method in H₃PO₄ are successfully exfoliated into water via the intercalation of alkanol amines. With volume fractions greater than 0.02 they are stacked into tactoids of few layers with a repeat distance in the order of 10 nm. The tactoids align into nematic liquid crystalline phases with irregularly wide interstices of empty liquid. Colloidal processing involves the freeze-drying of such anisotropic fluids and the dispersion of the restacked tacoids into aqueous dispersions of colloidal polymer particles of largely varying size which occupy the otherwise empty liquid between the α-ZrP tactoids and induce piling of the tactoids into columns. Real-time SAXS on drying films and TEM of the obtained coatings demonstrate that the stacked α-ZrP platelets and the polymer particles comprising liquid dry separately without polymer intercalation, while the morphology of the obtained composites can be tuned primarily by the size of the polymer colloids. Concomitant α-ZrP hydrolysis in the exfoliation step is scrutinized as a function of amine basicity and temperature. The role of zirconium based hydrolysis products in the hierarchical α-ZrP assembly is indirectly though consistently confirmed by opposing impacts of ultra-filtration and added oxoanions on the platelets’ spacing, smoothness and aggregation. HAADF-TEM imaging of scattered, singular platelets and XRD peak analysis of the pristine solid shed light on the α-ZrP synthesis. Coexisting flakes and lacunae, both similar in size to the intra-layer crystal domains, suggest the stitching of proto-α-ZrP flakes into extended layers in accordance with our observations on the aging behaviour of α-ZrP dispersions as well as with literature data on related systems.

Orientational ordering and layering of hard plates in narrow slitlike pores

We examine the ordering behavior of hard platelike particles in a very narrow, slitlike pore using the Parsons-Lee density functional theory and the restricted orientation approximation. We observe that the plates are orientationally ordered and align perpendicularly (face-on) to the walls at low densities, a first-order layering transition occurs between uniaxial nematic structures having n and n+1 layers at intermediate densities, and even a phase transition between a monolayer with parallel (edge-on) orientational order and n layers with a perpendicular one can be detected at high densities. In addition to this, the edge-on monolayer is usually biaxial nematic, and a uniaxial-biaxial nematic phase transition can be also seen at very high densities.

Microscope Observation of Morphology of Colloidally Dispersed Niobate Nanosheets Combined with Optical Trapping

Although inorganic nanosheets prepared by exfoliation (delamination) of layered crystals have attracted great attention as 2D nanoparticles, in situ real space observations of exfoliated nanosheets in the colloidally dispersed state have not been conducted. In the present study, colloidally dispersed inorganic nanosheets prepared by exfoliation of layered niobate are directly observed with bright-field optical microscopy, which detects large nanosheets with lateral length larger than several micrometers. The observed nanosheets are not strictly flat but rounded, undulated, or folded in many cases. Optical trapping of nanosheets by laser radiation pressure has clarified their uneven cross-sectional shapes. Their morphology is retained under the relation between Brownian motion and optical trapping.

Evolutions of packing properties of perfect cylinders under densification and crystallization

Cylindrical particles are ubiquitous in nature and industry, and a cylinder is a representative shape of rod-like particles. However, the disordered packing results of cylinders in previous studies are quite inconsistent with each other. In this work, we obtain the MRJ (maximally random jammed) packings and the MDRPs (maximally dense random packings) of perfect cylinders with the aspect ratio (height/diameter) 0.2 ≤ w ≤ 6.0 using the ASC (adaptive shrinking cell) algorithm and the IMC (inverse Monte Carlo) method, respectively. The optimal aspect ratio corresponding to the maximal packing density is w = 0.9 in the MRJ state, while the value is w = 1.2 in the MDRP state. Then we investigate the evolutions of packing properties of perfect cylinders under densification and crystallization. We compare the different final packing states generated via the two methods with different compression rates and order constraints. In the densification procedure, we generate jammed and random packings of cylinders with various compression rates via the ASC and IMC method, respectively. When decreasing the compression rate, we find that the packing density increases but the optimal w remains the same in both methods. In the crystallization procedure, the order constraint in the IMC method is gradually released which means the degree of order in the packings is allowed to increase, and we find that the optimal w shifts from 1.2 to 0.9 while the packing density increases as well. Meanwhile, the random packings evolve into the jammed packings in the crystallization procedure which reflects the competition mechanism between the randomness and jamming. These results also indicate that the optimal w is solely related to the degree of order in the cylinder packings but not determined by the protocol or packing density. Furthermore, a uniform shape elongation effect on the random-packing densities of various shaped particles is found via a new proposed definition of the scaled aspect ratio. Finally, a rough linear relationship between the mean and standard deviation of the reduced Voronoi cell volumes is obtained only for the random packings. Our findings should lead to a better understanding toward the jammed and random packings and are helpful in guiding the granular material design.

Hierarchical, Self-Healing and Superhydrophobic Zirconium Phosphate Hybrid Membrane Based on the Interfacial Crystal Growth of Lyotropic Two-Dimensional Nanoplatelets

We demonstrate a facile route to in situ growth of lyotropic zirconium phosphate (ZrP) nanoplates on textiles via an interfacial crystal growing process. The as-prepared hybrid membrane shows a hierarchical architecture of textile fibers (porous platform for fluid transport), ZrP nanoplatelets (layered scaffolds for chemical barriers), and octadecylamine (organic species for superhydrophobic functionalization). Interestingly, such a hybrid membrane is able to separate the oily wastewater with a high separation efficiency of 99.9%, even at in harsh environments. After being chemically etched, the hybrid membrane is able to restore its hydrophobicity autonomously and repeatedly, owing to the hierarchical structure that enables facile loading of healing agent. We anticipate that the concept of implanting superhydrophobic self-healing features in anisotropic structure of lyotropic nanoparticles will open up new opportunities for developing advanced multifunctional materials for wastewater treatment, fuel purification, and oil spill mitigation.

Electrical switching of birefringence in zirconium phosphate colloids with various solvents

Even though a graphene-oxide (GO) dispersion is attractive for electro-optical switching devices because of its high Kerr coefficient, it has several limitations such as chemical instability and optical loss due to absorption at visible wavelengths. Here we introduce the use of tetrabutylammonium-tethered α-zirconium phosphate (TBA-ZrP) colloid in various solvents for electro-optical switching devices; the TBA-ZrP colloid is chemically stable and optically transparent. We find that the electrical switching behavior of TBA-ZrP is sensitively dependent on the type of solvent. The TBA-ZrP colloid in acetone exhibits the highest effective Kerr coefficient and the fastest switching time, which is related to the unusual behavior of the viscosity of the TBA-ZrP colloid in acetone. Its viscosity is relatively low and less sensitive to concentration compared to ZrP in other solvents. This indicates that the motion of individual nanoparticles is relatively less restricted in acetone. These findings may be useful in developing electro-optical devices using lyotropic liquid crystal colloids.

Staged phase separation in the I-I-N tri-phase region of platelet-sphere mixtures

Mixtures of colloids with different sizes or shapes are ubiquitous in nature and extensively applied in industries. Phase transition pathways and kinetics in this model system should be investigated because of the difficulty in observing tri-phase coexistence in colloidal platelet-sphere mixtures. Similar to the polymer-sphere mixtures, the phase transition pathway has three main categories. Analytical results show a staged phase transition process in which the mixture first separates into one or two metastable phases, then further separates, and subsequently reaches tri-phase equilibrium. Unique to our system, and different from the gas-liquid-crystal coexistence in colloid-polymer mixtures, the platelet-sphere mixture reached a gas-liquid-liquid crystal (nematic) coexistence. Thus, the different phases are easy to distinguish using the birefringence of the liquid crystals. In addition, the volume fraction of the liquid crystal formation in the ZrP platelet suspensions is much lower than for the crystal formation in hard spheres.

Polydispersity reduction of colloidal plates via size fractionation of the isotropic-nematic phase transition

Controlling the size polydispersity of colloidal particles is important for their phase transitions, resulting structures, and properties. In this study, a fractionation method was established to control the polydispersity of colloidal plates based on the isotropic-nematic (I-N) phase transition. The size ratio of nanoplates between the N phase and the I phase (D_N/D_I) was relatively large, whereas the size polydispersities in both the N phase and the I phase were smaller than that of the original sample before fractionation. The degree of fractionation was dependent on the time since the phase transition began and the polydispersity of the original sample. A long time resulted in a small D_N/D_I and a small degree of polydispersity reduction. The experimental data confirmed a quadratic scaling of D_N/D_I with polydispersity that was predicted by simulations. Large to small particles were segregated sequentially by sedimentation because of self-assembly and gravity. The polydispersity reduction based on the I-N phase transition can be utilized to select nanoplates with a certain size with improved size monodispersity.

Graphene and Other 2D Colloids: Liquid Crystals and Macroscopic Fibers

Two-dimensional colloidal nanomaterials are running into renaissance after the enlightening researches of graphene. Macroscopic one-dimensional fiber is an optimal ordered structural form to express the in-plane merits of 2D nanomaterials, and the formation of liquid crystals (LCs) allows the creation of continuous fibers. In the correlated system from LCs to fibers, understanding their macroscopic organizing behavior and transforming them into new solid fibers is greatly significant for applications. Herein, we retrospect the history of 2D colloids and discuss about the concept of 2D nanomaterial fibers in the context of LCs, elaborating the motivation, principle and possible strategies of fabrication. Then we highlight the creation, development and typical applications of graphene fibers. Additionally, the latest advances of other 2D nanomaterial fibers are also summarized. Finally, conclusions, challenges and perspectives are provided to show great expectations of better and more fibrous materials of 2D nanomaterials. This review gives a comprehensive retrospect of the past century-long effort about the whole development of 2D colloids, and plots a clear roadmap - "lamellar solid - LCs - macroscopic fibers - flexible devices", which will certainly open a new era of structural-multifunctional application for the conventional 2D colloids.

Thermosensitive ZrP-PNIPAM Pickering Emulsifier and the Controlled-Release Behavior

Asymmetric Janus and Gemini ZrP-PNIPAM monolayer nanoplates were obtained by exfoliation of two-dimensional layered ZrP disks whose surface was covalently modified with thermosensitive polymer PNIPAM. The nanoplates largely reduced interfacial tension (IFT) of the oil/water interface so that they were able to produce stable oil/water emulsions, and the PNIPAM grafting either on the surface or the edge endowed the nanoplates rapid temperature responsivity. The ZrP-PNIPAM nanoplates proved to be thermosensitive Pickering emulsifiers for controlled-release applications.

Synthesis and Exfoliation of Discotic Zirconium Phosphates to Obtain Colloidal Liquid Crystals

Due to their abundance in natural clay and potential applications in advanced materials, discotic nanoparticles are of interest to scientists and engineers. Growth of such anisotropic nanocrystals through a simple chemical method is a challenging task. In this study, we fabricate discotic nanodisks of zirconium phosphate [Zr(HPO4)2·H2O] as a model material using hydrothermal, reflux and microwave-assisted methods. Growth of crystals is controlled by duration time, temperature, and concentration of reacting species. The novelty of the adopted methods is that discotic crystals of size ranging from hundred nanometers to few micrometers can be obtained while keeping the polydispersity well within control. The layered discotic crystals are converted to monolayers by exfoliation with tetra-(n)-butyl ammonium hydroxide [(C4H9)4NOH, TBAOH]. Exfoliated disks show isotropic and nematic liquid crystal phases. Size and polydispersity of disk suspensions is highly important in deciding their phase behavior.

Observation of isotropic-isotropic demixing in colloidal platelet-sphere mixtures

Mixtures of colloids with different sizes and shapes are ubiquitous in nature and industry. The possible existence of isotropic-isotropic (I1-I2) demixing of platelets and spheres remains an open question. Here we present direct experimental evidence of I1-I2 demixing using platelets with a very small thickness-to-diameter ratio mixed with silica spheres at the size ratio q = R(sphere)/R(disk) = 0.0901 ± 0.0004. The platelets cause the isotropic-to-nematic phase transition at a very low volume fraction because of their highly anisometric shape. The presence of silica spheres in the suspension accelerates the phase transition and packs the nematic phase more densely via depletion interaction. Increasing the sphere volume fraction to 0.0014, a tri-phase region emerges. This direct observation of I1-I2 demixing seems to validate the free-volume scaled particle theory and indicates the need for refinement of the fundamental measure density functional theory.

Interplay of anisotropy in shape and interactions in charged platelet suspensions

Motivated by the intriguing phase behavior of charged colloidal platelets, we investigate the structure and dynamics of charged repulsive disks by means of Monte Carlo simulations. The electrostatic interactions are taken into account through an effective two-body potential, obtained within the nonlinear Poisson-Boltzmann formalism, which has the form of anisotropic screened Coulomb potential. Recently, we showed that the original intrinsic anisotropy of the electrostatic potential in competition with excluded volume effects leads to a rich phase behavior that not only includes various liquid-crystalline phases but also predicts the existence of novel structures composed of alternating nematic-antinematic sheets. Here, we examine the structural and dynamical signatures of each of the observed structures for both translational and rotational degrees of freedom. Finally, we discuss the influence of effective charge value and our results in relation to experimental findings on charged platelet suspensions.

Hard-body models of bulk liquid crystals

Hard models for particle interactions have played a crucial role in the understanding of the structure of condensed matter. In particular, they help to explain the formation of oriented phases in liquids made of anisotropic molecules or colloidal particles and continue to be of great interest in the formulation of theories for liquids in bulk, near interfaces and in biophysical environments. Hard models of anisotropic particles give rise to complex phase diagrams, including uniaxial and biaxial nematic phases, discotic phases and spatially ordered phases such as smectic, columnar or crystal. Also, their mixtures exhibit additional interesting behaviours where demixing competes with orientational order. Here we review the different models of hard particles used in the theory of bulk anisotropic liquids, leaving aside interfacial properties and discuss the associated theoretical approaches and computer simulations, focusing on applications in equilibrium situations. The latter include one-component bulk fluids, mixtures and polydisperse fluids, both in two and three dimensions, and emphasis is put on liquid-crystal phase transitions and complex phase behaviour in general.

Thermo-sensitive discotic colloidal liquid crystals

We fabricated for the first time thermo-sensitive discotic liquid crystals by grafting poly(N-isopropylacrylamide) (PNIPAM) onto zirconium phosphate (ZrP) platelets using pre-irradiated polymerization. The I-N transition was investigated by adjusting the temperature for a single set of samples. We found that soft disks self-assemble into nematic liquid crystals in a wider thickness-over-diameter ratio than do hard disks.

Temperature-dependent isotropic-to-nematic transition of charged nanoplates

Studies of temperature-dependent phase behaviors of discotic colloids are found infrequently in the literature. We establish here the isotropic-to-nematic (I-N) transition phase diagram of charged platelets in the temperature-versus-volume fraction plane. We discover that the N phase can be melted by increasing temperature, and that coexistent samples are more sensitive to polydispersity at higher temperature and higher concentration.