In this paper, we present a comprehensive study of the sol-gel transitions and liquid crystal phase transitions in aqueous suspensions of positively charged colloidal gibbsite platelets at pH 4-5 over a wide range of particle concentrations (50-600 g/L) and salt concentrations (10(-4)-10(-1) M NaCl). A detailed sol-gel diagram was established by oscillatory rheological experiments. These demonstrate the presence of kinetically arrested states both at high and at low salt concentrations, enclosing a sol region. Birefringence and iridescence show that in the sol state nematic and hexagonal columnar liquid crystal phases are formed. The gel and liquid crystal structures are studied in further detail using small-angle X-ray scattering (SAXS) and cryo-focused ion beam/scanning electron microscopy (cryo-FIB-SEM). The gel formed at high salt concentration shows signatures of a sponge-like structure and does not display birefringence. In the sol region, by lowering the salt concentration and/or increasing the gibbsite concentration, the nematic phase gradually transforms from the discotic nematic (ND) into the columnar nematic (NC) with much stronger side-to-side interparticle correlations. Subsequently, this NC structure can be either transformed into the hexagonal columnar phase or arrested into a birefringent repulsive gel state with NC structure.

Antireflective coatings (ARCs) are applied to reduce surface reflections. We review coatings that reduce the reflection of the surface of the transparent substrates float glass, polyethylene terephthalate, poly(methyl methacrylate), and polycarbonate. Three main coating concepts exist to lower the reflection at the interface of a transparent substrate and air: multilayer interference coatings, graded index coatings, and quarter-wave coatings. We introduce and discuss these three concepts, and zoom in on porous quarter-wave coatings comprising colloidal particles. We extensively discuss the four routes for introducing porosity in quarter-wave coatings through the use of colloidal particles, which have the highest potential for application: (1) packing of dense nanospheres, (2) integration of voids through hollow nanospheres, (3) integration of voids through sacrificial particle templates, and (4) packing of nonspherical nanoparticles. Finally, we address the remaining challenges in the field of ARCs, and elaborate on potential strategies for future research in this area.

A comprehensive nanoscale understanding of layered double hydroxide (LDH) thermal evolution is critical for their current and future applications as catalysts, flame retardants and oxygen evolution performers. In this report, we applied in situ transmission electron microscopy (TEM) to extensively characterise the thermal progressions of nickel-iron containing (Ni-Fe) LDH nanomaterials. The combinative approach of TEM and selected area electron diffraction (SAED) yielded both a morphological and crystallographic understanding of such processes. As the Ni-Fe LDH nanomaterials are heated in situ, an amorphization occurred at 250°C, followed by a transition to a heterogeneous structure of NiO particles embedded throughout a NiFe 2 O 4 matrix at 850°C, confirmed by highresolution TEM and scanning TEM. Further electron microscopy characterisation methodologies of energy-filtered TEM were utilised to directly observe these mechanistic behaviours in real time, showing an evolution and nucleation to an array of spherical NiO nanoparticles on the platelet surfaces. The versatility of this characterisation approach was verified by the analogous behaviours of Ni-Fe LDH materials heated ex situ as well as parallel in situ TEM and SAED comparisons to that of an akin magnesium-aluminium containing (Mg-Al) LDH structure. The in situ TEM work hereby discussed allows for a state-of-the-art understanding of the Ni-Fe material thermal evolution. This is an important first, which reveals pivotal information, especially when considering LDH applications as catalysts and flame retardants.

The effect of a nonspherical particle shape and shape polydispersity on the structure of densely packed hard colloidal particles was studied in real space by confocal microscopy. We show that the first layer at the wall of concentrated size-monodisperse but shape-polydisperse polyhedral colloids exhibits significant deviations from a hexagonal lattice. These deviations are identified as bond-orientational fluctuations which lead to percolating ''mismatch lines.'' While the shape-induced geometrical frustration of the hexagonal symmetry suppresses translational order, bond-orientational order is clearly retained, indicating a hexaticlike structure of the polyhedral colloids. DOI: 10.1103/PhysRevLett.96.028304 PACS numbers: 82.70.Dd, 64.60.Cn, 64.70.ÿp Polydispersity is an intrinsic property of colloidal systems. For hard spheres it is well known that size polydispersity has a tremendous effect on the structure and dynamics [1][2][3][4][5][6]. However, the effect of the particle shape is a more general issue, as molecular systems are ''monodisperse'' in size but usually not spherical. The impact of shape is, for example, demonstrated in simulations that show the existence of a rotator-phase in hard hexamers, pentamers, and pentagons [7][8][9][10], being absent in hard spheres. The influence of shape polydispersity on crystallization has hardly been addressed, despite the fact that relevant systems like nanoparticles [11] and granular matter [12] often exhibit a distribution of particle shapes. Here, we use a colloidal model system to study the influence of only the particle shape on the structure of densely packed particles. In particular, the particles represent hard spheres that are monodisperse in size, but have a small random perturbation in shape.The model colloids are crosslinked and fluorescently labeled polymethylmethacrylate (PMMA) particles. Their preparation is described in Ref. [13]. As shown in the inset in Fig. 1(a), these particles clearly have a nonspherical, polyhedral shape, which was also observed using electron microscopy. Note, that ''polyhedral'' refers to the twodimensional (2D) cross section through the center of the three-dimensional particle. The particle shape can be characterized by a shape factor SF 4A p =P 2 p , with A p the projected particle area and P p the projected perimeter. The average SF for the polyhedrals is 0:76 0:09 compared to 1 for a perfect sphere. The random character of the nonsphericity is reflected by the polydispersity in shape of 11.8%. Comparison to regular polygons reveals that the polyhedrals are on average best described by pentagons. Remarkably, their size polydispersity is comparable to that of rather monodisperse colloidal spheres. Therefore, these polyhedrals provide an excellent model system to investigate the effect of shape and shape polydispersity. In our experiment, we compare the structure of the first layer at the wall of densely packed polyhedrals to one formed by spherical and size-monodisperse reference PMMA particles [inset Fig. 1 . Both sys...

Gelation is a common effect in aqueous suspensions of charged colloidal clay platelets at concentrations as low as 1 wt%. However, in systems of charged gibbsite [Al(OH) 3 ] platelets, gelation can be delayed to concentrations as high as 50 wt% depending on the ionic strength. We investigated the phase behaviour of this system approaching the state of gelation in the delicate region between attractive and repulsive states that originate from competition between Coulomb repulsion and van der Waals attraction. As a function of the ionic strength, isotropic-nematic, nematic-columnar and isotropiccolumnar phase separations were observed. Moreover, compression by gravitational forces allowed us to observe phase separation that is arrested by gelation in the homogeneous suspensions.

Colloidal platelets of hydrotalcite, a layered double hydroxide, have been prepared by coprecipitation at pH 11-12 of magnesium nitrate and aluminum nitrate at two different magnesium to aluminum ratios. Changing the temperature and ionic strength during hydrothermal treatment, the platelets were tailored to different sizes and aspect ratios. Amino-modified polyisobutylene molecules were grafted onto the platelets following a convenient new route involving freeze-drying. Organic dispersions in toluene were prepared of the particles with the largest size and highest aspect ratio. The colloidal dispersions prepared in this way showed isotropic-nematic phase transitions above a limiting concentration in a matter of days. The number density at the transition and the width of the biphasic region were determined and compared to theory. The orientation of the platelets in nematic droplets (tactoids) and at the isotropic-nematic interface were analyzed by polarization microscopy. It was observed that sedimentation induces a nematic layer in samples that are below the limiting concentration for isotropic-nematic phase separation. No nematic phase was observed in the initial aqueous suspensions of the ungrafted particles.

This paper describes the electro-optic response of a suspension of disk-like colloids. We have considered aqueous suspensions of Gibbsite platelets and measured the electrically induced birefringence in the broad frequency range 10(2)-10(8) Hz. When simply dispersed in an electrolyte solution, these particles orient with their major axis parallel to the electric field at all frequencies. The spectral dependence of their Kerr coefficient features three regimes: an electrokinetic α-relaxation within the kHz range, a conductive Maxwell-Wagner-O'Konski (MWO) relaxation having characteristic frequency in the 1-10 MHz range, and a nonzero high frequency asymptote. We quantitatively analyze the MWO spectral component and the high-frequency asymptote on the basis of a model developed for oblate particles. This analysis enables us to obtain the relevant particle properties: surface conductivity, zeta potential, and intrinsic Gibbsite birefringence. When the particles are dispersed in a mixture that also contains smaller spherical particles that have a charge of the same sign, their electric birefringence becomes negative at low frequency. This anomalous orientation of the platelets is analogous to that observed in mixtures of prolate and spherical particles, and demonstrates the anomalous birefringence as a universal property of suspensions of nonspherical particles when surrounded by smaller charged particles.

Rheological, optical and structural properties of colloidal suspensions of charge-stabilized gibbsite platelets across the sol-gel transition region are investigated. In this work we focus on samples with a low salt content (10 −4 M). While at a gibbsite concentration of 300 g l −1 , a nematic-columnar phase separation is observed, an arrested state with a nematic signature and highly elastic response has been observed for a concentration of 400 g l −1 . A temporal evolution of the structure of the arrested state, which leads to stronger interparticle correlations, has been observed on a timescale of 20 months. The results suggest that the arrested state develops into a glass with a columnar nematic structure.